Velocity Map Imaging Technique Research Articles

Rotational-mode specific effects on the stereo-requirement in the reaction of prealigned-CHD3(v1 = 1; |JK = |10 or |1 ± 1) with the chlorine atom.

Several aspects of the stereo-specific requirement in the title reaction are systematically investigated in a crossed-beam experiment using a time-sliced, velocity-mapped imaging technique. Specifically, we explored (1) the differential steric effect from pre-aligning two different reagent rotational states and (2) the effect from probing different product rotational states. In the reaction with an aligned JK=10 reagent at Ec = 3.2 kcal mol-1, the head-on geometry yields a predominantly backward-scattered CD3(00) + HCl(v = 0) product pair, whereas the side-on approach results in a pronounced sideway-scattered distribution. The alternative CD3(00) + HCl(v = 1) channel exhibits a sharply forward-scattering feature for both the collisional geometries. The branching of the two product channels shows sensitive dependency on the collisional geometries. Probing different rotational states of CD3(00) reveals little variation in pair-correlated angular distributions, yet yields notable effect on the correlated vibrational branching of the HCl(v = 0, 1) coproducts. Similar steric propensities hold at lower collisional energy of 1.3 kcal mol-1. In stark contrast, diminishing steric effects were observed in the reaction with an aligned 1±1 reagent. Such huge differential, K-dependent stereo-requirements are largely attributed to the distinct "shapes" of the two rotational states of the aligned CHD3(v1 = 1) reagents.

The study of Eu 4f76p6d autoionizing states with RIS and VMI techniques

The spectra and dynamic properties of Eu [Formula: see text] autoionizing states are studied systematically with the resonance-ionization spectroscopy (RIS) and the velocity-map imaging (VMI) techniques. The RIS technique is utilized to obtain the spectra of Eu [Formula: see text] autoionizing states with the three-step excitation, while the VMI technique is used to detect the dynamic process of autoionization, from which the branching ratio (BR) of ions and the angular distribution (AD) of ejected electrons from Eu [Formula: see text] autoionizing states are achieved. Not only the energy levels, line width and line shape are obtained, but also the q-reversal phenomenon is observed in the autoionization spectra. The BR represents energy distribution (ED) of ions by observing the radical information of the VMI image, while the AD, characterized by anisotropy parameters, reveals the differential cross-section of autoionization extracted from the angular information of the VMI image. Furthermore, the variation of BR and AD are analyzed within the entire region of autoionization resonance. Based on the results of BR between [Formula: see text] and [Formula: see text] ionic states, the possibility of the population inversion is discussed.

Note: A well-confined pulsed low-energy ion beam: Test experiments of Ar.

Here we report a pulsed low-energy ion beam source for ion-molecule reaction study, in which the ions produced by the pulsed electron impact are confined well in the spatial size of each bunch. In contrast to the ion focusing method to reduce the transverse section of the beam, the longitudinal section in the translational direction is compressed by introducing a second pulse in the ion time-of-flight system. The test experiments for the low-energy argon ions are performed. The present beam source is ready for applications in the ion-molecule reaction dynamics experiments, in particular, in combination with the ion velocity map imaging technique.

Time-resolved ion imaging at free-electron lasers using TimepixCam.

The application of a novel fast optical-imaging camera, TimepixCam, to molecular photoionization experiments using the velocity-map imaging technique at a free-electron laser is described. TimepixCam is a 256 × 256 pixel CMOS camera that is able to detect and time-stamp ion hits with 20 ns timing resolution, thus making it possible to record ion momentum images for all fragment ions simultaneously and avoiding the need to gate the detector on a single fragment. This allows the recording of significantly more data within a given amount of beam time and is particularly useful for pump-probe experiments, where drifts, for example, in the timing and pulse energy of the free-electron laser, severely limit the comparability of pump-probe scans for different fragments taken consecutively. In principle, this also allows ion-ion covariance or coincidence techniques to be applied to determine angular correlations between fragments.

Study of electron beam induced ion-pair dissociation dynamics of O2 using velocity slice imaging spectrometer

The ion-pair dissociation (IPD) of molecular oxygen due to 21–35 eV energy electron collision has been studied using the time sliced velocity map imaging technique. The threshold of the process and the kinetic energy and angular distribution of the fragment negative ions are measured. The IPD is found to be occurring due to pre-dissociation of a Rydberg state via ion-pair state for lower incident electron energies as well as from direct excitation to the ion-pair states for higher primary beam energy. The location and symmetry of the excited states are determined from the kinetic energy and angular distribution data respectively.

Multiphoton ionization dissociation dynamics of iodoethane studied with velocity map imaging technique

Halogenated alkanes destroy the ozone layer, and iodoethane is one of the important representative halogenated alkanes. Time-of-flight mass spectrometry and velocity map imaging technique are used for investigating the photoionization dissociation dynamics of iodoethane, induced by 800 nm femtosecond laser. The dissociation mechanisms of iodoethane are obtained and discussed by analyzing the velocity distributions and angular distributions of the fragment ions generated in the dissociation. The measurements by time-of-flight mass spectrometry show that iodoethane cations generates C2H5+, I+, CH2I+, C2H2+, C2H3+ and C2H4+. The fragments related to CI bond fragmentation are C2H5+ ions and I+ ions, and the dissociation mechanisms are C2H5I+ C2H5++I and C2H5I+ C2H5+I+ respectively. Comparison between the configurations before and after ionization shows that the CI bond length is 0.2220 nm before ionization and turns longer and becomes 0.2329 nm after ionization. This indicates that the CI bond becomes more unstable after ionization and is more prone to dissociation. Moreover, the velocity map images of C2H5+ and I+ ions are acquired, from which the speed and angular distribution of C2H5+ and I+ are obtained. The analysis of speed distribution of the fragment ions shows that there are two channels, i.e. high energy channel and low energy channel in the dissociation process for producing C2H5+ and I+ ion. The difference between the ratios of the high energy channel and the low energy channel is small, indicating that the high energy channel and the low energy channel of the two dissociation processes are similar. According to the further analysis of the angular distribution of the fragment ions, it is found that the anisotropy parameter of C2H5+ is close to 0 (isotropic), the production channel of which may correspond to the slow vibration predissociation process. The anisotropy parameters of I+ ions are higher, which may be due to the rapid dissociation process on the repulsive potential energy surface. In addition, the density functional theory is used to calculate the configuration change of the iodoethane molecule before and after ionization, the energy level and oscillator strength for the ionic state in order to obtain more insights into the photodissociation dynamics.

Sequential evaporation of water molecules from protonated water clusters: measurement of the velocity distributions of the evaporated molecules and statistical analysis.

Velocity distributions of neutral water molecules evaporated after collision induced dissociation of protonated water clusters H+(H2O)n≤10 were measured using the combined correlated ion and neutral fragment time-of-flight (COINTOF) and velocity map imaging (VMI) techniques. As observed previously, all measured velocity distributions exhibit two contributions, with a low velocity part identified by statistical molecular dynamics (SMD) simulations as events obeying the Maxwell-Boltzmann statistics and a high velocity contribution corresponding to non-ergodic events in which energy redistribution is incomplete. In contrast to earlier studies, where the evaporation of a single molecule was probed, the present study is concerned with events involving the evaporation of up to five water molecules. In particular, we discuss here in detail the cases of two and three evaporated molecules. Evaporation of several water molecules after CID can be interpreted in general as a sequential evaporation process. In addition to the SMD calculations, a Monte Carlo (MC) based simulation was developed allowing the reconstruction of the velocity distribution produced by the evaporation of m molecules from H+(H2O)n≤10 cluster ions using the measured velocity distributions for singly evaporated molecules as the input. The observed broadening of the low-velocity part of the distributions for the evaporation of two and three molecules as compared to the width for the evaporation of a single molecule results from the cumulative recoil velocity of the successive ion residues as well as the intrinsically broader distributions for decreasingly smaller parent clusters. Further MC simulations were carried out assuming that a certain proportion of non-ergodic events is responsible for the first evaporation in such a sequential evaporation series, thereby allowing to model the entire velocity distribution.

Correlated ion and neutral time of flight technique combined with velocity map imaging: Quantitative measurements for dissociation processes in excited molecular nano-systems.

The combination of the Dispositif d'Irradiation d'Agrégats Moléculaire with the correlated ion and neutral time of flight-velocity map imaging technique provides a new way to explore processes occurring subsequent to the excitation of charged nano-systems. The present contribution describes in detail the methods developed for the quantitative measurement of branching ratios and cross sections for collision-induced dissociation processes of water cluster nano-systems. These methods are based on measurements of the detection efficiency of neutral fragments produced in these dissociation reactions. Moreover, measured detection efficiencies are used here to extract the number of neutral fragments produced for a given charged fragment.

Photodissociation of van der Waals complexes of iodine X-I2 (X = I2, C2H4) via charge-transfer state: A velocity map imaging investigation.

The photodissociation of van der Waals complexes of iodine X-I2 (X = I2, C2H4) excited via Charge-Transfer (CT) band has been studied with the velocity map imaging technique. Photodissociation of both complexes gives rise to translationally "hot" molecular iodine I2 via channels differing by kinetic energy and angular distribution of the recoil directions. These measured characteristics together with the analysis of the model potential energy surface for these complexes allow us to infer the back-electron-transfer (BET) in the CT state to be a source of observed photodissociation channels and to make conclusions on the location of conical intersections where the BET process takes place. The BET process is concluded to provide an I2 molecule in the electronic ground state with moderate vibrational excitation as well as X molecule in the electronic excited state. In the case of X = I2, the BET process converts anion I2- of the CT state into the neutral I2 in the repulsive excited electronic state which then dissociates promptly giving rise to a pair of I atoms in the fine states 2P1/2. In the case of C2H4-I2, the C2H4 molecules appear in the triplet T1 electronic state. Conical intersection for corresponding BET process becomes energetically accessible after partial twisting of C2H4+ frame in the excited CT state of complex. The C2H4(T)-I2 complex gives rise to triplet ethylene as well as singlet ethylene via the T-S conversion.

Note: Ultraviolet photodissociation dynamics of o-bromofluorobenzene in 234-267 nm.

Photodissociation dynamics of o-bromofluorobenzene in the 234-267 nm range has been experimentally investigated using the DC-slice velocity map imaging technique. It is found that Br(2P3/2) atoms produced from repulsive singlet state surfaces via excitation of one or more 1ππ* excited states dominate the Br photofragments. The quantum yield of spin-orbit excited Br(2P1/2) atoms was found to be only ∼0.02, indicating weak spin-orbit couplings in the low-lying electronic states of o-bromofluorobenzene.

Contribution of resonance excitation on ionization of OCS molecules in strong laser fields

Photoelectron images from single ionization of OCS molecules are measured at 800 nm or 400 nm linearly polarized laser pulses (∼50 fs) using velocity-map imaging techniques. The resonant ionization of the molecules has been studied by tracing the electron energy shift in the photoelectron energy spectra at different laser intensities, and the contributions of three excited states at certain laser intensities and wavelengths are identified. During the ionization of OCS molecules, two excited states (1Δ and 1∏) are shifted into resonance at different 800 nm laser intensities, and the Rydberg state 4pσ ∏ is involved in the resonance for a 400 nm laser. The demonstration of resonant excitation contribution experimentally will help further the understanding of OCS ionization in strong laser fields.

Photodissociation Dynamics of OCS at 217 nm

The S(1D2)+CO(X1Σ+) product channel from photodissociation of OCS at 217 nm has been measured using the DC slice velocity map imaging (VMI) technique in combination with resonance enhanced multiphoton ionization (REMPI). Two different REMPI intermediate states (1F3 and 1P1) and several pump-probe laser polarization geometries are used to detect the angular momentum polarization of the photofragmented S(1D2). The molecular-frame polarization parameters, as well as the laboratory-frame anisotropy parameters, for individual rotational states of co-fragment CO, are determined using two different full quantum theories. The measured total kinetic energy release spectrum from photodissociation of OCS indicates two dissociation channels, corresponding to the fast and slow recoiling velocities of S(1D2), respectively. The slow channel is concluded to originate from an initial photoexcitation to the A(1A′) state, followed by a non-adiabatic transition to the ground state. The fast channel is found to follow a coherent excitation to A(1A′) and B(1A″) states, where contributions of the two states are almost equal at 217 nm. The determined alignment and anisotropy parameters further indicate that the slow channel follows an incoherent excitation, while the fast channel follows a coherent excitation to A(1A′) and B(1A″) states with a phase difference of π/2.

Measurement of the velocity of neutral fragments by the "correlated ion and neutral time of flight" method combined with "velocity-map imaging".

In the challenging field of imaging molecular dynamics, a novel method has been developed and implemented that allows the measurement of the velocity of neutral fragments produced in collision induced dissociation experiments on an event-by-event basis. This has been made possible by combining a correlated ion and neutral time of flight method with a velocity map imaging technique. This new method relies on a multiparametric correlated detection of the neutral and charged fragments from collision induced dissociation on one single detector. Its implementation on the DIAM device (Device for irradiation of biomolecular clusters) (Dispositif d'Irradiation d'Agrégats bioMoléculaires) allowed us to measure the velocity distribution of water molecules evaporated from collision induced dissociation of mass- and energy-selected protonated water clusters.

Soft X-ray spectroscopy of nanoparticles by velocity map imaging.

Velocity map imaging (VMI), a technique traditionally used to study chemical dynamics in the gas phase, is applied here to study X-ray photoemission from aerosol nanoparticles. Soft X-rays from the Advanced Light Source synchrotron, probe a beam of nanoparticles, and the resulting photoelectrons are velocity mapped to obtain their kinetic energy distributions. A new design of the VMI spectrometer is described. The spectrometer is benchmarked by measuring vacuum ultraviolet photoemission from gas phase xenon and squalene nanoparticles followed by measurements using soft X-rays. It is demonstrated that the photoelectron distribution from X-ray irradiated squalene nanoparticles is dominated by secondary electrons. By scanning the photon energies and measuring the intensities of these secondary electrons, a near edge X-ray absorption fine structure (NEXAFS) spectrum is obtained. The NEXAFS technique is used to obtain spectra of aqueous nanoparticles at the oxygen K edge. By varying the position of the aqueous nanoparticle beam relative to the incident X-ray beam, evidence is presented such that the VMI technique allows for NEXAFS spectroscopy of water in different physical states. Finally, we discuss the possibility of applying VMI methods to probe liquids and solids via X-ray spectroscopy.

Photodissociation dynamics of OCS at ∼210 nm: The role of c(23A″) state.

Photodissociation dynamics of carbonyl sulfide (OCS) in the deep ultraviolet region is investigated using a time-sliced ion velocity map imaging technique. The measured total kinetic energy release spectra from the photodissociation of OCS at ∼210 nm shows three dissociation channels to the fragment S(1D2), corresponding to low, medium, and high kinetic energy release (ET), respectively. The high ET channel is found to be a new dissociation channel opening with photolysis wavelength at ∼210 nm. Based on the aq(k)(p) polarization parameters as well as the anisotropy parameters β determined from the images of S(1D2), the dissociation of OCS to S(1D2) + CO at 210 nm is concluded to involve a direct vertical excitation of the triplet c(23A″) state from the ground state, followed by processes as: the low ET component arises from a non-adiabatic transition from the repulsive A(21A') state to the electronic ground state X(11A'); the medium ET component arises from a simultaneous excitation to two repulsive excited states; and the high ET component arises from the intersystem crossing from the triplet c(23A″) state to the repulsive A(21A') state. The present study shows that, due to the strong spin-orbit coupling between the triplet c(23A″) state and the repulsive A(21A') state, a direct excitation to c(23A″) significantly contributes to the photodissociation dynamics of OCS in the deep-UV region.

Photofragmentation dynamics and dissociation energies of MoO and CrO.

Neutral metal-containing molecules and clusters present a particular challenge to velocity map imaging techniques. Common methods of choice for producing such species-such as laser ablation or magnetron sputtering-typically generate a wide variety of metal-containing species and, without the possibility of mass-selection, even determining the identity of the dissociating moiety can be challenging. In recent years, we have developed a velocity map imaging spectrometer equipped with a laser ablation source explicitly for studying neutral metal-containing species. Here, we report the results of velocity map imaging photofragmentation studies of MoO and CrO. In both cases, dissociation at the two- and three-photon level leads to fragmentation into a range of product channels, some of which can be confidently assigned to particular Mo* (Cr*) and O atom quantum states. Analysis of the kinetic energy release spectra as a function of photon energy allows precise determination of the ground state dissociation energies of MoO (=44 064 ± 133 cm-1) and CrO (=37 197 ± 78 cm-1), respectively.

A velocity-map imaging study of methyl non-resonant multiphoton ionization from the photodissociation of CH3I in the A-band.

Chemical reaction dynamics and, particularly, photodissociation in the gas phase are generally studied using pump-probe schemes where a first laser pulse induces the process under study and a second one detects the produced fragments. Providing an efficient detection of ro-vibrationally state-selected photofragments, the resonance enhanced multiphoton ionization (REMPI) technique is, without question, the most popular approach used for the probe step, while non-resonant multiphoton ionization (NRMPI) detection of the products is scarce. The main goal of this work is to test the sensitivity of the NRMPI technique to fragment vibrational distributions arising from molecular photodissociation processes. We revisit the well-known process of methyl iodide photodissociation in the A-band at around 280 nm, using the velocity-map imaging technique in conjunction with NRMPI of the methyl fragment. The detection wavelength, carefully selected to avoid any REMPI transition, was scanned between 325 and 335 nm seeking correlations between the different observables-the product vibrational, translational and angular distributions-and the excitation wavelength of the probe laser pulse. The experimental results have been discussed on the base of quantum dynamics calculations of photofragment vibrational populations carried out on available ab initio potential-energy surfaces using a four-dimensional model.This article is part of the themed issue 'Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces'.

Imaging multiphoton ionization and dissociation of rotationally warm CO via the B+Σ1 and EΠ1 electronic states.

Pathways for formation of C+ and O+ ions when applying (2 + 1) resonance enhanced multiphoton ionization (REMPI) of CO via the B1Σ+ and E1Π electronic states are characterized with the velocity map imaging technique. By employing an unskimmed pulsed valve, it was possible to obtain sharp images for a wide range of initial CO J-states. Most of the atomic ion production pathways could be assigned as one- or two-photon dissociation of a series of vibrational levels of the CO+ X2Σ+ and A2Π states. Large enhancements in dissociation of particular CO+ vibrational states in these progressions could be accurately assigned to accidental resonances of the REMPI laser with CO+ X2Σ+-B2Σ+ transitions.

Anion photoelectron spectroscopy of deprotonated ortho-, meta-, and para-methylphenol.

The anion photoelectron spectra of ortho-, meta-, and para-methylphenoxide, as well as methyl deprotonated meta-methylphenol, were measured. Using the Slow Electron Velocity Map Imaging technique, the Electron Affinities (EAs) of the o-, m-, and p-methylphenoxyl radicals were measured as follows: 2.1991±0.0014, 2.2177±0.0014, and 2.1199±0.0014 eV, respectively. The EA of m-methylenephenol was also obtained, 1.024±0.008 eV. In all four cases, the dominant vibrational progressions observed are due to several ring distortion vibrational normal modes that were activated upon photodetachment, leading to vibrational progressions spaced by ∼500 cm-1. Using the methylphenol O-H bond dissociation energies reported by King et al. and revised by Karsili et al., a thermodynamic cycle was constructed and the acidities of the methylphenol isomers were determined as follows: ΔacidH298K0=348.39±0.25, 348.82±0.25, 350.08±0.25, and 349.60±0.25 kcal/mol for cis-ortho-, trans-ortho-, m-, and p-methylphenol, respectively. The excitation energies for the ground doublet state to the lowest excited doublet state electronic transition in o-, m-, and p-methylphenoxyl were also measured as follows: 1.029±0.009, 0.962±0.002, and 1.029±0.009 eV, respectively. In the photoelectron spectra of the neutral excited states, C-O stretching modes were excited in addition to ring distortion modes. Electron autodetachment was observed in the cases of both m- and p-methylphenoxide, with the para isomer showing a lower photon energy onset for this phenomenon.

Imaging of rotational wave-function in photodissociation of rovibrationally excited HCl molecules.

We demonstrate a visualization of quantum mechanical phenomena with the velocity map imaging (VMI) technique, combining vibrationally mediated photodissociation (VMP) of a simple diatomic HCl with the VMI of its H-photofragments. Free HCl molecules were excited by a pump infrared (IR) laser pulse to particular rotational J levels of the v = 2 vibrational state, and subsequently a probe ultraviolet laser photodissociated the molecule at a fixed wavelength of 243.07 nm where also the H-fragments were ionized. The molecule was aligned by the IR excitation with respect to the IR laser polarization, and this alignment was reflected in the angular distribution of the H-photofragments. In particular, the highest degree of molecular alignment was achieved for the J=1←0 transition, which exclusively led to the population of a single rotational state with M = 0. The obtained images were analyzed for further details of the VMP dynamics, and different J states were studied as well. Additionally, we investigated the dynamic evolution of the excited states by changing the pump-probe laser pulse delay; the corresponding images reflected dephasing due to a coupling between the molecular angular momentum and nuclear spin. Our measurements confirmed previous observation using the time-of-flight technique by Sofikitis et al. [J. Chem. Phys. 127, 144307 (2007)]. We observed a partial recovery of the originally excited state after 60 ns in agreement with the previous observation.