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Comparison of raw 2D and processed 3D images. The raw (a) and rapidly inverted (b) images for CD3+ ions. The inversion was accomplished with the ‘onion-peeling’ or ‘back-projection’ method described in this section. The purple and white boxed regions in (b) illustrate how different processes could be isolated from the CD3+ images and the corresponding yields used to define a control objective. On a 661 × 661 pixel test image, our algorithm could produce an inverted image in 50±5 ms, compared with 1.5 s for pBASEX51, 4.86 s for POP53 and 174 s for the iterative procedure52. For a full-sized 1040 × 1054 pixel image, the inversion time was still only around 600 ms, well below the image acquisition time. Additional efficiency was gained by separating the image acquisition and analysis steps. Thus, while one image was being acquired, the previous image was being inverted. As the exposure time is almost always longer than the image processing time, the overall time for the experiment was not affected by the addition of the inversion step into the control loop.
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Shaping ultrafast laser pulses using adaptive feedback can manipulate dynamics in molecular systems, but extracting information from the optimized pulse remains difficult. Experimental time constraints often limit feedback to a single observable, complicating efforts to decipher the underlying mechanisms and parameterize the search process. Here we...
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... inversion. In this control scheme, accurate feedback from the velocity map image, in the form of an ion yield from a particular portion of the image, often depends critically on the conversion of the 2D raw image to a slice through the centre of the full 3D momentum distribution. An example is displayed in Fig. 7, which shows the raw and inverted images of CD 3 þ ions produced in a laser- ethylene interaction. Although the outer structure is visible in the raw image on the left, the inner structure is nearly completely obscured by an unwanted azimuthal contribution that can easily lead to sub-optimal control results 26 ...
Citations
... Versatile tools (e.g., core-electron ionization [10][11][12], strong-field ionization [13][14][15], and charged particle collision [16][17][18]) were utilized to probe the isomerization of acetylene ions. So far, acetylene-vinylidene (A-V) isomerization has been widely documented in its ionic forms [19][20][21][22][23][24]. ...
First-principle calculations are employed to investigate the ultrafast isomerization of the acetylene cation and dication. We use the time-dependent density functional theory together with the Ehrenfest dynamics to track the coupled electron-nuclear dynamics. For both the acetylene cation and the dication, we observe nonadiabatic behaviors during the isomerization. We find that the charge transfer not only alters the electronic structure through nonadiabatic transitions, but also plays a key role in the subsequent hydrogen migration. We show that nonadiabatic transitions affect the structural modification of the excited potential energy surface, and also facilitate the ultrafast isomerization through the creation of a channel of increased negative charge that facilitates the proton movement. For the acetylene cation, we find a timescale for hydrogen isomerization of 66±4 fs, which is consistent with previous pump-probe experiments and on-the-fly calculations. For the dication, we find nonadiabatic transitions occur before the isomerization and identify a similar channel for the proton. Moreover, we find the formation of vinylidene-like structures is always accompanied by a characteristic charge separation on the carbon skeleton. These heuristics will be useful in identifying tautomers and motivating the ultrafast charge-transfer detection methods for future experiments.
... The hydrogen atoms can migrate along the C-C skeletal bonds and cause various important chemical reactions [11], e.g., 1,2-hydrogen shift and roaming processes. These processes have been extensively investigated by intense laser field ionization [12][13][14][15][16][17][18], which enables precise measurement of temporal and energetic information. However, strong laser pulses with rather long irradiation times could easily alter the profile of the molecular potential-energy surface (PES). ...
... As two of the simplest hydrocarbon molecules, acetylene and ethane have attracted much attention for their C-C bond cleavage dissociation [12][13][14][15][16]22,24,29,30]. Both the direct or symmetric fragmentation channel (C 2 H 2n 2+ → CH n + + CH n + ) (n = 1, 3) and the isomeric or asymmetric one (C 2 H 2n 2+ → CH + (n+1) + CH + (n−1) (n = 1, 3) are observed in the dication dissociation. ...
... However, the KER distribution of C 2 H 2 2+ asymmetric fragmentation is bimodal and involves isomerization on the PES of some higher excited states [22,27,30]. For the symmetric fragmentation, the hydrogen transfer process is not involved in the C-C bond cleavage of C 2 H 2+ 2 [15,27], but it is an important contributor to the case of C 2 H 2+ 6 [16,24]. A considerable proportion of the parent ion [CH 3 6 symmetric channel has a rather complicated reaction path as compared to the one-step dissociation of the C 2 H 2+ 2 case. ...
The C-C bond cleavage dissociation of ethylene dication produced by 18-keV/u Ne8+ impact is investigated by combining experimental measurement and theoretical calculation. Using cold target recoil ion momentum spectroscopy, two channels, i.e., the symmetric fragmentation, C2H42+ → CH2++CH2+, and the isomeric one, C2H42+ → CH++CH3+, are clearly identified and thus their kinetic-energy release (KER) distributions are determined. The average KER values are then compared with the theoretical results obtained by quantum chemical calculations, which provide reaction paths of both channels on the potential-energy surfaces of different molecular states. It is found that the hydrogen transfer process and excited-state dynamics still play crucial roles in the formation of C-C bond cleavage channels, but in a different way in comparison with the cases of acetylene C2H2 and ethane C2H6.
... Pulse characteristics were determined using a second-harmonicgeneration frequency-resolved-optical-grating (SHG-FROG) [44] device. Our version of VMI [45][46][47] integrates the momentum image of a given m/q time-of-flight peak over many laser shots by fully powering the detector within a specific time window. For a typical trial pulse, we collected VMI data for 5,000 laser shots for D 2 H + and 35,000 laser shots for D + 3 in order to obtain similar statistics for each ion. ...
... (E) The unshaped spectral density and phase for comparison, again with the same color scheme. feedback to drive a genetic algorithm that optimizes the pulse shapes to a control objective [46,47]. The raw VMI data is inverted on-the-fly to recover a slice through the center of the three-dimensional momentum distribution using a modified "onion-peeling" algorithm as described by Rallis et al. [47]. ...
... In addition, the increase in the D 2 H + :D + 3 ratio was due to an increase in D 2 H + yield, not a reduction of D + 3 yield. In some similar experiments, this combination of indicators has been a signature of a barrier-suppression mechanism [46]. Several of the theoretical efforts with ethane [31,34,35] identified one or more transition states in the dissociation process. ...
An adaptive learning algorithm coupled with 3D momentum-based feedback is used to identify intense laser pulse shapes that control H 3 + formation from ethane. Specifically, we controlled the ratio of D2H+ to D 3 + produced from the D3C-CH3 isotopologue of ethane, which selects between trihydrogen cations formed from atoms on one or both sides of ethane. We are able to modify the D2H+: D 3 + ratio by a factor of up to three. In addition, two-dimensional scans of linear chirp and third-order dispersion are conducted for a few fourth-order dispersion values while the D2H+ and D 3 + production rates are monitored. The optimized pulse is observed to influence the yield, kinetic energy release, and angular distribution of the D2H+ ions while the D 3 + ion dynamics remain relatively stable. We subsequently conducted COLTRIMS experiments on C2D6 to complement the velocity map imaging data obtained during the control experiments and measured the branching ratio of two-body double ionization. Two-body D 3 + + C 2 D 3 + is the dominant final channel containing D 3 + ions, although the three-body D + D 3 + + C 2 D 2 + final state is also observed.
... The ionization and dissociation processes can be manipulated using shaped-pulse laser fields. The chemical bonds of molecules can be broken and rearranged irradiated by shaped laser fields, which enables us to further manipulate chemical reactions [122,123]. The ionization processes, such as above-threshold ionization [124,125], nonsequential double ionization [126,127], high-harmonic spectroscopy [128], are also manipulated by variety of shaped-pulse laser fields. ...
This review focuses on the properties of the light fields that are more useful in applications. We review recent means of generating shaped-pulse light field, by which matters can be steered toward the desired products, thereby allowing the coherent control in terms of effectiveness, selectivity and manipulation. Applications of these light fields are discussed, including bioscience, laser machining, novel material fabrication, trace material detection and military. © 2021 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
... Recently, numerous studies have been conducted on the dissociation of molecular ions produced by various ionizing projectile or radiation [6][7][8][9][10][11][12][13][14][15][16] and KERs for some specific dissociation channels have been measured, which advance insight into the proton/hydrogen migration [12], isomerization [13,14], fragmentation mechanism [15,16], etc. ...
... Recently, numerous studies have been conducted on the dissociation of molecular ions produced by various ionizing projectile or radiation [6][7][8][9][10][11][12][13][14][15][16] and KERs for some specific dissociation channels have been measured, which advance insight into the proton/hydrogen migration [12], isomerization [13,14], fragmentation mechanism [15,16], etc. ...
The fragmentation of C2H4²⁺ produced by 300 eV electron impact has been investigated. Both 2-body and 3-body fragmentation channels were observed with an ion–ion coincidence technique. Using the recoil ion momentum spectroscopy, the momentum vector of each ionic fragment was measured and the kinetic energy release (KER) distributions for different fragmentation channels were obtained. For 2-body fragmentation channels, the branching ratios and the peak values of the KER distributions were compared with previous photon ionization experiments. The electronic state information of the parent ion C2H4²⁺ was inferred by comparing the present measured KER values and the predictions in previous works. For 3-body fragmentation channels, the dissociation mechanisms were analyzed according to the signature on the Newton diagram.
... 23 For example, the isomerization reaction C 2 H 2+ 2 → C + + CH + 2 was enhanced by suppressing the barrier between the acetylene-like initial state and the vinylidene-like final state at the appropriate time. 59 Another way a bond-rearrangement reaction can occur in a polyatomic molecule is via the roaming mechanism. [60][61][62][63][64][65][66][67][68][69][70] In the roaming process, a transient neutral atomic or molecular fragment explores the region of the parent molecule before reacting to form the final product. ...
... 80 In one approach, the laser pulse shapes are optimized using three-dimensional momentum based feedback derived from the VMI data to drive an adaptive learning algorithm. 59,79 In a second approach, the laser pulse shape is systematically scanned through a reduced set of parameters, while the D 2 H + /D + 3 ratio, obtained from the VMI data, is monitored. ...
... The pulse shapes are then optimized using a genetic algorithm, as described in previous publications. 59,79 In the present experiment, the pulse was parameterized in the frequency domain and controlled via 16 spectral phase "genes" spread over the oscillator bandwidth. The number of genes was selected to keep the search space reasonably sized. ...
Using the CD3OH isotopologue of methanol, the ratio of D2H⁺ to D3+ formation is manipulated by changing the characteristics of the intense femtosecond laser pulse. Detection of D2H⁺ indicates a formation process involving two hydrogen atoms from the methyl side of the molecule and a proton from the hydroxyl side, while detection of D3+ indicates local formation involving only the methyl group. Both mechanisms are thought to involve a neutral D2 moiety. An adaptive control strategy that employs image-based feedback to guide the learning algorithm results in an enhancement of the D2H⁺/D3+ ratio by a factor of approximately two. The optimized pulses have secondary structures 110–210 fs after the main pulse and result in photofragments that have different kinetic energy release distributions than those produced from near transform limited pulses. Systematic changes to the linear chirp and higher order dispersion terms of the laser pulse are compared to the results obtained with the optimized pulse shapes.
... Acetylene (C 2 H 2 ) is one of the simplest stable hydrocarbons widely existing in the planetary atmosphere and interstellar medium. In recent years, acetylene dication (C 2 H 2 2+ ) has been extensively studied both theoretically [4][5][6][7][8][9][10] and experimentally as a prototype for investigating superfast deprotonation [11], proton migration (molecular isomerization) [12][13][14][15][16][17][18][19], and H 2 + formation [20][21][22] [23,25] and an intense laser field [12,24] have revealed that the isomerization process can occur through the A 2 g + state of [HCCH] + by absorbing photons in two steps on a time scale of about 50 fs, or through the 1 3 state of [HCCH] 2+ by absorbing multiple photons in one step. ...
... Since the mass of the hydrogen atom is very low, the deprotonated and protonated channels overlap strongly in the TOF correlation map. Deuteration is an alternative way [15,19,[33][34][35][36] to better separate the explored channels, which, however, requires highly demanding target molecules. Therefore, a general method is needed to disentangle fragmentation channels of charged hydrocarbons with small differences in the mass of the fragments. ...
We report experimental investigations of two-body fragmentation of C2H22+ induced by 1 keV electron collision utilizing an ion momentum imaging spectrometer. With the ion-ion coincidence measurement, dissociation channels C2H22+→H++C2H+ (deprotonation) and C2H22+→H2++C2+ (H2+ formation) are directly identified, while the symmetric breakup C2H22+→CH++CH+ channel and vinylidene decarbonation C2H22+→C++CH2+ channel are not well separated in the measured time-of-flight (TOF) correlation map. In this work, by taking advantage of the independence of kinetic energy release (KER) on the dissociation angle, we are able to disentangle the events from the TOF map. Consequently, KER distributions for all four fragmentation channels are deduced, and the relative branching ratios are precisely determined from the measurements. By comparing the measured KER values with the previous calculated potential energy surfaces, pathways for the fragmentation channels are assigned.
... However, they are slightly larger than KERs obtained in intense laser fields [17]. This is probably due to the drastic modification of the dication PES by the intense laser interaction [33], e.g., suppressing the dissociation barrier towards the H 3 + fragments. ...
Formation mechanism of H3+ ions from doubly charged hydrocarbons (CH4 and C2H4) is investigated by combining charged particle (300 eV electrons and 3 keV/u Ar8+ ions) collision experiments and quantum chemistry calculations. The kinetic energy release (KER) distribution for each H3+ loss process was measured with the cold target recoil ion momentum spectroscopy. The good agreement between the mean KER and corresponding theoretical reverse activation energy, related to a transition state and the asymptote of the dissociation products, provides information on the H3+ formation dynamics. A H2 roaming mechanism is proposed for the formation of H3+. Since the hydrocarbons in the interaction with charged particles have direct interests in astronomical environments, the finding of this work would be helpful in the understanding of the energetics and dynamics of H3+ formation in space.
... For that, acetylene has drawn great attention to recent subjects of ion-and laser-induced experiments. A number of fragmentation channels have been studied in a wide variety of laser, electron, and ion-acetylene interactions, and corresponding dynamics have been reconstructed in different energy ranges to unravel the proton migration and/or fragmentation mechanism of acetylene in ionic forms [8][9][10][13][14][15][16][17][18]. Previously the three-body fragmentation mechanisms (sequential and concerted breakup) of linear triatomic molecular ions of, e.g., CO 2 3+ [19][20][21] and OCS 3+ [22], have been clearly disentangled. ...
Three-fragment dissociation dynamics of C2H22+ dications and C2H23+ trications induced by electron capture of slow (3-keV/u) Ar8+ ions are investigated. Using cold target recoil ion momentum spectroscopy, the complete kinematic information and thus kinetic energy releases are determined for the three-fragment channels, C2H22+→H++C2++H and C2H23+→H++C2++H+. Then by analyzing the complete kinematics with a Dalitz plot and Newton diagram, different fragmentations, i.e., concerted or sequential pathway, are identified. For dications, the sequential fragmentation C2H22+→H++C2H+→H++C2++H is dominant. However, the trications mainly dissociate via the synchronous concerted fragmentation. The sequential pathway C2H23+→H++C2H2+→H++C2++H+, which was found to be significant at higher collision velocities, is not observed here. This distinction reveals the important role of projectile velocity on the fragmentation dynamics for some specific channels.
... The development of ultrafast laser technology has enabled time-resolved studies of femtosecond molecular dynamics and provided tools to control them. Many of the suggested control schemes are based on the modification of molecular potential curves by the electric field of an optical or infrared laser pulse [1][2][3][4][5][6]. This requires the field to be strong enough to be achieved. ...
Laser-induced dissociation of a photoionized oxygen molecule is studied employing an extreme-ultraviolet-pump–near-infrared-probe (EUV-NIR pump-probe) technique. A combination of a narrow-band 11th harmonic pump centered at 17.3 eV and a moderate-intensity NIR probe restricts the dissociation dynamics to the pair of low-lying cationic states, a4Πu and f4Πg. The measured kinetic energies of the O+ fragments reveal contributions from one-, two-, and three-photon dissociation pathways (1ω, 2ω, and 3ω) involving these two states. While the yields of the two- and three-photon channels initially rise and then decrease as a function of EUV-NIR delay, the yield of the single-photon pathway rises slower but keeps increasing over the whole delay range studied. This behavior reflects the evolving probability density of the ionic nuclear wave packet at the internuclear distances, where it can undergo resonant 3ω and 1ω transitions from the a4Πu to the f4Πg state of O2+.
