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3??????Gas-phase molecular spectroscopy
... Over the past two decades chemical sensing using laser absorption spectroscopy (LAS) in the molecular fingerprint region from 3 to 20 µm has been established as a powerful in-situ diagnostic tool for molecular plasmas [1][2][3][4][5][6]. The non-invasive, selective and time resolved detection and quantification of transient and stable molecular species provides important information on the gas phase composition and chemistry of complex gas mixtures in electric discharges. ...
... The expressions usually employed are discussed in [41]. The NEA defined in (3) and provided for the CEAS experiments would correspond to the "per scan" category in [41]. The present paper is intended to give an overview of recent achievements in mid-IR chemical sensing based on the application of QCLAS techniques. ...
... Identification and quantification of species produced in gaseous fluorocarbon (C x F y ) containing environments, e.g., in plasma processes, is another field of application of high-resolution IR-LAS [68]. Specific extension to p-QCLAS has so far been reported for the transient molecule CF 3 [57] and stable fluorocarbon molecules (CF 4 , C 3 F 8 [69,70]). In both cases the experiments were carried out under low pressure conditions. ...
The recent availability of thermoelectrically cooled pulsed and continuous wave quantum and inter-band cascade lasers in the mid-infrared spectral region has led to significant improvements and new developments in chemical sensing techniques using in-situ laser absorption spectroscopy for plasma diagnostic purposes. The aim of this article is therefore two-fold: (i) to summarize the challenges which arise in the application of quantum cascade lasers in such environments, and, (ii) to provide an overview of recent spectroscopic results (encompassing cavity enhanced methods) obtained in different kinds of plasma used in both research and industry.
Progress in the development of compact semiconductor-based mid-infrared light sources, more specifically of quantum cascade lasers (QCLs), has been astonishingly rapid in the 2 decades since their first realisation. Their performance makes them superior to conventional sources and has led to significant improvements and new developments in chemical sensing techniques encompassing cavity enhanced methods. The aim of this compilation is to provide an overview about useful combinations of QCLs with optical cavities and to highlight recent achievements thereby focussing on potential sensing applications.
We present a theoretical study of the thermodynamical properties of polypeptides in the gas phase. The aim of this work is a better understanding of the fundamental mechanisms involved in protein folding. A statistical approach based on Monte Carlo algorithms applied in generalised ensembles, such as Replica Exchange Method or Wang-Landau method, has been used to sample the rugged energy landscape of those molecules. The peptides were composed of 2 to 20 amino acids and have been modelised by the AMBER 96 forcefield. Simulations have been realised in strong relation with experimental progress of the group. We thus tried to understand the influence of the secondary structure on photofragmentation mechanism, the role of entropy in the stabilisation of beta sheets and the effect of intense electric field on peptide conformation.
High-level ab initio calculations employing the multireference configuration interaction and coupled clusters methods with a correlation-consistent sequence of basis sets have been used to obtain accurate potential energy curves for the complex of the sodium cation with the iodine atom. Potential curves for the first two electronic Lambda-S states have very different characters: the potential for the 2pi state has a well depth of approximately 10 kcal/mol, while the 2sigma state is essentially unbound. This difference is rationalized in terms of the anisotropic interaction of the quadrupole moment of the iodine atom with the sodium cation, which is stabilizing in the case of the 2pi state and destabilizing in the case of the 2sigma state. The effects of spin-orbit coupling have been accounted for with both ab initio and semiempirical approaches, which have been found to give practically the same results. Inclusion of spin-orbit interactions does not affect the X(omega = 32) ground state, which retains its 2pi character, but it results in two omega = 12 spin-orbit states, with mixed 2sigma and 2pi characters and binding energies roughly half of that of the ground spin-orbit state. Complete basis set (CBS) extrapolations of potential curves, binding energies, and equilibrium geometries were also performed, and used to calculate a number of rovibronic parameters for the Na+...I* complex and to parameterize model potentials. The final CBS-extrapolated and zero-point vibrational energy-corrected binding energy is 10.2 kcal/mol. Applications of the present results for simulations of NaI photodissociation femtosecond spectroscopy are discussed.
The group for molecular kinetics and spectroscopy at ETH Zürich investigates the fundamental physical-chemical primary processes of chemical reactions. We have developed a conceptually new approach to derive these primary processes of intramolecular kinetics on time scales leading into the femtosecond and subfemtosecond domain on the basis of infrared spectroscopy with high frequency resolution but without short-time resolution. Selected applications include intramolecular wavepacket dynamics of chemical functional groups of isolated, individual molecules and IR-laser chemistry of molecules under infrared multiphoton excitation, hydrogen bond tunneling dynamics in hydrogen fluoride clusters (HF)2 and the tunneling stereomutation of prototypical chiral molecules. One of the greatest current challenges is the elucidation of the influence of the parity violating weak interaction mediated by the Z-boson of high energy physics on the dynamics of chiral molecules.
Diode laser spectra of the rare gas - spherical top van der Waals complexes Ar-CH4 and Kr-CH4 were measured in the wavelength region near 1310 cm-1 and assigned. The most prominent lines of both complexes exhibit three dense but well resolved RP0, QR0, and RQ0 branches, correlated to the R(0) transition of the triply degenerate bending vibration v4 of methane, CH4. A model Hamiltonian based on Coriolis coupled states was applied for the assignment, analysis and fitting of the spectra to within the experimental accuracy of ≈ 15 MHz. The rotational B constants of the upper and lower states determined from the three allowed branches appeared to be strongly correlated. The precision in the determination of the rotational B constants of the two complexes was substantially increased by additional recording of several weak transitions in the nearly forbidden QP0 and RR0 branches, which were fitted together with the allowed transitions. The separation between the rare gas atom and the methane molecule in the ground vibrational state was determined to be 3.999 Å and 4.094 Å for Ar-CH4 and Kr-CH4, respectively. The measured small values of the splitting between the K=0 and the K = ±1 levels in the vibrationally excited state (0.39 cm-1 and 0.67 cm-1 for Ar-CH4 and Kr-CH4, respectively), which characterizes the anisotropy of the intermolecular potential, indicated that Kr-CH4 and Ar-CH4 together with Ne-SiH4 represent examples close to the free rotor limit, where the spherical top CH4 is almost free to rotate within the complex. In comparison, the previously analyzed Ar-SiH4 van der Waals molecule is closer to the hindered rotor limit.
A molecular beam Fourier transform microwave spectrometer, designed for the study of chemical reactions within electrical discharges, is described in detail. Applications include the production of carbonyl fluoride, ketene, acetic acid, and difluorocarbene. For the production of carbonyl fluoride and ketene with 1,1-difluoroethylene and carbon dioxide as precursor molecules a reaction path via a 2+2 cycloaddition is proposed.
We report the assignment and analysis of all stable monosubstituted isotopomers of trifluoroacetic acid. The 13C and 18O isotopomers were observed in natural abundance. The rotational constants and quartic centrifugal constants are presented. The rotational constants are used to derive a partial substitution structure and a complete r0 structure for future comparison with the corresponding values in hydrogen bridged bimolecules containing trifluoroacetic acid as a subunit. The deuterium nuclear quadrupole coupling constants are derived from the hfs-splittings of low-J rotational transitions of the CF3COOD isotopomer. The results of ab initio quantum chemical calculations are presented, which were carried out to assist in the assignment of the rotational spectra of the isotopomers and for comparison with the experimental molecular parameters.
The pure rotational b-type spectrum of the van der Waals complex Ne-CO has been measured using a pulsed jet, intracavity millimeter wave spectrometer. The millimeter wave generation is based on the OROTRON principle. The high sensitivity of the spectrometer allowed measurements of R(J), K = 1 ← 0 transitions between 108 and 150 GHz of the Ne isotopomers 20Ne-CO and 22Ne-CO. This new millimeter wave data set together with the microwave data in the literature, i.e. a-type microwave transitions, yield in a fit to an asymmetric rotor a reliable set of ground state constants. These are for 20Ne-CO: A = 107127.021(14) MHz, B = 3479.6597(95) MHz, and C = 3039.5387(93) MHz. For both 20Ne-CO and 22Ne-CO, a global fit to a near-symmetric rotor was performed, taking into account the infrared and microwave transition frequencies from the literature and the millimeter wave measurements of the present work.
The pure rotational spectra of CS and its isotopomers 12 C 33 S, 12 C 34 S, 13 C 32 S, 13 C 33 S, 13 C 34 S including the very rare isotopomer 12 C 36 S were observed. The rotational spectra include ground and vibrationally excited transitions up to v = 16. The new measurements have been performed with the Cologne terahertz spectrometer covering the frequency region from 259 to 1075 GHz. These newly observed rotational transitions together with earlier data were fitted to a Dunham-type Hamiltonian. The obtained isotopically invariant parameters include vibrational and rotational expansion coefficients.
Using the new high-resolution (~8 km s-1) echelle spectrograph on the 3.5 m telescope at the Apache Point Observatory, we have begun a high-sensitivity survey of the diffuse interstellar bands (DIBs) in a large sample of reddened stars. Now that we are 2 years into this long-term survey, our sample includes over 20 reddened stars that show at least one of the DIBs that had been suggested to be caused by C, based on the gas-phase measurement of the C spectrum by J. P. Maier's group. The high-quality astronomical data from this larger sample of stars, along with the spectroscopic constants from the new laboratory work recently reported by Maier's group, have enabled us to examine more carefully the agreement between C and the DIBs. We find that none of the C bands match the DIBs in wavelength or expected profile. One of the DIBs (λ5748) attributed to C is actually a stellar line. The two strongest DIBs attributed to C (λ6270 and λ4964) are not correlated in strength, so they cannot share the same carrier. On the whole, we find no evidence supporting the hypothesis that C is a carrier of the DIBs.
We report on the detection with the Infrared Space Observatory (ISO), for the first time in the circumstellar medium, of the polyacetylenic chains C4H2 and C6H2 and of benzene (C6H6) in the direction of the proto-planetary nebula CRL 618. Surprisingly, the abundances of di- and triacetylene are only a factor of 2-4 lower than that of C2H2. Benzene is 40 times less abundant than acetylene. We suggest that the chemistry in CRL 618 has been strongly modified by the UV photons coming from the hot central star and by the shocks associated with its high-velocity winds. All the infrared bands arise from a region with kinetic temperatures between 200 and 250 K, probably the photodissociation region associated with the dense torus that surrounds the central star. C4H2 and C6H2 have also been detected in CRL 2688, so it seems that C-rich proto-planetary nebulae are the best organic chemistry factories in space.
We present high-resolution CO images of the Galactic center region taken with the 2 × 2 focal-plane array receiver mounted on the 45 m telescope of Nobeyama Radio Observatory. We have collected about 44,00012C16O (J = 1-0) spectra and over 13,00013C16O (J = 1-0) spectra with a 34'' (1.4 pc) grid spacing. The 12CO mapping area is roughly -15 ≤ l ≤ +34 and -06 ≤ b ≤ +06, which covers almost the full extent of the molecular gas concentration in the Galactic center. These CO images demonstrate extremely complex distribution and kinematics of molecular gas in the Galactic center. While its large-scale behavior can be attributed to the well-known coherent features, bright CO emission arises from a number of compact (d ≤ 10 pc) clouds with large velocity widths (ΔV ≥ 30 km s-1). The small-scale structure of molecular gas is characterized by filaments, arcs, and shells. The boisterous molecular gas kinematics there may be a result of violent release of kinetic energy by a number of supernova explosions and/or Wolf-Rayet stellar winds.
The vibrational spectrum of HCP (phosphaethyne) is studied and analyzed in terms of a 1:2 resonance effective Hamiltonian. The parameters of the model Hamiltonian are determined by fitting 361 out of the first 370 energy levels obtained from diagonalization of the full Hamiltonian, which is based on a newly calculated potential-energy surface with near spectroscopic accuracy. It is demonstrated that all features characteristic of the approach to the HCP↔CPH isomerization, such as the strong mixing between the bending and CP-stretching motions, the appearance of “isomerization states” (large amplitude bending motion) at intermediate energies, and the diagnostically significant appearance of a zig–zag pattern in the energy spacings between neighboring levels within each polyad, are quantitatively reproduced by the effective Hamiltonian. The semiclassical analysis of the model Hamiltonian for specific combinations of the HC-stretch and polyad quantum numbers explains all of the observed features of the full Hamiltonian in terms of stable and unstable periodic orbits. In particular, the birth of the isomerization states is found to be related to a saddle-node bifurcation of the classical phase space. The connection with the “polyad phase sphere” representation of quantum polyads is also discussed.
Interstellar space contains matter that is concentrated into cool regions known as interstellar clouds, which can be many light years in extent [1]. These clouds consist of material in the form of gas and tiny dust particles. The matter is mainly gaseous, and the dominant species in the gas is hydrogen, in either atomic or molecular form. Clouds come in a variety of gas densities ranging from 101–104 cm−3 with temperatures mainly in the range 10–100 K. The denser clouds are often associated with star formation; local portions of matter, known as protostars, collapse and heat up until nuclear fusion reactions begin to burn. Prom spectroscopic studies of the interstellar medium, astronomers have determined the composition of the gas and of the dust particles. In the more diffuse clouds, the gaseous matter can be studied by optical absorption spectroscopy using background stars as sources of light. The gas appears to be mainly atomic, although concentrations of diatomic molecules, chiefly H2, are detectable. The gas is overwhelmingly neutral; the fractional ionization of ∼ 10−4 is due mainly to carbon atoms ionized by UV radiation. Many unidentified spectral lines remain, chiefly of a larger width than the assigned features. These lines, known as the diffuse interstellar bands, represent one of the major unsolved mysteries of spectroscopy and may be caused by large but particularly stable molecules [2].
We report on the production of fluoroacetylene by an elimination reaction of 1,2-fluoroethene as precursor in an electrical discharge. The substance was identified and investigated by molecular beam Fourier transform microwave spectroscopy. Also fluorodiacetylene and with deuterium, D2, as additional precursor fluoroacetylene-d1 and fluorodiacetylene-d1 were synthesized in the beam plasma. The hfs of the rotational transitions provided spin-rotation and deuterium nuclear quadrupole coupling constants. The experimental quadrupole coupling constants are compared to vibrationally corrected ab initio values.
Jet-cooled isolated diastereoisomeric pairs formed between R- and S-2-naphthyl-1-ethanol and a series of optically active terpenes have been investigated by studying both the fluorescence excitation spectra and the fluorescence decays. While the homo- and heterochiral complexes with α-pinene and borneol can be resolved spectrally, no spectral enantiodifferentiation was obtained in the case of camphene and camphor complexes. Chirality-dependent fluorescence decays were however observed for these complexes which exhibit either longer lifetimes than the bare chromophore for the diasteroisomers involving camphene, or shorter for those with camphor. The enantioselective quenching of fluorescence of 2-naphthylethanol by R- and S-camphor has also been observed in n-hexane solution and has been shown to be due to a singlet-singlet energy transfer taking place at a rate lower than diffusion. In contrast, the energy transfer to R- and S-camphorquinone and biacetyl was shown to be diffusion controlled and nonenantioselective.
The pure rotational spectrum of NaNH2 (X̃1A1) has been recorded using millimeter/submillimeter direct absorption techniques. This study is the first gas-phase detection of this molecule in the laboratory. NaNH2 was generated by the reaction of sodium vapor and NH3 in the presence of a DC discharge. The Ka = 0, 1, 2, 3, 4, and 5 components in 10 separate rotational transitions were measured, identifying the molecule as a planar, near-prolate asymmetric top with C2v, symmetry. The data were analyzed using an S-reduced Hamiltonian, and rotational constants A, B, and C were determined, as well as various centrifugal distortion parameters. An r0 structure was additionally calculated for the molecule. NaNH2 may be produced in circumstellar or interstellar gas via the radiative association reaction Na+ + NH3 → NaNH+3 + hv, followed by dissociative electron recombination. Theoretical calculations by Petrie suggest this pathway may be sufficiently fast to synthesize detectable concentrations of NaNH2.
Complexes H2O⋯ClF and H2O⋯F2 were detected by means of their ground-state rotational spectra in mixtures of water vapour with chlorine monofluoride and difluorine, respectively. A fast-mixing nozzle was used in conjunction with a pulsed-jet, Fourier-transform microwave spectrometer to preclude the vigorous chemical reaction that these dihalogen species undergo with water. The ground-state spectra of seven isotopomers (H216O⋯35ClF, H216O⋯37ClF, H218O⋯35ClF, D216O⋯ 35ClF, D216O⋯37ClF, HDO⋯35ClF and HDO⋯37ClF) of the ClF complex and five isotopomers (H2O⋯F2, H218O⋯ F2, D2O⋯F2, D218O⋯F2 and HDO⋯ F2) of the F2 complex were analysed to yield rotational constants, quartic centrifugal distortion constants and nuclear hyperfine coupling constants. These spectroscopic constants were interpreted with the aid of simple models of the complexes to give effective geometries and intermolecular stretching force constants. Isotopic substitution showed that in each complex the H2O molecule acts as the electron donor and either ClF or F2 acts as the electron acceptor, with nuclei in the order H2O⋯ClF or H2O⋯ F2. For H2O⋯ClF, the angle φ etween the bisector of the HOH angle and the O⋯Cl internuclear line has the value 58.9(16)°, while the distance r(O⋯Cl) = 2.6081(23) Åcorresponding quantities for H2O⋯F2 are φ = 48.5(21)° and r(O⋯Fi) = 2.7480(27) Å, where Fi indicates the inner F atom. The potential energy V(φ) as a function of the angle φ was obtained from ab initio calculations at the aug-cc-pVDZ/MP2 level of theory for each complex by carrying out geometry optimisations at fixed values of φ in the range ±80°. The global minimum corresponded to a complex of Cs symmetry with a pyramidal configuration at O in each. The function V(φ) was of the double-minimum type in each case with equilibrium values φe = ±55.8° and ±40.5° for H2O⋯ClF and H2O⋯F2, respectively. The barrier at the planar C2v conformation was V0 = 174cm-1 for H2O⋯ClF and 7cm-1 for H2O⋯F2. For the latter complex, the zero-point energy level lies above the top of the barrier.
High-resolution Fourier transform spectra of the laser-induced fluorescence of 63Cu37Cl2 produced in a cell have been recorded following excitation of a single vibronic level of the E2Πu electronic state. Fluorescence occurs in combination bands to a broad spread of levels in the ground electronic state. A global vibronic model is proposed for the ground state based on an effective Hamiltonian, which fits the experimental data (2782 fluorescence lines, lower state quantum numbers: v1 = 0–6, v2 = 0–2, v3 = 0–6, and J = 4 –80 ) to 0.019 cm−1 rms error. Vibrational, rotational and Renner–Teller parameters are obtained (e.g., ω2 = 95.195(36) cm−1, Be = 0.055106(3) cm−1, ϵ = −0.1893(28)). A revised value for the equilibrium internuclear distance Cu–Cl is deduced: re(Cu–Cl) = 0.20341(3) nm. The energy diagram of vibronic levels in the ground state is plotted up to 4000 cm−1.
The pure rotational spectrum of the KS radical (X2Πi) has been recorded using millimeter/submillimeter direct absorption techniques in the range 248-409 GHz. This work is the first laboratory detection of KS by any method. The species was created by the reaction of potassium vapor and CS2 under DC discharge conditions. Twenty-three rotational transitions were measured in both spin-orbit components, Ω=1/2 and Ω=3/2; lambda-doubling splittings were resolved in each substate. These data were analyzed with a 2Π case (a) Hamiltonian, and the rotational, spin-orbit, and lambda-doubling parameters have been determined. The lambda-doubling p-constant was found to be anomalously large, indicating the presence of a 2Σ state that is close by. The detection of KCl in the circumstellar shell of IRC +10216 indicates that potassium is present in the gas phase and may be in the form of other species such as KS.
Precise ab initio calculations of the rovibrational structures of the A2E″ and X2E′ electronic states of Na3 prompt a new vibrational assignment of the A ← X transition and provide the basis for the rotational analysis of the vibrational band A(vs = 1, vb, = 0. va = 0) ← X(0,0,0) by means of high-resolution optical-optical double resonance. The calculations, which use the single-surface adiabatic approach, reproduce our experimental data only if. as required by theory, a geometric phase of π under pseudorotation around the equilateral configuration is imposed. We consider this the first verification of Berry's phase in high-resolution molecular spectroscopy.
Time-resolved Fourier transform IR emission spectroscopy, capable of 10-8 s and 0.1 cm-1 spectral resolution, has been used to study the collisional deactivation of highly vibrationally excited SO2 by bath-gas molecules Ar, N2, O2, CO2 and SF6. The vibrationally excited SO2 were initially prepared with 32,500 cm-1 energy in the X̃1A1 state by the pulsed 308 nm laser excitation followed by internal conversion. The entire collisional deactivation process of the excited SO2 was monitored by time-resolved IR emission spectra through the IR active transitions. The average energy, 〈E〉, of excited SO2 was extracted from the IR emission bands using known vibrational constants and selection rules. 〈E〉 is further used to derive the average energy loss per collision, 〈ΔE〉, by each of the bath-gas molecules. The results show that 〈ΔE〉 increases from mono- and diatomic quenchers to more complex polyatomic molecules, as V-V energy transfer contributes to V-T/R. For all bath molecules, 〈ΔE〉 increases with 〈E〉 and displays a marked increase at 〈E〉 ≈ 20,000 cm-1. The observed threshold behavior most likely arises from intramolecular vibronic coupling within SO2 and implies the importance of long range interaction in intermolecular energy transfer.
Mid-infrared photodissociation spectra of H2O+−Arn (n = 1−14) complexes have been recorded in the vicinity of the OH stretch vibrations of the water cation. The rovibrational structure of the transitions in the dimer spectrum (n = 1) are consistent with a planar, proton (H)-bound HOHAr equilibrium geometry. The slightly translinear intermolecular bond in the ground vibrational state is characterized by a bond angle φ0 = 175(5)°, an interatomic HAr separation R0 = 1.929(15) Å, and an intermolecular stretching force constant ks ∼ 29 N/m. The assignment of the vibrational transitions is confirmed by spectra of partly deuterated species. The relaxation dynamics depend strongly on the excited vibrational state and do not obey statistical theories. Analysis of the spin-rotation constants indicates that the electromagnetic properties of the H2O+ cation in its 2B1 ground electronic state are not significantly affected by the formation of the intermolecular bond to Ar. The vibrational bands in the spectra of larger clusters (n = 2−14) are assigned to OH stretch fundamentals and their combination bands with the intermolecular HAr stretch modes. The observed systematic band shifts as a function of cluster size provide information about the cluster's geometries and the occurrence of structural isomers. The most stable trimer (n = 2) geometry has two equivalent intermolecular H-bonds. This stable trimer core is further solvated by two Ar ligands attached to opposite sites of the 2py orbital of oxygen (n = 3,4) and subsequently by less strongly bound Ar ligands (n = 5−14) to form an Ar solvation shell, probably around an interior H2O+ ion.
Fluorescence excitation, resonant two-photon ionisation (R2PI) and IR-UV ion-depletion spectroscopy were used to study the structures of 2-phenylacetamide (2PA) and its dimer and water clusters in their S0, S1 and ionic states. The n = 1–3 hydrates have ‘daisy chain’ structures in which the water molecules link the amide NH and CO sites. The dimer is symmetric, connected ia two strong NH···OC hydrogen bonds. Hydrogen-bonded NH and OH modes of the 1:1 hydrates are strongly coupled, to the extent that the symmetric combination is almost IR forbidden. Examination and reinterpretation of previously published IR data on the 1:1 hydrates of formamide and 2-pyridone revealed similar coupling. Solvation affects the photophysics of 2PA as it does N-benzylformamide; the isolated 2PA molecule does not appear to fluoresce but its clusters do. Hydrogen bonding to the amide carbonyl group could account for this behaviour by raising the energy of the lowest amide (n,π*) states above that of the fluorescent S1 (π,π*) level.
A variant of pulsed-field-ionisation zero-kinetic-energy photoelectron spectroscopy (PFI-ZEKE-PES) has been developed to determine the position of ionic energy levels with an accuracy limited by the bandwidth of the tunable photoexcitation source. The development relies on the ability of resolving the transitions to the high Rydberg states (n similar or equal to 200) which contribute to each line in a PFI-ZEKE photoelectron spectrum using a near-Fourier-transform-limited extreme ultraviolet (XUV) laser system (bandwidth <250 MHz). Thwe lowest ionisation potentials of Ar and N-2 have been determined from Rydberg-state-resolved PFI-ZEKE-PE spectra to be (127109.825<plus/minus>0.015) cm(-1) and (125667.028 +/-0.015) cm(-1), respectively. (C) 2001 Published by Elsevier Science B.V.
The (22(0)1-00(0)0) combination band of CO(2) around 6348 cm(-1) is studied with diode-laser spectroscopy, and linestrengths, collision-broadening parameters, and pressure shifts are determined. Linestrengths are modeled with a third-order Herman-Wallis expansion, and discrepancies with values reported in the literature are explained. Copyright 2000 Academic Press.
The high resolution far infrared absorption spectrum of IBr was recorded
with a Fourier transform spectrometer. The fundamental 1-0
vibration-rotation band and the 2-1 and 3-2 hot bands were recorded at a
resolution of 0·001 25 cm-1. The infrared continuum was provided
by synchrotron radiation emission from the Max-I storage ring. For high
resolution - 1 spectroscopy at 250 cm-1 synchrotron radiation is about 5
times brighter than a conventional infrared glower. This increase in
flux at the detector resulted in a corresponding increase in the
signal-to-noise ratio and a much improved infrared spectrum. - 1
We report on a coherent control experiment to selectively excite a well localized nuclear probability density in the electronic ground state of isolated, jet cooled potassium dimer molecules. The basic idea of this control scheme is to take advantage of the motion of a vibrational wave packet created in an intermediate excited electronic state, from which a Franck-Condon window to the desired final state in the ground state emerges. To realize this concept we apply a resonant stimulated Raman process, in which the two involved laser pulses are separated by a time delay τ1 and their wavelengths are independently tunable. The method enables us to excite ground state vibrational wave packets, whereby the vibrational energy of the wave packet and the position of its generation along the internuclear separation coordinate of the ground state potential energy surface can in principle be freely chosen by the experimentalist. The results we present for the chosen model system K2 strongly support the idea that this control concept might be one possible approach to selectively populate a desired chemical reaction channel in the electronic ground state of a molecule.
Multiphoton transitions can be reached by many routes through a
continuum of virtual levels. We show that the transition probability of
two-photon and multiphoton processes can be controlled by tailoring the
shape of an ultrashort excitation pulse, so that the many paths leading
to the final state interfere to give a desired probability amplitude. We
analyze the effect of pulse shapes on N-photon absorption as well as on
Raman transitions. We show theoretically that certain tailored dark
pulses do not excite the system at all, while other shaped pulses induce
transitions as effectively as transform limited pulses, even when their
peak amplitudes are greatly reduced. These results are confirmed
experimentally for two-photon transitions in atomic cesium.
Enhancement of the production of cold molecules via photoassociation is considered for the Cs2 system. The employment of chirped picosecond pulses is proposed and studied theoretically. The analysis is based on the ability to achieve impulsive excitation which is given by the ultracold initial conditions where the nuclei are effectively stationary during the interaction with a field. The appropriate theoretical framework is the coordinate-dependent two-level system. Matching the pulse parameters to the potentials and initial conditions results in full Rabi cycling between the electronic potentials. By chirping the laser pulse, adiabatic transfer leading to the population inversion from the ground to the excited state is possible in a broad and tunable range of internuclear distance. Numerical simulations based on solving the time-dependent Schrödinger equation (TDSE) were performed. The simulation of the photoassociation of Cs2 from the ground 3Σu+ to the excited 0g- state under ultracold conditions verifies the qualitative picture. The ability to control the population transfer is employed to optimize molecular formation. Transfer of population to the excited 0g- surface leaves a void in the nuclear density of the ground 3Σu+ surface. This void is either filled by thermal motion or by quantum “pressure” and it is the rate-determining step in the photoassociation. The spontaneous-emission process leading to cold-molecules is simulated by including an optical potential in the TDSE. Consequently, the rate of cold molecule formation in a pulsed mode is found to be larger than that obtained in a continuous-wave mode.
The emission and absorption spectra of interstellar molecules are reviewed with special consideration of recent observational and technical advances in the shorter submillimeter wave region of the electromagnetic spectrum. Single-dish observations have contributed in the past probably most of the information about the structure of interstellar molecular clouds.
At present about 120 interstellar molecules have been identified in interstellar clouds and circumstellar envelopes, evidence of a rich and diversified chemistry. CO, the most abundant interstellar molecule and other diatomic molecules and radicals are found throughout molecular clouds, whereas the more complex molecules are found in high-density cores, which are often the sites of active star formation. These locations represent prime targets for the search for larger molecules, such as glycine. The ignition of young stars is accompanied by strong heating of the surrounding material by radiation and/or shocks, leading to photoevaporation of molecules depleted on dust grains driving a "hot core" chemistry, traceable by its rich organic chemistry and its prevailing high excitation conditions (up to about 2000 cm-1).
However, in the list of detected interstellar molecules many simple hydrides are still missing, e.g. SH, PH, PH2, etc., which constitute the building blocks for larger molecules. With the technological opening of the terahertz region (ν ∼1 THz corresponds to λ ∼0.3 mm) to both laboratory and interstellar spectroscopy, great scientific advances are to be expected. Amongst these will be the direct detection of the lowest rotational transitions of the light hydrides, the low energy bending vibrations of larger (linear) molecules, and possibly the ring-puckering motion of larger ring molecules such as the polycyclic (multiring) aromatic hydrocarbons.
Pulsed-laser-ablated first-row transition metal atoms scandium through nickel dissociate sufficient N2 for atom combination reactions to form MN molecules during condensation at 10 K. The MN molecules are identified by nitrogen-15 substitution and density functional calculations. In the case of chromium and iron, stable bent NCrN and NFeN molecules are formed. Scandium is the best example of the dimetal dinitride (ScN)2, which is an open rhombus with fully reduced dinitrogen. Density functional theory calculations provide vibrational frequencies that help support the identification of these new molecules.
The rotational spectra of 1,1-dichloroethane in the ground state of the CH3CH, CH3CH and CH3CH isotopomers and in four low frequency excited states of the main species have been investigated in the range 6–250 GHz. Measurements have been carried out using molecular beam and waveguide Fourier transform microwave spectrometers as well as Stark and source modulation spectrometers. Accurate rotational and centrifugal distortion constants have been obtained. The complete Cl-nuclear quadrupole coupling tensors have been determined from the analysis of the resolved hyperfine structure for the three observed isotopomers. From the small A–E splittings observed in the first torsional excited state, the barrier to internal rotation of the methyl group has been determined to be 4.260(9) kcal mol−1.
The rotational spectra of all mono-substituted -isotopomers of equatorial piperidine and of the piperidine–water complex were analyzed and their rotational, centrifugal distortion, and quadrupole coupling constants were derived. Further a partial rS-structure of piperidine was determined and compared with an optimized structure on MP2/6-31G** level. The observed spectrum of the piperidine–water complex is in agreement with two possible structures. Only one of them is supported by dipole moment estimates from line intensities and is consistent with calculations on MP2/6-31G** level. In this structure the N⋯H–O hydrogen bond and the water molecule are located in the symmetry plane of piperidine. The free water proton is directed into the same direction as the amino proton. Common assumptions like unchanged structures of monomers, linear hydrogen bond, and symmetry restrictions were tested by ab initio calculations.
This paper describes a computer-accessible catalog of submillimeter, millimeter, and microwave spectral lines in the frequency range between 0 and 10 000 GHz (i.e. wavelengths longer than 30 μm). The catalog can be used as a planning guide or as an aid in the identification and analysis of observed spectral lines in the interstellar medium, the Earth’s atmosphere, and the atmospheres of other planets. The information listed for each spectral line includes the frequency and its estimated error, the intensity, the lower state energy, and the quantum number assignment. The catalog is continuously updated and at present has information on 331 atomic and molecular species and includes a total of 1 845 866 lines. The catalog has been constructed by using theoretical least-squares fits of published spectral lines to accepted molecular models. The associated predictions and their estimated errors are based upon the resultant fitted parameters and their covariance. Future versions of this catalog will add more atoms and molecules and update the present listings as new data appear. The catalog is available on-line via anonymous FTP at spec.jpl.nasa.gov and on the world wide web at http: //spec.jpl.nasa.gov.
Using 0.002 cm−1 resolution Fourier transform absorption spectra of an 17O enriched ozone sample, an extensive analysis of the ν3 band together with a partial identification of the ν1 band of the 16O17O17O isotopomer of ozone has been performed for the first time. The experimental rotational levels of the (001) and (100) vibrational states could be satisfactorily reproduced using an Hamiltonian matrix which takes into account the observed rovibrational resonances. Actually, as for other Cs-type ozone isotopomers [J.-M. Flaud and R. Bacis, Spectrochimica Acta, A54 (1998) 3–16], the (001) rotational levels are involved in both Coriolis and Fermi type resonances with the levels from the (100) vibrational state. Using an Hamiltonian matrix which takes explicitly into account these resonances precise vibrational energies and rotational and coupling constants were deduced and the following band centers ν0(ν3)=1017.5336 cm−1 and ν0(ν1)=1080.153 cm−1 were obtained for the ν3 and ν1 bands respectively.
The extension of high-resolution observation of the electronic emission spectrum of 14N2 toward the infrared domain is presented. To date, rotational analysis of the widely investigated spectrum of the N2 molecule was done in a spectral domain ranging from 2500 cm−1 to the UV. We have recorded for the first time the infrared part of the 14N2 spectrum from 1250 to 2250 cm−1, using the Fourier transform spectrometer of Laboratoire de Photophysique Moléculaire (LPPM) at an unapodized resolution of 0.0043 cm−1. A complete rotational analysis is performed for the (1 → 0), (2 → 1), (0 ← 1), (1 ← 2) bands of the B3Πg–W3Δu system, not included in any previous analysis. Spectroscopic parameters for the v = 0, v = 1, v = 2 levels of the B3Πg and the W3Δu states, consistent with those previously reported but with improved accuracy, are obtained from the experimental wavenumbers by a nonlinear least-squares procedure.
The emission spectrum of ZrCl has been observed in the 1800–12 000 cm−1 region using a Fourier transform spectrometer. The molecules were excited in a microwave discharge of a mixture of helium and a trace of ZrCl4 vapor. In addition to the C4Δ–X4Φ transition reported previously, numerous new bands observed in the near infrared have been classified into two electronic transitions, [7.3]2Δ–a2Φ and [9.4]2Φ–a2Φ. Five new bands observed in the 6700–7400 cm−1 region have been assigned as 2Δ3/2–2Φ5/2 and 2Δ5/2–2Φ7/2 subbands of a new electronic transition, [7.3]2Δ–a2Φ. The two subbands of the [9.4]2Φ–a2Φ transition were previously observed by G. Phillips, S. P. Davis, and D. C. Galehouse [Astrophys. J. Suppl. 43, 417–434 (1980)], who tentatively labeled them as 2Π1/2–2Π1/2 and 2Π3/2–2Π3/2 subbands. A number of new bands involving higher vibrational levels have been identified for these two subbands. A rotational analysis of a number of bands of both transitions has been obtained and spectroscopic constants have been determined. Global perturbations have been observed in both spin components of the a2Φ state. The assignment of the observed electronic states has been discussed in light of recent theoretical calculations.
The E2Σ+ → C2Π Rydberg–Rydberg transition of 14N16O near 8492 cm−1 has been studied by Fourier transform spectrometry in the emission from a dc excited supersonic jet expansion and from a dc discharge under equilibrium conditions. The same transition has also been observed in laser-induced stimulated emission. Line wavenumbers of the 0–0, 1–1, and 2–2 bands, together with data for previously published near-infrared transitions, have been reduced to consistent sets of rovibronic term values for v = 0, 1, and 2 of the A2Σ+, D2Σ+, E2Σ+, and C2Π states which frequently serve as intermediates in the multiphoton excitation of higher Rydberg levels of NO.
The (0, 0) band of the A4Π–X4Σ− transition of MoN, between 590 and 635 nm, has been studied using the sub-Doppler technique of intermodulated laser-induced fluorescence spectroscopy. Spectra taken at a resolution of about 60 MHz showed resolved hyperfine structure, which is caused principally by the unpaired 5sς electron in the ground state interacting with large magnetic moment of the 9542Mo and 9742Mo nuclei with nuclear spin I = 5/2. The hyperfine structures of the ground state vary with J in the manner expected for spin uncoupling from case (aβ) to case (bβJ). This band is notable for its complex appearance due to a combination of the high-spin multiplicity transition and the presence of seven isotopic species. Least-squares fits of the observed hyperfine line positions have yielded comprehensive sets of rotational, spin, and hyperfine parameters for the A4Π and X4Σ− states of 95MoN and 97MoN. The hyperfine parameters are interpreted and give information about the electron distribution in the molecule. Improved molecular constants of other isotopomers with zero Mo nuclear spin are also reported.
Most of a large gap in the laboratory rovibrational spectrum of H3+ between the ν2 fundamental and the first overtone has been filled using a recently automated color center laser spectrometer which scans between 3000 and 4200 cm−1. A narrow-bore liquid-nitrogen-cooled He/H2 discharge is used to produce rotationally and vibrationally hot H3+. With the high-temperature plasma and the improved sensitivity of the spectrometer we were able to probe very high-energy rovibrational levels, several in the region of the barrier to linearity where theoretical calculations are expected to break down.
For the purpose of atmospheric applications, the collisional relaxation of the J=5.5←4.5 rotational line of 14NO (551.53 GHz) has been investigated in the 235 to 345-K temperature range. Experiments were performed with N2 and O2 as buffer gases, which allow the derivation of the air-induced broadening parameters. Using a video-type spectrometer, small but significant departures from the usual Voigt profile have been demonstrated. These departures are characteristic of narrowed lineshapes: they have been fitted using the speed-dependent Galatry profile that takes account of both the dependence of relaxation rates on molecular speeds and the molecular diffusion via velocity changing collisions. Retrieved broadening parameters, as well as their temperature dependencies, are in agreement with previous determinations dealing with rovibrational lines: they are fairly reproduced by semi-classical calculations that consider exact collisional trajectories. The detailed lineshape properties have been studied thanks to some further experiments using atomic buffer gases, namely He, Ar, and Xe. If we discard the case of He-induced relaxation, theoretical arguments suggest that observed line narrowings are mainly related to the speed dependence of relaxation and that molecular diffusion plays a negligible role.
All literature vibration–rotational and pure rotational transition energies for the ground X1Σ+ electronic state of H35Cl, H37Cl, D35Cl, and D37Cl, along with the entire collection of electronic B1Σ+ → X1Σ+ emission data for the four isotopomers, have been used in a least-squares fit of compact analytic Born–Oppenheimer potential functions for the B1Σ+ and X1Σ+ electronic states. Additional functions related to the adiabatic and nonadiabatic corrections have also been determined. Separate least-squares fits were made according to the hamiltonian operators of J. K. G. Watson (J. Mol. Spectrosc. 80, 411 (1980)) and R. M. Herman and J. F. Ogilvie (Adv. Chem. Phys. 103, 187 (1998)). The results from the separate analyses demonstrate clearly that the two hamiltonian operators are essentially equivalent, both achieving equally satisfactory representations of the spectral data, and furnishing virtually identical Born–Oppenheimer potential functions. Fully quantum-mechanical vibrational eigenvalues and rotational perturbation series parameters Bv–Ov are presented for the lower levels of the X1Σ+ ground state for which infrared and/or microwave data are available (v" ≤ 7 for H35Cl and H37Cl, v" ≤ 10 for D35Cl and D37Cl). These parameters collectively reproduce the corresponding spectroscopic line positions included in our fit to within the uncertainties of the measurements.
In this study, a supersonic beam of NiF was produced by the reaction of SF6 with a dc discharge-sputtering source of nickel atoms. The laser-induced fluorescence excitation spectrum of a 2Π3/2–2Π3/2 transition has been recorded in the range of 500–520 nm and rotational structure of 506.5-nm band analyzed under the 30 K rotational temperature. Our data are consistent with a 2Π3/2 ground state for NiF. The lifetime of this band is measured.
The pressure broadening coefficient for O2 in the state has been measured to be 2.006±0.064 MHz/Torr at 290 K with a submillimeter-wave spectrometer with a backward wave oscillator as radiation source, where the uncertainty indicates one standard error from a least-squares fit. The broadening parameter by nitrogen is also measured. The temperature dependence is examined in the range 246–290 K. These results are useful for deriving the altitude distribution of O2 generated by a photochemical reaction from ozone in the upper stratosphere and mesosphere.
Gas-phase WO was generated by laser vaporization and its electronic spectra investigated by cavity ringdown laser absorption spectroscopy (CRLAS). The tungsten isotopic structure in the F–X 0–0 transition at 23405 cm−1 is clearly resolved, and its analysis reveals that the strong interstate interactions previously reported in the matrix are also present in the free molecule in the gas phase.