Velocity Map Imaging Research Articles

Vacuum ultraviolet photodissociation dynamics of N2O+hv→N2(X1Σg+)+O(1S) in the short wavelength tail of D1Σ+ band

Vacuum ultraviolet photodissociation dynamics of N2O+hv→N2(X1Σg+)+O(1S0) in the short wavelength tail of D1Σ+ band has been investigated using the time-sliced velocity-mapped ion imaging technique by probing the images of the O(1S0) photoproducts at a set of photolysis wavelengths including 121.47 nm, 122.17 nm, 123.25 nm and 123.95 nm. The product total kinetic energy release distributions, vibrational state distributions of the N2(X1Σg+) photofragments and angular anisotropy parameters have been obtained by analyzing the raw O(1S0) images. It is noted that additional vibrationally excited photoproducts (3≤v≤8) with a Boltzmann-like feature start to appear except the non-statistical component as the photolysis wavelength decreases to 123.25 nm, and the corresponding populations become more pronounced with decreasing of the photolysis wave-length. Furthermore, the vibrational state specific anisotropy parameter β at each photolysis wavelength exhibits a drastic fluctuation near β=1.75 at v<8, and decreases to a minimum as the vibrational quantum number further increases. While the overall anisotropy parameter β for the N2(X1Σg+)+O(1S0) channel presents a roughly monotonical increase from 1.63 at 121.47 nm to 1.95 at 123.95 nm. The experimental observations suggest that there is at least one fast nonadiabatic pathway from initially prepared D1Σ+ state to the dissociative state with bent geometry dominating to generate the additional vibrational structures at high photoexcitation energies.

Coincident angle-resolved state-selective photoelectron spectroscopy of acetylene molecules: a candidate system for time-resolved dynamics.

The acetylene-vinylidene system serves as a benchmark for investigations of ultrafast dynamical processes where the coupling of the electronic and nuclear degrees of freedom provides a fertile playground to explore the femto- and sub-femto-second physics with coherent extreme-ultraviolet (EUV) photon sources both on the table-top as well as free-electron lasers. We focus on detailed investigations of this molecular system in the photon energy range 19-40 eV where EUV pulses can probe the dynamics effectively. We employ photoelectron-photoion coincidence (PEPICO) spectroscopy to uncover hitherto unrevealed aspects of this system. In this work, the role of excited states of the C2H2+ cation, the primary photoion, is specifically addressed. From photoelectron energy spectra and angular distributions, the nature of the dissociation and isomerization channels is discerned. Exploiting the 4π-collection geometry of the velocity map imaging spectrometer, we not only probe pathways where the efficiency of photoionization is inherently high but also perform PEPICO spectroscopy on relatively weak channels.

Energy partitioning and spin-orbit effects in the photodissociation of higher chloroalkanes.

We investigate the photodissociation dynamics of the C-Cl bond in chloroalkanes CH3Cl, n-C3H7Cl, i-C3H7Cl, n-C5H11Cl, combining velocity map imaging (VMI) experiment and direct ab initio dynamical simulations. The Cl fragment kinetic energy distributions (KEDs) from the VMI experiment exhibit a single peak with maximum close to 0.8 eV, irrespective of the alkyl chain length and C-Cl bond position. In contrary to CH3Cl, where less than 10% of the available energy is deposited into the internal excitation of the CH3 fragment, for all higher chloroalkanes around 40% to 60% of the available energy goes into the alkyl fragment excitation. We apply the classical hard spheres and spectator model to explain the energy partitioning, and compare the classical approach with direct ab initio dynamics simulations. The alkyl chain appears to be a soft, energy absorbing unit. We further investigate the role of the spin-orbit effects on the excitation and dynamics. Combining our experimental data with theory allows us to derive the probability of the direct absorption into the triplet electronic state as well as the probabilities for intersystem crossing. The results indicate an increasing direct absorption into the triplet state with increasing alkyl chain length.

Vibrational predissociation versus intramolecular vibrational energy redistribution (IVR) in rare gas⋯dihalogen complexes: IVR identified in Ar⋯I2(B, ν′) using velocity-map imaging

Contributions from competing relaxation pathways can be difficult to identify, but direct evidence for IVR in the dissociation of excited-state Ar···I2(B, v') complexes was obtained using ion product velocity map imaging.

Unveiling the coexistence of cis- and trans-isomers in the hydrolysis of ZrO2: A coupled DFT and high-resolution photoelectron spectroscopy study.

High-resolution anion photoelectron spectroscopy of the ZrO3H2 - and ZrO3D2 - anions and complementary electronic structure calculations are used to investigate the reaction between zirconium dioxide and a single water molecule, ZrO2 0/- + H2O. Experimental spectra of ZrO3H2 - and ZrO3D2 - were obtained using slow photoelectron velocity-map imaging of cryogenically cooled anions, revealing the presence of two dissociative adduct conformers and yielding insight into the vibronic structure of the corresponding neutral species. Franck-Condon simulations for both the cis- and trans-dihydroxide structures are required to fully reproduce the experimental spectrum. Additionally, it was found that water-splitting is stabilized more by ZrO2 than TiO2, suggesting Zr-based catalysts are more reactive toward hydrolysis.

Experimental and theoretical investigation of resonances in low-energy NO-H2 collisions.

The experimental characterization of scattering resonances in low energy collisions has proven to be a stringent test for quantum chemistry calculations. Previous measurements on the NO-H2 system at energies down to 10 cm-1 challenged the most sophisticated calculations of potential energy surfaces available. In this report, we continue these investigations by measuring the scattering behavior of the NO-H2 system in the previously unexplored 0.4 cm-1-10 cm-1 region for the parity changing de-excitation channel of NO. We study state-specific inelastic collisions with both para- and ortho-H2 in a crossed molecular beam experiment involving Stark deceleration and velocity map imaging. We are able to resolve resonance features in the measured integral and differential cross sections. Results are compared to predictions from two previously available potential energy surfaces, and we are able to clearly discriminate between the two potentials. We furthermore identify the partial wave contributions to these resonances and investigate the nature of the differences between collisions with para- and ortho-H2. Additionally, we tune the energy spreads in the experiment to our advantage to probe scattering behavior at energies beyond our mean experimental limit.

Coulomb explosion imaging for gas-phase molecular structure determination: An ab initio trajectory simulation study.

Coulomb explosion velocity-map imaging is a new and potentially universal probe for gas-phase chemical dynamics studies, capable of yielding direct information on (time-evolving) molecular structure. The approach relies on a detailed understanding of the mapping between the initial atomic positions within the molecular structure of interest and the final velocities of the fragments formed via Coulomb explosion. Comprehensive on-the-fly ab initio trajectory studies of the Coulomb explosion dynamics are presented for two prototypical small molecules, formyl chloride and cis-1,2-dichloroethene, in order to explore conditions under which reliable structural information can be extracted from fragment velocity-map images. It is shown that for low parent ion charge states, the mapping from initial atomic positions to final fragment velocities is complex and very sensitive to the parent ion charge state as well as many other experimental and simulation parameters. For high-charge states, however, the mapping is much more straightforward and dominated by Coulombic interactions (moderated, if appropriate, by the requirements of overall spin conservation). This study proposes minimum requirements for the high-charge regime, highlights the need to work in this regime in order to obtain robust structural information from fragment velocity-map images, and suggests how quantitative structural information may be extracted from experimental data.

Excited-state dynamics of CH2I2 and CH2IBr studied with UV-pump VUV-probe momentum-resolved photoion spectroscopy.

We perform time-resolved ionization spectroscopy measurements of the excited state dynamics of CH2I2 and CH2IBr following photoexcitation in the deep UV. The fragment ions produced by ionization with a vacuum-ultraviolet probe pulse are measured with velocity map imaging, and the momentum resolved yields are compared with trajectory surface hopping calculations of the measurement observable. Together with recent time-resolved photoelectron spectroscopy measurements of the same dynamics, these results provide a detailed picture of the coupled electronic and nuclear dynamics involved. Our measurements highlight the non-adiabatic coupling between electronic states, which leads to notable differences in the dissociation dynamics for the two molecules.

Probing time delay of strong-field resonant above-threshold ionization* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11574101, 11674116, 11774111, and 11934006), the Open Fund of Hubei Provincial Key Laboratory of Optical Information and Pattern Recognition (Grant No. 201902), and the International Cooperation Program of Hubei Innovation Fund (Grant No. 2019AHB052).

The high-resolution three-dimensional photoelectron momentum distributions via above-threshold ionization (ATI) of Xe atoms are measured in an intense near circularly polarized laser field using velocity map imaging and tomography reconstruction. Compared to the linearly polarized laser field, the employed near circularly polarized laser field imposes a more strict selection rule for the transition via resonant excitation, and therefore we can selectively enhance the resonant ATI through certain atomic Rydberg states. Our results show the self-reference ionization delay, which is determined from the difference between the measured streaking angles for nonadiabatic ATI via the 4f and 5f Rydberg states, is 45.6 as. Our method provides an accessible route to highlight the role of resonant transition between selected states, which will pave the way for fully understanding the ionization dynamics toward manipulating electron motion as well as reaction in an ultrafast time scale.

Post extraction inversion slice imaging for 3D velocity map imaging experiments

ABSTRACT An experimental configuration for velocity map ion imaging experiments is presented, in which a pulsed voltage defocusses the ion Newton sphere along the time-of-flight axis. This significantly spreads the times-of-flight for ions with the same mass-to-charge ratio, allowing for either sliced or three-dimensional velocity imaging with high slicing resolution along the time-of-flight axis. The technique is coupled to an event-triggered, position-sensitive sensor, enabling full three-dimensional Newton sphere imaging at high count rates, with significantly improved slicing resolution (∼1–2%) compared to previous DC slicing approaches. Good slice imaging conditions can be brought about at relatively high extraction voltages, circumventing issues regarding image size, stray fields, and poor detection efficiency when operating at low extraction voltages. The method, termed Post Extraction Inversion Slice Imaging (PEISI) was optimised through ion trajectory simulations and experimentally verified on the well-studied photodissociation of OCS at around 230 nm. We demonstrate that this approach is suitable for recording full 3D angular distributions of systems lacking an axis of cylindrical symmetry in the detector plane, where conventional image inversion techniques are invalid. This method could be useful in a range of systems lacking cylindrical symmetry, including studies into angular momentum polarisation and bimolecular scattering.

Cation−π Complexes of Silver Studied with Photodissociation and Velocity-Map Imaging

Ag+(aromatic) ion-molecule complexes of benzene, toluene, or furan are generated in the gas phase by laser vaporization in a supersonic expansion. These ions are mass selected in a time-of-flight spectrometer and studied with ultraviolet laser photodissociation and photofragment imaging. UV laser excitation results in dissociative charge transfer (DCT) for these ions, producing neutral silver atom and the respective aromatic cation as the photofragments. Velocity-map imaging and slice imaging techniques are employed to investigate the kinetic energy release in these photodissociation processes. In each case, DCT produces significant kinetic energy, and evidence is also found for excitation of the internal rovibrational degrees of freedom for the molecular cations. Analysis of the kinetic energy release together with the known ionization energies of silver and the molecular ligands provides new information on the cation-π bond energies.

Disentangling the H2 E, F(1Σ g +) (v′=0−18)←X(1Σ g +)(v″=3−9)(2+1) REMPI spectrum via 2D velocity-mapped imaging

We have measured the two-photon Resonance-Enhanced Multiphoton Ionization (REMPI) spectrum of H by producing the hydrogen molecule via photolysis of HCO. H is produced with rovibrational energies up to 30,000 cm, at a maximum observed of (9,5). In the well-resolved jet-cooled HCO spectrum, ortho and para H are selectively produced with near 100% selectivity. Over 470 REMPI transitions were identified in the EF−X band system, many of which have not been seen previously, including transitions to EF levels not yet accurately measured. Speed distributions were obtained for each REMPI transition using Velocity-mapped imaging, aiding in the assignment by providing a precise measurement of the internal energy of the H molecule.

Time-resolved site-selective imaging of predissociation and charge transfer dynamics: the CH3I B-band

The predissociation dynamics of the 6s (B2E) Rydberg state of gas-phase CH3I were investigated by time-resolved Coulomb-explosion imaging using extreme ultraviolet (XUV) free-electron laser pulses. Inner-shell ionization at the iodine 4d edge was utilized to provide a site-specific probe of the ensuing dynamics. The combination of a velocity-map imaging (VMI) spectrometer coupled with the pixel imaging mass spectrometry (PImMS) camera permitted three-dimensional ionic fragment momenta to be recorded simultaneously for a wide range of iodine charge states. In accord with previous studies, initial excitation at 201.2 nm results in internal conversion and subsequent dissociation on the lower-lying A-state surface on a picosecond time scale. Examination of the time-dependent yield of low kinetic energy iodine fragments yields mechanistic insights into the predissociation and subsequent charge transfer following multiple ionization of the iodine products. The effect of charge transfer was observed through differing delay-dependencies of the various iodine charge states, from which critical internuclear distances for charge transfer could be inferred and compared to a classical over-the-barrier model. Time-dependent photofragment angular anisotropy parameters were extracted from the central slice of the Newton sphere, without Abel inversion, and highlight the effect of rotation of the parent molecule before dissociation, as observed in previous works. Our results demonstrate the ability to perform three-dimensional ion imaging at high event rates and showcase the potential benefits of this approach, particularly in relation to further time-resolved studies at free-electron laser facilities.

Velocity Map Imaging VUV Angle-Resolved Photoemission on Isolated Nanosystems: Case of Gold Nanoparticles

We present an angle-resolved photoelectron spectroscopy study on isolated gold nanoparticles in the photon energy range between 6 and 12 eV performed at the DESIRS beamline of Synchrotron SOLEIL wi...

Dissociative electron attachment to carbon dioxide

Our experimental progresses on the reaction dynamics of dissociative electron attachment (DEA) to carbon dioxide (CO2) are summarized in this review. First, we introduce some fundamentals about the DEA dynamics and provide an epitome about the DEAs to CO2. Second, the experimental technique developments are described, in particular, on the high-resolution velocity map imaging apparatus in which we put a lot of efforts during the past two years. Third, our findings about the DEA dynamics of CO2 are surveyed and briefly compared with the others’ work. At last, we give a perspective about the applications of the DEA studies and highlight the inspirations in the production of molecular oxygen on Mars and the catalytic transformations of CO2.

Duality of roaming mechanism in H2CO

Chemical Physics The phenomenon of roaming in chemical reactions (that is, bypassing the minimum energy pathway from unlikely geometries) has attracted a great deal of attention in the chemical reaction dynamics community over the past decade and still demonstrates unexpected results. Using velocity-map imaging of state-selected H2 products of H2CO photodissociation, Quinn et al. discovered the bimodal structure of rotational distribution of the other product fragment, CO. Quasiclassical trajectories showed that this bimodality originates from two distinctive reaction pathways that proceed by the trans or cis configuration of O–C–H⋯H, leading to high or low rotational excitations of CO, respectively. Whether such a mechanism is present in the many other chemical reactions for which roaming reaction pathways have been reported is yet to be determined. Science , this issue p. [1592][1] [1]: /lookup/doi/10.1126/science.abc4088

Clustering and multiphoton effects in velocity map imaging of methyl chloride

We have investigated different effects caused by clustering and multiphoton processes in the photodissociation of CHCl using velocity map imaging (VMI). Single-photon dissociation of CHCl at 193.3 nm yields CH and Cl fragments probed subsequently by resonance enhanced multiphoton ionisation (REMPI) at 333.52 nm and 235.31 nm, respectively. In the clusters, the CH fragments are strongly ‘caged’ losing most of their kinetic energy. Consequently, the VMI is dominated by a sharp central peak. On the other hand, the Cl fragments lose a broad range of energies in the cluster, yielding a blurred isotropic ‘central blob’ in VMI. At an increased 193 nm photon flux, another central feature appears in the CH images for isolated CHCl molecules. This ‘hour-glass’ shaped image corresponds to the CHCl multiphoton ionisation at 193 nm and subsequent CHCl ion photodissociation at 333 nm. The ion photodissociation is a parallel process with the anisotropy β-parameter values between +1.3 and +2.0. The total kinetic energy release (TKER) distribution suggests a high CH vibrational excitation. The CHCl ion photodissociation is discussed and compared to the photodissociation of similar CHBr ion.

A velocity map imaging study of multiphoton photodissociation and photoionisation dynamics in niobium oxide, NbO

Velocity map imaging has transformed our capabilities in terms of determining photofragmentation and photoionisation thresholds. Here, we present an investigation into the multi-photon dissociation and photoionisation dynamics of niobium monoxide (NbO) probed using velocity-map imaging via the C 4Σ– ← X 4Σ– and B 4Π ← X 4Σ- transitions. Analysis of the kinetic energy release spectra permits determination of refined values of the dissociation energies for NbO and NbO+ of 60,650 ± 124 and 57,414 ± 124 cm−1, respectively, in good agreement with previous values using other methods. With the same instrument, we have further recorded photoelectron images which exhibit a long progression assigned to the production of the ground state, X 3Σ− NbO+ (v +), for which a vibrational constant of 1002 ± 11 cm−1 is determined.

A velocity map imaging apparatus optimised for high-resolution crossed molecular beam experiments

We present the design of a Velocity Map Imaging apparatus tailored to the demands of high-resolution crossed molecular beam experiments employing Stark or Zeeman decelerators. The key requirements for these experiments consist of the combination of a high-relative velocity resolution for large ionisation volumes and a broad range of relatively low lab-frame velocities. The SIMION software package was employed to systematically optimise the electrode geometries and electrical configuration. The final design consists of a stack of 16 tubular electrodes, electrically connected with resistors, which is divided into three electric field regions. The resulting apparatus allows for an inherent velocity blurring of less than for NO ions originating from a ionisation volume, which is negligible in a typical crossed beam experiment. The design was recently employed in several state of the art crossed-beam experiments, allowing the observation of fine details in the velocity distributions of the scattered molecules.

Charge transfer dynamics in Ar+ + CO

Differential cross sections for the charge transfer reaction between Ar+ and CO have been measured using three-dimensional velocity map imaging in a crossed beam setup at the two relative collision energies 0.55 and 0.74 eV. We find dominant forward scattering with CO+ product ions predominantly in the vibrational levels =6,7 of the electronic ground state X. This is indicative of a direct resonant mechanism for the two argon spin-orbit states. At both collision energies also an isotropic distribution with product ions exhibiting high internal excitation is observed. This is more pronounced at the higher collision energy, where the first electronically excited state A becomes accessible. We conclude that the A-state is partially populated by the product ions at 0.74 eV collision energy and suggest that the isotropic distribution stems from the formation of a charge-transfer complex, in concurrence with previously performed studies.