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Probing the Electronic Structures and Relative Stabilities of Monomagnesium Oxide Clusters MgOx- and MgOx (x=1-4): A Combined Photoelectron Imaging and Theoretical Investigation
Abstract
The electronic and structural properties of small monomagnesium oxide clusters, MgOx(-) and MgOx (x = 1-4), have been investigated using a synergistic approach combining photoelectron imaging spectroscopy and first principles electronic structure calculations. The adiabatic detachment energy (ADE) and vertical detachment energy (VDE) of MgOx(-) clusters along with the photoelectron angular distributions (PADs) are determined experimentally. The measured PADs of the clusters are dependent on both the orbital symmetry and electron kinetic energies. Density-functional theory (DFT) calculations were performed to explore the optimized geometries of neutral and anionic MgOx clusters. The theoretical ADE and VDE values calculated according to the optimized geometries are in good agreement with our experimental measurements. In addition, MgO(-) and MgO4 clusters are found to have enhanced relative stability in the corresponding anionic and neutral series, based on both theoretical parameters and the experimental cluster distribution.
... Vanadium oxides, in particular, are the most versatile metal oxide catalysts predominantly used in industrial catalytic processes [17,18], making the understanding of the nature of vanadium-oxygen bonds highly valuable. Experiments on metal oxides in the gas phase can provide valuable molecular models with which to explore the fundamental bonding mechanism and their dependence on electronic structure without the interference from solvation and other environmental factors [3,[19][20][21][22]. ...
... pure vanadium rod. To obtain smaller oxide clusters, a high-purity helium gas (typically 50 psi) seeded with 5% N 2 O was used as a carrier gas to react with laser-ablated V atoms [19]. The formed neutral and charged clusters were expanded through a conical nozzle, and then collimated by a skimmer. ...
The electronic structure of the diatomic VO anion was explored by combining the photoelectron-imaging spectroscopy and high-level theoretical calculations. The electron affinity (EA) of VO is determined to be 1.244 ± 0.025 eV from the vibrationally resolved photoelectron spectrum acquired at 532 nm. The anisotropy parameter (β) for the EA defined peak is measured to be 1.59 ± 0.02, indicating that it is the 9σ electron attachment leading to the formation of the ground state of VO−. The present imaging experiment provides direct evidence that the ground state of VO− is X3Σ− with a (3π)4(8σ)2(9σ)2(1δ)2 electron configuration, which resolves the significant discrepancy in previous experiment and theory. In addition, the molecular orbitals and bonding involved in the anionic VO cluster are also examined based on the present high-level theoretical calculations.
... In principle, AMS of 26 Al completely avoids atomic isobars when selecting the elemental anion Al − for injection because Mg has a negative EA (Andersen et al. 1999 unprecedented suppression of MgO − of >10 14 at ∼50% overall transmission of AlO − through the ion guide and allows to exploit the benefits of higher overall detection efficiency (Lachner et al. 2021). Despite a published VDE of 1.61 eV (Cheng et al. 2013), MgO − is suppressed by up to 10 5 by simple passage through pure He and 16 O − is found in the ion beam exiting the cooler. While clearly suggesting dissociation reactions, this behavior is still not fully understood and e.g., not observed for MnO − , a diatomic anion with similar EA (see below). ...
A setup for ion-laser interaction was coupled to the state-of-the-art AMS facility VERA five years ago and its potential and applicability as a new means of isobar suppression in accelerator mass spectrometry (AMS) has since been explored. Laser photodetachment and molecular dissociation processes of anions provide unprecedented isobar suppression factors of >10 ¹⁰ for several established AMS isotopes like ³⁶ Cl or ²⁶ Al and give access to new AMS isotopes like ⁹⁰ Sr, ¹³⁵ Cs or ¹⁸² Hf at a 3-MV-tandem facility. Furthermore, Ion-Laser InterAction Mass Spectrometry has been proven to meet AMS requirements regarding reliability and robustness with a typical reproducibility of results of 3%. The benefits of the technique are in principle available to any AMS machine, irrespective of attainable ion beam energy. Since isobar suppression via this technique is so efficient, there often is no need for any additional element separation in the detection setup and selected nuclides may even become accessible without accelerator at all.
... We explored this question by utilizing the photoelectron spectroscopy, which has been proven to be a powerful approach to directly probe the electronic properties of atoms and clusters [27][28][29][30][31][32][33][34][35][36][37][38][39][40] . Herein, we present direct experimental observations on the features of the electron-atom interaction in Eu − . ...
Direct experimental determination of precise electron affinities (EAs) of lanthanides is a longstanding challenge to experimentalists. Considerable debate exists in previous experiment and theory, hindering the complete understanding about the properties of the atomic anions. Herein, we report the first precise photoelectron imaging spectroscopy of europium (Eu), with the aim of eliminating prior contradictions. The measured EA (0.116 ± 0.013 eV) of Eu is in excellent agreement with recently reported theoretical predictions, providing direct spectroscopic evidence that the additional electron is weakly attached. Additionally, a new experimental strategy is proposed that can significantly increase the yield of the lanthanide anions, opening up the best opportunity to complete the periodic table of the atomic anions. The present findings not only serve to resolve previous discrepancy but also will help in improving the depth and accuracy of our understanding about the fundamental properties of the atomic anions.
... Therefore, finding a superatomic cluster with numbers of unpaired electrons and valence electrons that are similar to those of the corresponding RE is the most important step in mimicking the magnetic properties of REs, which is also the focus of this study. Spectroscopy combined with high-level theoretical calculations has been shown to be a powerful tool to directly examine the electronic structures of atoms and clusters (18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28). Herein, we used photoelectron imaging spectroscopy to explore the possibility of mimicking the magnetic properties of the REs. ...
Significance
Superatom concept has been found to be promising in designing atomic clusters to mimic the chemistry of scarce or expensive elements. Also, longstanding interest and challenge exist in searching for suitable candidates to mimic the valuable properties of rare earth elements owing to their highly important application in modern technologies and extremely low yield. Herein, by using photoelectron imaging spectroscopy, we provide direct experimental evidence that well-designed boron-doped clusters, namely LaB and NdB, could be promising candidates to mimic the magnetic properties of corresponding rare earth atoms Nd and Eu, respectively, because the same numbers of valence electrons, unpaired electrons, and magnetic moments are found in both of the counterparts. This finding opens up an exciting possibility for rare earth mimicry using the superatom concept.
The existence of abundant 4f electrons significantly increases the complexity and difficulty in precisely determining the geometric and electronic structures of the lanthanide oxide clusters. Herein, by combining the photoelectron imaging spectroscopy and density functional theory (DFT) calculations, the electronic structure of GdO was investigated. An electron affinity (EA) of 1.16 ± 0.09 eV is obtained, and the measured anisotropy parameter (β) provides direct experimental evidence about the orbital symmetry of the detached electron in GdO⁻. DFT calculations have been employed to acquire the optimized geometries of the GdOn−1/0 (n = 2-4) clusters, and multiple activated oxygen species, which are radical, peroxide, superoxide, triradical, and ozonide radical, are found in these oxide clusters. Simulated photoelectron spectra (PES) and of the GdOn−1/0 (n = 2-4) clusters are examined, which may stimulate further experimental investigations on the gadolinium oxide clusters. In addition, the valence molecular orbitals (MOs) of these clusters are also discussed to reveal the interaction between the lanthanide metal (Gd) and O atoms.
We present a joint photoelectron imaging spectroscopic and theoretical investigation on the triatomic ZrC2(-) anion. Vibrationally resolved spectrum was acquired at 532 nm photon energy. Electron affinity for the neutral ZrC2 cluster was determined to be 1.60 ± 0.07 eV. The CCSD(T) level of theory was used to explore the ground-state geometries and vibrational frequencies of the anionic and neutral ZrC2 clusters. Our vibrationally resolved PES reveals two vibrational frequencies (564.6 and 1774.0 cm(-1)) of the neutral ZrC2 cluster, which correspond to the symmetric Zr-C2 and C-C stretching modes, and these experimental findings are in good agreement with the calculated values. Additionally, the molecular orbitals and chemical bonding in the anionic and neutral ZrC2 clusters are also discussed to disclose the interaction between the transition metal atom (Zr) and C2 unit.
In the current review of the journal Structural Chemistry, the contents of the issues 4–6 for the calendar year 2012 are related to thermochemistry. A brief thermochemical commentary is added to the summary of each paper.
Metal clusters featuring closed supershells or aromatic character usually exhibit remarkably enhanced stability in their cluster series. However, not all stable clusters are subject to these fundamental constraints. Here, by employing photoelectron imaging spectroscopy and ab initio calculations, we present experimental and theoretical evidence on the existence of unexpectedly stable open-shell clusters, which are more stable than their closed-shell and aromatic counterparts. The stabilization of these open-shell Al-Mg clusters is proposed to originate from the S-P molecular orbital coupling, leading to highly stable species with increased HOMO-LUMO gaps, akin to s-p hybridization in organic carbon atom that is beneficial to form stable species. The introduction of such coupling effect highlighted here not only shows the limitations of the conventional closed-shell model and aromaticity but provides the possibility to design valuable building blocks.
The low-lying electronic states, X(2)Π and A(2)Σ(+) of CaO(+) and X(2)Σ(+) and A(2)Π of CaO(-), have been determined at the MRCI+Q level of theory with the aug-cc-pV5Z(O) and cc-pCV5Z(Ca) basis sets. The two states of CaO(+) are close within <0.1 eV and coupled via spin-orbit effect. The X(2)Σ(+) and A(2)Π states of CaO(-) are energetically separated by <1 eV such that the first excited state is close to the electronic ground state of neutral CaO and unstable with respect to electron detachment. Using the potential energy curves and the spin-orbit coupling terms, the vibronic energy levels of these ions have been determined. The ionization energy and the electron affinity of CaO are calculated at 6.79 and 0.79 eV, respectively. The photoelectron spectra of CaO(-) and CaO have also been simulated.
The concept of aromaticity has been advanced beyond the framework of organic chemistry, and multiple aromaticity (σ, π, and δ) has been observed to account for the highly symmetric structures or unusual stability of the clusters. In the present study, the electronic structures and chemical bonding of small monolanthanum boride clusters are investigated using photoelectron imaging spectroscopy and first principles electronic structure calculations. Accurate electron affinities of 1.32 ± 0.04 and 1.13 ± 0.06 eV for the neutral LaB2 and LaB3 clusters are obtained by the vibrationally-resolved photoelectron spectra of the LaB2(-) and LaB3(-) clusters, respectively. It is shown that LaB2(-) and LaB3 exhibit enhanced stability in their respective cluster series, as evidenced from the calculated removal energies and HOMO-LUMO gaps. Molecular orbital analysis discloses that these two clusters possess doubly aromatic characters (σ and π), responsible for their enhanced stability. Interestingly, unlike conventional σ-, π-, and δ-aromaticity formed by the delocalization of unhybridized p or d orbitals, the σ and π delocalized molecular orbitals shown here are formed through the effective overlap between the 5d atomic orbital of the La atom and the p orbitals of the remaining boron atoms, representing an intriguing d-p hybridized aromaticity.
Highly correlated ab initio methods are used to investigate the lowest MgO2+ electronic states. Our computations confirm the existence of the strongly bent (MgO2+X∼2A2) form and the weakly bound l-MgOO+(X∼4Σ-) charge quadrupole complex. For both isomers, the three-dimensional potential energy surfaces (3D-PESs) of their electronic ground states are mapped in internal coordinates not far from their respective equilibrium geometries. Then a set of spectroscopic parameters is derived using second order perturbation theory. The rovibrational spectra are also deduced variationally. One-dimensional cuts of the 3D-PESs of the lowest doublet and quartet electronic states of MgO2+ along the RMgO and ROO stretchings and bending are calculated, covering both the molecular and the asymptotic regions. These curves are used later for discussing the metastability of this cation and to propose a plausible mechanism for the Mg++O2 atmospheric ion–molecule reaction.
In the past, basis sets for use in correlated molecular calculations have largely been taken from single configuration calculations. Recently, Almlöf, Taylor, and co‐workers have found that basis sets of natural orbitals derived from correlated atomic calculations (ANOs) provide an excellent description of molecular correlation effects. We report here a careful study of correlation effects in the oxygen atom, establishing that compact sets of primitive Gaussian functions effectively and efficiently describe correlation effects i f the exponents of the functions are optimized in atomic correlated calculations, although the primitive (s p) functions for describing correlation effects can be taken from atomic Hartree–Fock calculations i f the appropriate primitive set is used. Test calculations on oxygen‐containing molecules indicate that these primitive basis sets describe molecular correlation effects as well as the ANO sets of Almlöf and Taylor. Guided by the calculations on oxygen, basis sets for use in correlated atomic and molecular calculations were developed for all of the first row atoms from boron through neon and for hydrogen. As in the oxygen atom calculations, it was found that the incremental energy lowerings due to the addition of correlating functions fall into distinct groups. This leads to the concept of c o r r e l a t i o n c o n s i s t e n t b a s i s s e t s, i.e., sets which include all functions in a given group as well as all functions in any higher groups. Correlation consistent sets are given for all of the atoms considered. The most accurate sets determined in this way, [5s4p3d2f1g], consistently yield 99% of the correlation energy obtained with the corresponding ANO sets, even though the latter contains 50% more primitive functions and twice as many primitive polarization functions. It is estimated that this set yields 94%–97% of the total (HF+1+2) correlation energy for the atoms neon through boron.
Contracted Gaussian basis sets for molecular calculations are derived from uncontracted (12,8) and (12,9) sets for the neutral second row atoms, Z=11–18, and for the negative ions P−, S−, and Cl−. Calculations on Na...2p63p, 2P and Mg...2p63s3p, 3P are used to derive contracted Gaussian functions to describe the 3p orbital in these atoms, necessary in molecular applications. The derived basis sets range from minimal, through double‐zeta, to the largest set which has a triple‐zeta basis for the 3p orbital, double‐zeta for the remaining. Where necessary to avoid unacceptable energy losses in atomic wave functions expanded in the contracted Gaussians, a given uncontracted Gaussian function is used in two contracted functions. These tabulations provide a hierarchy of basis sets to be used in designing a convergent sequence of molecular computations, and to establish the reliability of the molecular properties under study.
In this article we present a new method for reconstructing three-dimensional (3D) images with cylindrical symmetry from their two-dimensional projections. The method is based on expanding the projection in a basis set of functions that are analytical projections of known well-behaved functions. The original 3D image can then be reconstructed as a linear combination of these well-behaved functions, which have a Gaussian-like shape, with the same expansion coefficients as the projection. In the process of finding the expansion coefficients, regularization is used to achieve a more reliable reconstruction of noisy projections. The method is efficient and computationally cheap and is particularly well suited for transforming projections obtained in photoion and photoelectron imaging experiments. It can be used for any image with cylindrical symmetry, requires minimal user's input, and provides a reliable reconstruction in certain cases when the commonly used Fourier-Hankel Abel transform method fails. (C) 2002 American Institute of Physics.
In the past, basis sets for use in correlated molecular calculations have largely been taken from single configuration calculations. Recently, Almlöf, Taylor, and co‐workers have found that basis sets of natural orbitals derived from correlated atomic calculations (ANOs) provide an excellent description of molecular correlation effects. We report here a careful study of correlation effects in the oxygen atom, establishing that compact sets of primitive Gaussian functions effectively and efficiently describe correlation effects if the exponents of the functions are optimized in atomic correlated calculations, although the primitive (sp) functions for describing correlation effects can be taken from atomic Hartree–Fock calculations if the appropriate primitive set is used. Test calculations on oxygen‐containing molecules indicate that these primitive basis sets describe molecular correlation effects as well as the ANO sets of Almlöf and Taylor. Guided by the calculations on oxygen, basis sets for use in correlated atomic and molecular calculations were developed for all of the first row atoms from boron through neon and for hydrogen. As in the oxygen atom calculations, it was found that the incremental energy lowerings due to the addition of correlating functions fall into distinct groups. This leads to the concept of correlation consistent basis sets, i.e., sets which include all functions in a given group as well as all functions in any higher groups. Correlation consistent sets are given for all of the atoms considered. The most accurate sets determined in this way, [5s4p3d2f1g], consistently yield 99% of the correlation energy obtained with the corresponding ANO sets, even though the latter contains 50% more primitive functions and twice as many primitive polarization functions. It is estimated that this set yields 94%–97% of the total (HF+1+2) correlation energy for the atoms neon through boron.
Recent advances in experimental techniques now enable researchers to produce in a laboratory clusters of atoms of desired composition from any of the elements of the periodic table. This has created a new area of research into novel materials since clusters cannot be regarded either as a "large" molecule or as a fragment of the bulk. Both experimental and theoretical studies are revealing unusual properties that are not ob served in solid state environments. The structures of micro-clusters are found to be significantly distorted from the most symmetric arrangement, some even exhibiting pentagonal symmetry commonly found in icosahedric structures. The unusual stability of certain clusters, now described as "magic number species", shows striking similarities with the nuclear shell structure. The relative stabilities of clusters depend not only on the composition of the clusters but also on their charged states. The studies on spontaneous fragmentation of mUltiply charged clusters, commonly referred to as Coulomb explosion, illustrate the role of electronic bonding mechanisms on stability of clusters. The effect of foreign atoms on geometry and stability of clusters and the interaction of gas atoms with clusters are showing promise for an indepth understanding of chemisorption and catalysis. The magnetic and optical properties are dependent not only on cluster size but also on its geometry. These findings have the potential for aiding industry in the area of micro-electronics and catalysis.
This chapter describes building blocks of new materials. The chapter reviews some earlier work on clusters that relates to the development of the concept. A mass spectra of the generated clusters showed that clusters containing 2, 8, 18, 20, 40, 58, atoms have been more prominent than other sizes These numbers have been called magic numbers and experiments on other alkali metals revealed similar magic species, indicating that they were not specific to sodium. As alkali atoms are monovalent, the prominence in the mass spectra could be due to either the electronic counts or other features such as special geometries at these sizes. Further experiments on the dissociation of clusters showed that the magic sizes are the favored fragmentation products and that it required more energy to fragment magic species. All these observations indicated that (1) the magic clusters are very stable species and (2) their stability is probably rooted in electronic counts. The chapter illustrates that Knight and co-workers also proposed that the stability of magic clusters can be understood within a simple jellium picture. As the concept of superatoms relies on the electronic structure and electron counts, the model is considered in detail in the chapter.
SCF and MCSCF wave functions for the low-lying 3,1∑+ and 3,1Π states of MgO are presented and discussed. It is argued that a viable description of these states is possible if the 3∑+ and 3,1Π states are treated at the SCF level while the 1- and 2- 1∑+ states are treated with a limited MCSCF expansion.
We report a photoelectron spectroscopic investigation of a series of divanadium-oxide clusters V2Ox- (x=3-7). Well-resolved spectra were obtained at three photon energies (355, 266, and 193 nm), revealing the structural and electronic evolution as the number of oxygen atoms increases in the cluster series. A behavior of sequential oxidation was observed in V2Ox- for x up to 5: low binding energy features with primarily V 3d characters were disappearing in numbers and simultaneously shifting to higher binding energies with increasing oxygen content as a result of V[right arrow]O charge transfers. Finally, for V2O6- and V2O7-, the photoelectron spectra exhibit very-high-binding-energy features characteristic of O 2p characters. Vibrationally resolved spectra were obtained for the ground-state features of V2O4- and V2O6-, with a spacing of 1090 cm-1 (V2O4) and 800 cm-1 (V2O6), which are assigned to V-O stretching vibrations. Electron affinities are reported for V2O3 to V2O7, and those of 5.61 eV for V2O6 and 5.38 eV for V2O7 are among the highest electronic affinities ever reported. The data are compared with previous theoretical calculations.
Extended measurements are reported for rovibronic transitions between the strongly mixed low-lying X1Σ+, a3Π and A1Π electronic states of the MgO molecule in the 1640–3510 cm−1 region. The measurements were made with the Faraday laser magnetic resonance (LMR) technique utilizing a CO laser and a CO overtone laser. The new observed lines include the A1Π-X1Σ+ (0–0, 0–1, 1–1, 1–2, 1–3, 2–2, 2–4, 3–3) bands as well as the much weaker a3Π0.1–X1Σ+ (0–0, 1–0) intercombination bands. The present highly precise LMR data provide a basis for performing an extensive deperturbation analysis including the spin-orbit, orbit-rotation and Zeeman interactions between the rovibronic levels of the X1Σ+, a3Π and a1Π states. Thereby the present data are combined with reliable earlier experimental results, in particular for the transitions involving higher lying vibronic states, in order to obtain the effective deperturbed molecular constants and perturbation parameters for the three lowest lying electronic states of MgO to as high an accuracy as possible. The absolute standard deviation of the LMR data is 6 × 10−4 cm−1 (18 MHz), which is in accord with the accuracy of our measurements. The set of deperturbed molecular parameters reproduces all measured lines with an overall standard deviation of 0·87 relative to the experimental uncertainties. For perturbations such as spin-orbit interaction there is good agreement with the predictions of large scale ab initio calculations.
1 Cluster and Nanoscale Science: Overview and Perspective.- 2 Quantum and Classical Size Effects in Thermodynamic Properties.- 3 Photoelectron Spectroscopy.- 4 Quantum Tunneling of the Magnetization in Molecular Nanoclusters.- 5 Magnetism of Free and Supported Metal Clusters.- 6 Magnetism in Free Clusters and in Mn12O12-Acetate Nanomagnets.- 7 Size Effects in Catalysis by Supported Metal Clusters.- 8 Delayed Ionization.- 9 Cluster Dynamics: Influences of Solvation and Aggregation.- 10 Future Directions.
This chapter discusses several experimental studies conducted on cluster ions. Cluster ions comprise a nonrigid assembly of components having properties between those of large gas phase molecules and the bulk condensed state. Research on the properties of cluster ions is valuable in obtaining a complete understanding of the forces between ions and neutral atoms or molecules, where, for example, data on energetics provide a direct measure of the depth of the potential well of interaction between the ion and the collection of neutral molecules. There are three types of experimental approaches to produce and study these cluster ions depending on different degrees of physical isolation, that is, clusters moving freely in space (gas phase studies), clusters supported upon a substrate surface, and clusters existing in solids and liquids. Moreover, the use of cluster ions for fuel injection into fusion reactors and cluster ions are also present in other high-temperature systems such as those designed as sources of energy employing magnets and hydrodynamics.
Potential curves for the ground and various excited states of the MgO molecule have been calculated using the multi-reference configuration interaction (MRD CI) method. The electronic structure of the states is analyzed in terms of their semi-diffuse, ionic, Rydberg, and valence character and the electric dipole moments of the low-lying states are given. Special emphasis is placed upon the ionic-covalent interactions which give rise to numerous avoided crossings, primarily at large internuclear separations. The character of the low-lying 1Σ+ states which have distinct multiconfigurational nature is carefully investigated.
We have experimentally demonstrated that the electronic ground state of MgO is X1Σ+. The lowest energy excited state was shown to be a3Πi, for which the following constants were determined (1σ uncertainty in parentheses): View Within Article The a3Πi state was observed and characterized from its perturbations of the X1Σ+ (vX > 2) levels. These perturbations were detected in the photoluminescence from the B1Σ+ state induced by the 476.5-, 496.5-, and 514.5-nm lines of an argon ion laser. The three laser lines were shown to coincide with a total of eight strong and assignable molecular lines belonging to the B1Σ+ - X1Σ+ system of the three isotopic species 24Mg16O, 25Mg16O, and 26Mg16O. The 496.5-nm line excited the less abundant 26Mg16O species almost exclusively. Rotationally resolved photoluminescence from eight B1Σ+ (vB, JB) levels was observed into vibronic levels X1Σ+ (vX = 0–7) and A1Π (vA = 0–8). X1Σ+ ∼ a3Πi perturbations appeared as level shifts often accompanied by extra transitions into levels of dominants a3Πi character; a total of 20 extra lines were assigned. A1Π ∼ X1Σ+ perturbations were also observed, and six extra lines were assigned. Measurement of the relative intensities of B1Σ+ - A1Π and B1Σ+ - X1Σ+ photoluminescence originating simultaneously from a single (vB, JB) level permitted determination of the transition moment ratio View Within Article The electronically excited A1Π state was present in our Mg + N2O flame in sufficient concentration to record the MgO B1Σ+ ← A1Π excitation spectrum using a cw Rhodamine 6G dye laser. This observation raises the possibility of direct population monitoring of other electronically excited species in flames.
Gas-phase thermochemical values D°(Mg+-OH) = 75 ± 4 kcal/mol, IP(MgOH) = 7.3 ± 0.1 eV, D°(Mg+-O) = 53 ± 3 kcal/mol, and IP(MgO) = 7.9 ± 0.1 eV have been experimentally determined by photodissociation measurements and charge-transfer reactions with Fourier transform mass spectrometry. These results, together with previously determined thermochemical data, are used to determine other thermochemical properties of MgOH and MgO. Our value D°(Mg-OH) = 67 ± 6 kcal/mol agrees well with previous experimental values. However, D°(Mg-O) = 59 ± 5 kcal/mol is much lower than previous experimental results, which vary widely, but is in good agreement with a recent ab initio CI calculation.
This book provides a comprehensive account of the fundamental properties of metal-oxide surfaces and their interaction with atoms, molecules, and overlayers. It provides both a general overview of the basic properties of metal oxides and an extensive compilation of the experimental and theoretical work performed on single crystal oxide surfaces. There are seven chapters in all, treating topics such as the geometric structure of metal-oxide surfaces, the electronic structures of both transition- and non-transition-metal-oxide surfaces, and molecular adsorption on oxide surfaces. An extensive list of references, covering nearly one thousand citations, is provided.
It is shown that the recently proposed QCI method including all single and double substitutions has essentially the same computational requirements as the more complete CCSD approach. If properly formulated, the CCSD equations contain at most quadratic terms in the excitation amplitudes.
A recently constructed cluster based photoelectron spectrometer is described. This instrumentation is unique in that it enables the kinetic energy analysis of electrons ejected from both anions and neutral clusters. This capability permits the investigation of discrete electronic levels in all charge states (anionic, neutral, and cationic). A laser vaporization plasma reactor cluster source affixed with a sublimation cell is employed to produce a variety of metal clusters, and the resulting cluster distributions are analyzed with time-of-flight mass spectrometry. The corresponding electronic structure is analyzed with a ``magnetic bottle'' photoelectron spectrometer. Examples of instrument performance operating in both anion photodetachment and neutral multiphoton ionization (MPI) modes are provided. In the case of neutral MPI, the corresponding product distribution is collected with a Wiley-McLaren [Rev. Sci. Instrum. 26, 1150 (1955)] mass spectrometer mounted perpendicular to the magnetic bottle photoelectron spectrometer.
Ab initio calculations based on Nesbet's symmetry and equivalence restriction version of the LCAO—MO—SCF method, using an extended double-zeta basis set, have been performed at a number of internuclear separations for six configurations, in order to account for the features of the spectrum of MgO and to ascertain the nature of the ground state. Most arguments coming from both experimental data and theoretical calculations corrected by semiempirical estimates of correlation energy differences between the configurations, are in favor of a closed-shell 1Σ+ ground state, although the lowest Hartree—Fock state is a less correlated 3II open-shell state. Higher-lying observed states (D1Δ and C1Σ−) as well as unobserved ones up to the first ionization limit are accounted for by variational or reliable virtual orbital calculations, and they are presented in an energy-level diagram. In this connection, the properties of the variational 2II and 2Σ+ low-lying levels of MgO+ are discussed. It is then suggested that, in view of this, the unclassified ultraviolet bands should most likely be partly assigned to triplet—triplet transitions involving the very low-lying 3II state. Finally, reasonable predictions about other as yet unobserved band systems are made.
We have experimentally demonstrated that the electronic ground state of MgO is X1Sigma+. The lowest energy excited state was shown to be a3Pii, for which the following constants were determined (1sigma uncertainty in parentheses): Te2623 (7) cm-1 omegae648 (5) omegaexe3.9 (9) Be0.5022 (13) alphae0.0042 (8) (Pekeris' relation) A-64 (1) The a3Pii state was observed and characterized from its perturbations of the X1Sigma+ (vX > 2) levels. These perturbations were detected in the photoluminescence from the B1Sigma+ state induced by the 476.5-, 496.5-, and 514.5-nm lines of an argon ion laser. The three laser lines were shown to coincide with a total of eight strong and assignable molecular lines belonging to the B1Sigma+ - X1Sigma+ system of the three isotopic species 24Mg16O, 25Mg16O, and 26Mg16O. The 496.5-nm line excited the less abundant 26Mg16O species almost exclusively. Rotationally resolved photoluminescence from eight B1Sigma+ (vB, JB) levels was observed into vibronic levels X1Sigma+ (vX = 0-7) and A1Pi (vA = 0-8). X1Sigma+ ~ a3Pii perturbations appeared as level shifts often accompanied by extra transitions into levels of dominants a3Pii character; a total of 20 extra lines were assigned. A1Pi ~ X1Sigma+ perturbations were also observed, and six extra lines were assigned. Measurement of the relative intensities of B1Sigma+ - A1Pi and B1Sigma+ - X1Sigma+ photoluminescence originating simultaneously from a single (vB, JB) level permitted determination of the transition moment ratio Te2623 (7) cm-1 omegae648 (5) omegaeXe3.9 (9) betae0.5022 (13) alphae0.0042 (8) (Pekeris' relation) A-64 (1) The electronically excited A1Pi state was present in our Mg + N2O flame in sufficient concentration to record the MgO B1Sigma+
The application of a genetic algorithm, for optimizing the geometries of stoichiometric and non-stoichiometric MgO clusters, bound by a simple Coulomb-plus-Born–Mayer potential, is investigated. The genetic algorithm is shown to be efficient and reliable for finding, reproducibly the global minima for these clusters. The variation of the structures of MgO clusters are investigated as a function of the formal charges (±q) on the ions—ranging from q=1 to q=2. In agreement with previous studies, lower charges are found to favour compact, rocksalt-like cuboidal clusters, while the higher formal charges favour hollow pseudo-spherical structures. Hexagonal stacks are also found to be stable for small (MgO)N clusters with N=3n. Comparisons are made with experimental mass spectral
abundances and the results of previous empirical calculations, as well as with more sophisticated model potential and ab initio calculations. Finally, possible ways in which the genetic algorithm search method could be coupled with more accurate calculation methods are discussed.
The MgO B1Σ+-a3Πi (0-0) and (0–1) and D1Δ-a3Π (0-0) and (1-1) intercombination bands have been observed, rotationally analyzed, and reduced to molecular constants by a nonstandard procedure which made extensive use of an elaborate but highly constrained effective Hamiltonian model. The MgO a3Π state is important because it is low lying (Te = 2620.6 cm−1), it correlates to ground state atoms (unlike the X1Σ+ state), it is the lower state of the exceedingly complex near UV triplet-triplet bands, and many Mg + oxidant reactions significantly populate various , components of the a3Π state. The fit model, observed transitions, and computed eigenvalues and eigenvectors for MgO a3Π v = O and 1 will aid in the analysis of triplet-triplet bands and will be especially valuable in providing transition frequencies and relative , , and J-dependent rotational linestrength factors for population monitoring. The present results reaffirm the validity and utility of various semiempirical relationships between fine structure, Λ-doubling, and perturbation parameters. Moreover, by showing that the Λ-doubling in the a3Π state is dominated by interactions with the X1Σ+ and b3Σ+ states, an ab initio prediction of the location of the as yet unobserved b3Σ+ state is confirmed. The B1Σ+-a3Π transition borrows its oscillator strength from the B1Σ+-X1Σ+ and B1Σ+-A1Π transitions. The pattern of a3Π, A1Π, and X1Σ+ vibrational levels is such that, for v ≤ 4, the predominant perturber characters admixed into the a3Π, va level are vx = va + 3 and vA = va. Interference effects between transition amplitudes borrowed from B-X and B-A transitions will cause intensity to be transferred from to or vice versa. These e-level interference effects will be strongly dependent on vB, va, J, and . Although the pattern of interference effects will appear complicated, all such effects are determined, in sign and magnitude, by the following product of eight signed quantities.
Ab initio calculations at CASSCF and MRCI PS levels are used to determine the dissociation energy for the X 1Σ+ state of MgO, which adiabatically dissociates to the ground state 1Sg of magnesium and to the excited 1Dg state of oxygen, as well as other spectroscopic parameters. Emphasis is placed upon the problem of properly selecting an adequate active space in CASSCF calculations and upon the improvements obtained in MRCI by selecting perturbatively the most important contributions to the total wavefunction and evaluating the remaining ones only by perturbational method. Through a procedure based on stabilizing the computed dissociation energy, values of 3.87 eV (MRCI PS) and 4.20 eV (CASSCF) are obtained. These values compare with the experimental value of 3.76±0.13 eV.
This book contains papers on physical and chemical phenomena of solid clusters. The papers cover the atomic and electronic structure, dynamics, stability, fragmentation, optical properties, interaction with adsorbates, astrochemistry and van der Waals forces of clusters. (LSP)
Gas phase (MgO)+n and (MgO)nMg+ clusters (n≤90) were produced in a gas aggregation source and studied by using laser‐ionization time‐of‐flight mass spectrometry. The abundance maxima observed in the mass spectra indicate that the clusters form compact cubic structures similar to pieces of the MgO crystal lattice. The abundance maxima of the metal‐rich clusters show an interesting dependence on the ionization wavelength that appears to be due to different fragmentation pathways for the cluster ions and neutrals, and may be indicative of excess electron behavior analogous to that observed in solid state color centers. Calculations of cluster structures and stabilities made with an ionic model were useful in obtaining qualitative information about the primary fragmentation channels and cluster electronic properties, but also indicate that covalent bonding interactions must be included to obtain quantitatively accurate results.
Neutral (MgO)n clusters are produced in a molecular beam by laser vaporization in a pulsed-nozzle cluster source. These clusters are ionized via multiphoton absorption from either an ultraviolet excimer laser or a far-infrared free electron laser. While ultraviolet ionization produces mass spectra consistent with previous measurements, infrared ionization produces higher molecular weight ions from the same nascent source distribution. Ultraviolet ionization occurs by direct electronic excitation/ionization, while infrared ionization occurs by vibrational excitation followed by thermionic electron emission. In both cases, prominent masses are observed corresponding to cubic nanocrystals with near equal x:y:z dimensions. By tuning the IR wavelength while recording the mass-resolved ion yield, vibrational spectra are obtained revealing two resonances near 16 and 22 microns. Clusters up to 300 atoms in size are studied, and spectra exhibit a gradual variation with size, converging to positions near to, but not matching the bulk phonon frequencies. Structural implications of these vibrational spectra are investigated. © 2002 American Institute of Physics.
The chemically bound superoxide molecule MgO2+ has been studied by photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer over the spectral range 247–540 nm. The experimental observations are consistent with ab initio calculations showing the ground state of MgO2+ to be of Mg2+O2− superoxide character [Chem. Phys. Lett. 203, 215 (1993)]. Through the visible and near UV spectral range 280 nm<λ<540 nm, we observe weak continuum absorption with evidence for two overlapping bound–free absorption bands, assigned as 1 2B1←1 2A2 and 2 2B1←1 2A2 in C2v symmetry. These bands correspond to radiative charge-transfer transitions of the form Mg2+O2−→Mg+O2. Both Mg+ and MgO+ fragments are observed, with a clear threshold for branching to MgO+ at a photolysis wavelength of 330 nm. This threshold yields limiting values for the MgO2+ bond dissociation energies of D0″(MgO+–O) ⩽ (3.75±0.04) eV and D0″(Mg+–O2) ⩽ (1.13±0.11) eV. For photolysis wavelengths λ<280 nm we observe a third structured absorption band showing a clear vibrational progression with an excited state vibrational mode spacing of ωe = 520±15 cm−1. This electronic band is assigned as 3 2B1←1 2A2, with the upper state correlating to an excited electronic state of O2−. The vibrational progression is tentatively assigned to the ν2 (Mg–O) symmetric stretch mode of the complex (a1). © 1998 American Institute of Physics.
We present the results of photoelectron velocity-map imaging experiments for the photodetachment of small negatively charged BimGan (m = 1–2, n = 0–2), and Pbn (n = 1–4) clusters at 527 nm. The photoelectron images reveal new features along with their angular distributions in the photoelectron spectra of these clusters. We report the vertical detachment energies of the observed multiple electronic bands and their respective anisotropy parameters for the BimGan and Pbn clusters derived from the photoelectron images. Experiments on the BiGan clusters reveal that the electron affinity increases with the number of Ga atoms from n = 0 to 2. The BiGa2− cluster is found to be stable, both because of its even electron number and the high electron affinity of BiGa2. The measured photoelectron angular distributions of the BimGan and Pbn clusters are dependent on both the orbital symmetry and electron kinetic energies. Density-functional theory calculations employing the generalized gradient approximation for the exchange-correlation potential were performed on these clusters to determine their atomic and electronic structures. From the theoretical calculations, we find that the BiGa2−, Bi2Ga3− and Bi2Ga5− (anionic), and BiGa3, BiGa5, Bi2Ga4 and Bi2Ga6 (neutral) clusters are unusually stable. The stability of the anionic and neutral Bi2Gan clusters is attributed to an even-odd effect, with clusters having an even number of electrons presenting a larger gain in energy through the addition of a Ga atom to the preceding size compared to odd electron systems. The stability of the neutral BiGa3 cluster is rationalized as being similar to BiAl3, an all-metal aromatic cluster.
The closed‐shell CCSD equations are reformulated in order to achieve superior computational efficiency. Using a spin adaptation scheme based on the unitary group approach (UGA), we have obtained a new set of equations that greatly improves our previous formulation. Based on this scheme we have also derived equations for the closed‐shell configuration interaction including all single and double excitations (CISD) case. Both methods have been implemented and tested. For a range of test cases the new CCSD method is more efficient than the earlier CCSD method. The new closed‐shell CISD procedure is faster than the shape‐driven (SD)GUGA algorithm and the new CCSD scheme is less than two times more computation intensive than SDGUGA CISD per iteration.
Extended measurements are reported for rovibronic transitions between the strongly mixed low-lying X 1 Σ + , a 3 Π and A 1 Π electronic states of the MgO molecule in the 1640-3510 cm -1 region. The measurements were made with the Faraday laser magnetic resonance (LMR) technique utilizing a CO laser and a CO overtone laser. The new observed lines include the A 1 Π-X 1 Σ + (0-0, 0-1, 1-1, 1-2, 1-3, 2-2, 2-4, 3-3) bands as well as the much weaker a 3 Π 0,1 -X 1 Σ + (0-0, 1-0) intercombination bands. The present highly precise LMR data provide a basis for performing an extensive deperturbation analysis including the spin-orbit, orbit-rotation and Zeeman interactions between the rovibronic levels of the X 1 Σ + , a 3 Π and a 1 Π states. Thereby the present data are combined with reliable earlier experimental results, in particular for the transitions involving higher lying vibronic states, in order to obtain the effective deperturbed molecular constants and perturbation parameters for the three lowest lying electronic states of MgO to as high an accuracy as possible. The absolute standard deviation of the LMR data is 6 × 10 -4 cm -1 (18 MHz), which is in accord with the accuracy of our measurements. The set of deperturbed molecular parameters reproduces all measured lines with an overall standard deviation of 0·87 relative to the experimental uncertainties. For perturbations such as spin-orbit interaction there is good agreement with the predictions of large scale ab initio calculations.
We report here our study on development of adsorbents suitable for capturing CO2 from synthesis gas (syngas) at high temperatures (>100 °C). Our adsorbents are based on double salts of MgCO3 and K2CO3 in which samples with different ratios of Mg:K were prepared by the wet mixing method of magnesium nitrate and potassium carbonate. The adsorbents were characterized by X-ray diffraction analysis, thermogravimetric analysis (TGA), and N2 adsorption and desorption at 77 K. The morphology of the samples was observed with scanning electron microscopy (SEM). CO2 adsorption experiments were performed at four temperatures, 300 °C, 350 °C, 375 °C, and 400 °C, which correspond to the operating temperature range of gases exiting a typical water gas shift reactor and entering a gas turbine in the Integrated Gasification Combined Cycle (IGCC) process. The CO2 adsorption amounts were 1.65 wt%, 8.47 wt%, 8.55 wt%, and 0.63 wt% respectively at a CO2 partial pressure of 100 kPa. These adsorbents also exhibited excellent cycling stability both in temperature swing adsorption and pressure swing adsorption operation. The kinetics of CO2 adsorption at different temperatures was obtained by using the linear driving force (LDF) model and a diffusion activation barrier of 21 kJ/mol was also inferred.Graphical abstractHighlights► Successful synthesis of adsorbents for CO2 capture at high temperatures. ► Excellent cycling stability in temperature/pressure swing. ► Kinetics of CO2 adsorption at different temperatures obtained with LDF model.
Valence and excited states of the anions formed by the polar molecules LiH, LiF, LiCl, NaH, NaF, NaCl, BeO, and MgO are calculated by the recently developed electron-attachment equation-of-motion coupled-cluster (EA-EOMCC) method. The LiH−, NaH−, and LiF− anions are found to possess two excited dipole-bound states and the LiCl−, NaF−, NaCl−, BeO−, and MgO− anions have three excited dipole-bound states. The critical values of the dipole moment required to sustain the first, second, etc., excited state appears to be specific for each class of polar molecules.
The physical and chemical properties of cluster systems at the subnano and nanoscale are often found to differ from those of the bulk and display a unique dependence on size, geometry, and composition. Indeed, most interesting are systems which have properties that vary discontinuously with the number of atoms and composition, rather than scale linearly with size. This realm of cluster science where “one atom makes a difference” is undergoing an explosive growth in activity, and as a result of extensive collaborative activities through theory at VCU and experiment at PSU, our groups are recognized as pioneers in this area in which we have been active for many years. Herein we provide an overview of the field with primary focus on our joint undertakings which have spawned the superatom concept, giving rise to a 3-D periodic table of cluster elements and the prospect of using these as building blocks of new nanoscale materials with tailored properties.
The stability and electronic properties of anionic and neutral PbxIny clusters containing up to 5 Pb and up to 7 In atoms have been investigated using negative ion photodetachment spectroscopy along with first-principles electronic structure studies within a gradient corrected density functional approach. Through studies of the detachment energies, gaps in the electronic spectrum, variations in binding energy, and nature of the electronic states, two families of stable species are identified. PbIn3−, Pb2In2, and Pb3In2 exhibit enhanced stability compared to their neighbors and the stability is linked to the aromatic character identified in their molecular orbitals. On the other hand, PbIn5− and Pb2In4 exhibit enhanced stability associated with filled electronic shells within a confined nearly free electron gas.
The electronic structure and stability of neutral and negatively charged BixIny (x = 1−4, y = 1−6) clusters are investigated through anionic photoelectron spectroscopy employing magnetic bottle and photoelectron velocity map imaging experiments. Experimental and theoretical adiabatic and vertical detachment energies of the anionic species containing up to 4 Bi and 4 In atoms are deduced from first principles calculations. Among the BixIny series, many clusters are found to exhibit special stability in the mass spectra, exhibit a large gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO−LUMO gap), and a large formation energy. This stability is rationalized by different mechanisms. Bi2In− is classified as a gas phase Zintl species despite only having three atoms, making it the smallest possible case. Bi3In2−, with 12 valence electrons and a closo structure in agreement with Wade’s rule, is similar to Bi3Ga2−, a gas phase Zintl analogue of Sn52−. Bi4In− and Bi4In2 are both found to follow Wade’s rule, indicating gas phase Zintl clusters. BiIn3 is a cyclic planar molecule similar to BiGa3 and BiAl3, all-metal aromatic systems, and BiIn5 is a 20 electron closed shell Jellium species. Additionally, an even−odd oscillation of the HOMO−LUMO gaps, formation energies, and adiabatic electron affinities are found correlating with the open-shell/closed-shell nature of the clusters.
Vibrationally resolved photoelectron spectra of MgO- and ZnO- have been recorded at several photon energies under varied experimental conditions. Peaks in these highly structured spectra have been assigned to photodetachment transitions from the MgO- and ZnO- ground state (X2Σ+) to vibrational progressions in the ground and several low lying neutral excited states. In addition, a high-temperature MgO- spectrum shows spectral features due to photodetachment from an excited electronic state of the MgO- anion, which has been assigned to an A2Π anionic state. From the MgO- spectra, the electron affinity of the MgO ground state (X1Σ+) is determined to be 1.630 (0.025) eV. Four electronic excited states of MgO, a3Π, A1Π, b3Σ+, and B1Σ+, were found to lie 2510, 3390, 8390, and 20 000 cm-1 above the X1Σ+ neutral ground state, respectively. The excited MgO- A2Π anion state was found to lie 4791 cm-1 above the MgO- X2Σ+ anion ground state. The photoelectron spectra of ZnO-, presented here at higher photon energies, extend a previous photoelectron study by Fancher et al. to the first two excited neutral states, a3Π and A1Π, which have been found to lie 2460 and 4960 cm-1 above the X1Σ+ ground state, respectively. From Franck−Condon analyses of the well-resolved vibrational progressions for each electronic transition, equilibrium internuclear distances and fundamental vibrational frequencies of the MgO and ZnO neutral electronic states were determined. Moreover, because the sources employed produced vibrationally hot anions, the bond length and vibrational frequencies of both the MgO- ground and excited states were found from the vibrational hot band transitions.
Gas-phase thermochemical values D° (Mg+-OH) = 75 {plus minus} 4 kcal/mol. IP (MgOH) = 7.3 {plus minus} 0.1 eV, D°(Mg{sup +}-O) = 53 {plus minus} 3 kcal/mol and IP(MgO) = 7.9 {plus minus} 0.1 eV have been experimentally determined by photodissociation measurements and charge-transfer reactions with Fourier transform mass spectrometry. These results, together with previously determined thermochemical data, are used to determine other thermochemical properties of MgOH and MgO.
The effects of irradiation of 45KeV argon ions on (100) cleavage faces of KCl, NaCl and LiF has been studied. The sputtered positive ions have been analysed in mass and energy with the specific aim of studying their polyatomic or cluster components. The mass spectra, of 1 a.m.u. resolution reveal the relative positive ion yield to be predominantly composed of the alkali ion M('+) with the second most abundant species (by an order of magnitude) being the dimer M(,2)('+). The spectra also show a variety of lower yield metal and alkali halide ion clusters of the form M(,n)('+) and M(,n)X(,n -1)('+), where M is the cation and X the anion of the alkali halide. Traces of metal oxide, halide cations and doubly charged potassium halide species are also present. During the course of irradiation of insulators a positive surface charge develops on the surface and the resultant surface potential distorts the energy spectra. A two component surface potential model based on the surface heating effect of the irradiation beam enables the observed energy spectra to be deconvoluted and the true energy distribution to be estimated. The corrected results of the monomer M('+) distribution show an E('-2) dependence at high energies in agreement with simple collision cascade theory. The cluster energy profiles also give an E('-m) dependence with the index m increasing with increasing cluster size. The high energy tail of the M(,2)('+), MX('+) and M(,2)X('+) species suggest a collisional mechanism responsible for their ejection. A computer simulation of a simple surface correlated collision sequence model is shown to be able to generate the M(,2)('+) species and give predictions of their energy distributions in good agreement with experiments.
Coaxial Zn/ZnO nanocables and ZnO nanotubes have been fabricated via a thermal reduction route using ZnS powder as the source material. The samples were characterized using X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectrometry. The as-synthesized Zn/ZnO nanocables consisted of a metallic core (Zn) ≈50 nm in diameter and a semiconductor outer shell (ZnO) ≈5 nm in thickness and several micrometers in length. A good epitaxial relationship between the Zn core and ZnO shell was observed, and misfit dislocations were observed at the Zn/ZnO interface, which accommodated the relatively large lattice mismatch. The outer diameter and wall thickness of the ZnO nanotubes are ≈60 and ≈10 nm, respectively. The possible formation mechanisms for the Zn/ZnO nanocables and ZnO nanotubes are discussed.
We approximate the exchange-correlation energy of density functional theory as a controlled extrapolation from the slowly varying limit. While generalized gradient approximations (GGA's) require only the local density and its first gradient as input, our meta-GGA also requires the orbital kinetic energy density. Its exchange energy component recovers the fourth-order gradient expansion, while its correlation energy is free of self-interaction error. Molecular atomization energies and metal surface energies are significantly improved over GGA, while lattice constants are little changed.
Ordered mesoporous carbon supported MgO (Mg-OMC) materials were synthesized by the carbonization of sulfuric-acid-treated silica/triblock copolymer/sucrose/Mg(NO3)2 composites. In the current approach, triblock copolymer P123 and sucrose were employed as both structure-directing agents for the self-assembly of rice husk ash silica solution and carbon precursor. Sulfuric acid was used to cross-link P123 and sucrose in the as-synthesized composites in order to improve the carbon yield. The synthesized Mg-OMC was characterized by X-ray diffraction, N2 adsorption–desorption isotherm method, X-ray photoelectron spectroscopy, scanning electron microscope equipped with energy dispersive X-ray analysis and transmission electron microscopy. The thermal stability of Mg-OMC was verified by CO2-temperature programmed desorption, which confirmed the chemisorption of CO2 on MgO. The CO2 adsorption capacity of Mg-OMC-1 was observed to be 92 mg/g of sorbent which is comparable with that of the well established CO2 sorbents.
Gas phase (MgO)
n
+
and (MgO)
n
Mg+ clusters were produced in a gas aggregation source and studied by using laser-ionization time-of-flight mass spectrometry. A MgO molecule apparently serves as the nucleus for cluster growth, to which Mg and O atoms add. The heat generated by the formation of metal-oxygen bonds, and that added to the cluster by ionization leads to the production of clusters with the stoichiometry of the stable high-temperature oxide. The abundance maxima observed in the mass spectra indicate that the clusters form compact cubic structures similar to pieces of the MgO crystal lattice. The primary fragmentation channel responsible for the observed patterns is probably the loss of MgO monomers.
ZnO nanorods are prepared by a hydrothermal process with cetyltrimethylammonium bromide (CTAB) and zinc powder at 182°C. The
samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy
(TEM). The gas sensing properties of the materials have been investigated. The results indicate that the as-prepared ZnO nanorods
are uniform with diameters of 40–80 nm and lengths about 1 μm, the relatively high sensitivity and stability of these sensors
made from ZnO nanorods demonstrate the potential for developing a new class of stable and very sensitive sensors.
A new type of ion gun is described which greatly improves the resolution of a nonmagnetic time‐of‐flight mass spectrometer. The focusing action of this gun is discussed and analyzed mathematically. The validity of the analysis and the practicability of the gun are demonstrated by the spectra obtained. The spectrometer is capable of measuring the relative abundance of adjacent masses well beyond 100 amu.
The 2Σ+ and 2Π states of MgO− and the 1Σ+, 1Π, and 3Π states of MgO are studied using the ACPF approach. The computed spectroscopic constants are in good agreement with the available experimental data. The computed Franck–Condon factors and photodetachment overlaps are compared with experiment.