Velocity Map Imaging Research Articles

Imaging Rovibrational Excitation of Scattered YO Molecules in Inelastic Collisions with Kr and Ne.

Energy transfer between atoms and molecules is fundamental to many physical and chemical processes, and understanding the mechanisms and outcomes of energy transfer is crucial for various applications in physics and chemistry. Here, the rovibrational excitation of YO(X 2Σ+) molecules with the collision of Kr and Ne has been studied in the laser-ablation crossed beam and time-sliced ion velocity map imaging setup in combination with the resonance enhanced multiphoton ionization scheme. Significant changes in the angular distribution for different rovibrational excitations of YO molecules are observed with the collision of Kr. The sharp forward distribution for low rovibrational excitation of YO(v' = 0, 1) molecules suggest that the weak attractive potential between Kr and YO is dominant at large impact parameters. Comparatively, the strong sideway distribution for highly rovibrationally excited YO(v' = 1, 2, 3, and 5) is due to rainbow scattering from the stronger attractive potential of Kr···YO at relatively small impact parameters. The more isotropic angular distribution in the highly rovibrationally excited YO(v' = 11) indicates the formation of a short-lived complex. A change in the angular distribution of scattered YO with different rovibrational excitations was also observed in the collisions of Ne. For YO as a heteronuclear diatomic molecule, collisions of the Y- and the O-end of YO with rare gases would affect the contribution of inelastic processes at different impact parameters.

Imaging the Ion-Molecule Reaction Dynamics of O- + CD4.

While neutral reactions involved in methane oxidation have been intensively studied, much less information is known about the reaction dynamics of the oxygen radical anion with methane. Here, we study the scattering dynamics of this anion-molecule reaction using crossed-beam velocity map imaging with deuterated methane. Differential scattering cross sections for the deuterium abstraction channel have been determined at relative collision energies between 0.2 and 1.5 eV and ab initio calculations of the important stationary points along the reaction pathway have been performed. At lower collision energies, direct backscattering and indirect complex-mediated reaction dynamics are observed, whereas at higher energies, sideways deuterium stripping dominates the reaction. Above 0.7 eV collision energy, a suppressed cross section is observed at low product ion velocities, which is likely caused by the endoergic pathway of combined deuteron/deuterium transfer, forming heavy water. The measured product internal energy is attributed mainly to the low-lying deformation and out-of-plane bending vibrations of the methyl radical product. The results are compared with a previous crossed-beam result for the reaction of oxygen anions with nondeuterated ̧methane and with the related neutral-neutral reactions, showing similar dynamics and qualitative agreement.

Wavelength-Dependent Dynamics of the O(1D2) Channel in the 1Δu State Photodissociation of CO2.

The wavelength-dependent dynamics of the O(1D2) channel, formed by photoexcitation of CO2 to the 1Δu state at 143.53-153.03 nm, is investigated by using the time-sliced velocity-mapped ion imaging method. The measured ionic peaks of the O(1D2) images are analyzed to determine the total kinetic energy release (TKER) spectra and image anisotropy parameters. The structures observed in the TKER spectra can be directly assigned to the ro-vibrational state distributions of the counter CO photofragments. Compared to those observed at 157 and 155 nm, the highly rotationally excited CO photofragments still obviously appear in v = 0 and 1, but the fraction of rotational excitations is significantly reduced. Conversely, the CO photofragments exhibit substantially higher vibrational excitations, implying that the nearly linear 21A' state also contributes to dissociation in addition to the bend configuration. The image anisotropy parameters display an extremely slow decreasing trend with an increase of the CO ro-vibrational state besides those for the highest ro-vibrationally excited CO photofragments. Nevertheless, the nonaxial recoil effect, suggested in previous photodissociation studies of CO2 and other triatomic molecular systems, is still appropriate to explain the observations of internal energy dependences of image anisotropy parameters.

Early microvascular coronary endothelial dysfunction precedes pembrolizumab-induced cardiotoxicity. Preventive role of high dose of atorvastatin.

Immune checkpoint inhibitors (ICIs) exhibit remarkable antitumor activity and immune-related cardiotoxicity of unknown pathomechanism. The aim of the study was to investigate the ICI class-dependent cardiotoxicity in vitro and pembrolizumab's (Pem's) cardiotoxicity in vivo, seeking for translational prevention means. Cytotoxicity was investigated in primary cardiomyocytes and splenocytes, incubated with ipilimumab, Pem and avelumab. Pem's cross-reactivity was assessed by circular dichroism (CD) on biotechnologically produced human and murine PD-1 and in silico. C57BL6/J male mice received IgG4 or Pem for 2 and 5weeks. Echocardiography, histology, and molecular analyses were performed. Coronary blood flow velocity mapping and cardiac magnetic resonance imaging were conducted at 2weeks. Human EA.hy926 endothelial cells were incubated with Pem-conditioned media from human mononuclear cells, in presence and absence of statins and viability and molecular signaling were assessed. Atorvastatin (20mg/kg, daily) was administered in vivo, as prophylaxis. Only Pem exerted immune-related cytotoxicity in vitro. Pem's cross-reactivity with the murine PD-1 was confirmed by CD and docking. In vivo, Pem initiated coronary endothelial and diastolic dysfunction at 2weeks and systolic dysfunction at 5weeks. At 2weeks, Pem induced ICAM-1 and iNOS expression and intracardiac leukocyte infiltration. At 5weeks, Pem exacerbated endothelial activation and triggered cardiac inflammation. Pem led to immune-related cytotoxicity in EA.hy926 cells, which was prevented by atorvastatin. Atorvastatin mitigated functional deficits, by inhibiting endothelial dysfunction in vivo. We established for the first time an in vivo model of Pem-induced cardiotoxicity. Coronary endothelial dysfunction precedes Pem-induced cardiotoxicity, whereas atorvastatin emerges as a novel prophylactic therapy.

Observation of Renner-Teller and predissociation coupled vibronic intensity borrowing in dissociative electron attachment to OCS.

We use a time-of-flight-based velocity map imaging method to look into the dissociative electron attachment to a linear OCS molecule at electron beam energies ranging from 4.5 to 8.5eV. The conical time-gated wedge slice imaging method is utilized to extract fragments' slice images, kinetic energy (KE), and angular distributions, which provide a complete kinematic understanding of this experiment on the dissociative electron attachment process. We observe that the formation of S- is relatively higher than the O- product. Three distinct dissociative KE bands of S-/OCS have been observed for the 5.0 and 6.5eV resonance positions. We notice a prominent rovibrationally coupled bimodality for each KE band in the variation of the most probable KE values. When the electron energy is changed from 5.5 to 6.0eV, we observed vibronic intensity borrowing in the highest momentum band of S- via the Σ → Π symmetric dipole-forbidden transitions within the 1.5eV energy gap. Multiple peaks in the angular distributions of S- and their modeling indicate the presence of Renner-Teller vibronic splitting. Using Q-Chem's implemented complex absorbing potential-equation of motion-electron affinity coupled cluster singles and doubles aug-cc-pVDZ+4s3p level of multireference-based electronic structure theory, we confirm the presence of OCS temporary negative ion bending vibrations and Renner-Teller vibronic splittings for the Π symmetric states. Additionally, we notice the presence of a non-radiative predissociation continuum (bringing down the rotational spectrum) and speed-dependent angular anisotropy in the S- fragmentation. Our findings at the resonance of OCS at 6.5eV closely align with the prediction of vibronic intensity borrowing by Orlandi and Siebrand [J. Chem. Phys. 58, 4513 (1973)].

Probing the geometric and electronic structures of the transition metal oxides RhOn–1/0 (n = 1–4) clusters

The photoelectron spectroscopies of RhOn– (n = 1–2) were obtained via using the photoelectron velocity-map imaging (PE-VMI) approach. The experimental values of the adiabatic detachment energy (ADE) and vertical detachment energy (VDE) for RhO– were reported to be 1.58 ± 0.02 eV. The experimental AED and VDE values of RhO2– were reported to be 2.70 ± 0.02 eV and 2.79 ± 0.02 eV, respectively. The vibrational frequencies of RhO– and RhO2– measured from photoelectron spectra (PES) were 817(76) cm−1 and 932(55) cm−1, respectively. Based on the density functional theory (DFT), the RhOn–1/0 (n = 1–4) clusters were investigated. The optimized configurations of corresponding ground states and low-lying clusters were discovered. Meanwhile, the simulated photoelectron spectroscopy (PES) of RhOn– (n = 1–4) and the theoretical ADE and VDE values of RhOn– (n = 1–4) clusters were unveiled to assist future experimental studies of Rhodium oxide clusters. Moreover, the associated molecular orbitals (MOs), natural population analysis (NPA) and bond order analysis have been utilized to investigate the chemical bonding in these groups.

Ta+ and Nb+ + CO2: intersystem crossing in ion-molecule reactions.

The reactions of Ta+ and Nb+ with CO2 proceed only by a highly efficient oxygen atom transfer reaction to the respective oxide at room temperature in the gas phase. Although the product spin states are not determined, thermochemistry dictates that they must be different from ground state quintet Ta+ and Nb+, implying that intersystem crossing (ISC) has occurred. Recent reactive scattering experiments found dominant indirect dynamics for the reaction with Ta+ hinting at a bottleneck along the reaction path. The question on the nature of the bottleneck, whether it involves a crossing point or a transition state, could not be finally answered because theory located both close to each other. Here, we aim at shedding further light onto the impact of intersystem crossing on the reaction dynamics and ultimately the reactivity of transition metal ion reactions in the gas phase. We employ a combination of thermal kinetics for Ta+ and Nb+ with CO2 using a selected-ion flow tube (SIFT) apparatus and differential scattering cross sections for Nb+ + CO2 from crossed-beam velocity map imaging. The reaction with niobium again shows dominant indirect dynamics and in general very similar dynamics compared to Ta+ + CO2. At thermal energies, both reactions show sub-collisional rate constants with small negative temperature dependencies. Experiments are complemented by high level quantum chemical calculations of the minimum energy pathway. Statistical modelling well-reproduces the experimental thermal rate constants, and suggests that the Nb+ reaction is rate-limited by the intersystem crossing at thermal energies.

Theoretical Investigation of Bimolecular Carbon Chain Growth Reactions in the Interstellar Media.

Detection of molecular anions in interstellar media implies that negatively charged species play a prominent role in astrochemical reactions. Among the observed species, carbon chain anions are important, as they can be precursors for the production of complex organic molecules. These anions can form either via direct electron attachment to the corresponding neutral species or through chain growth reactions of smaller anions, resulting in longer chains. In a recent study, crossed beam experiments coupled with velocity map imaging techniques were used to investigate the carbon chain growth reaction C2n- + C2H2 → C2n+2Hm- + H2-m (n = 1-3, m = 0,1). Products and branching ratios were established from experimental data. In the present work, electronic structure calculations and on-the-fly direct dynamics simulations were used to study these reactions. Energy profiles were investigated by using different density functional methods. Direct trajectory simulations were performed at the experimental collision energies using the B3LYP/6-31+G* level of theory. Trajectory analysis showed a variety of reaction pathways, and detailed atomic-level reaction mechanisms are presented.

Ion core switching during photodissociation dynamics via the Rydberg states of XeAr

The heterodimer, XeAr, is a classic example of a weakly bound van der Waals molecule, which has a variety of accessible bound excited states that exhibit complex interactions. In this study, XeAr has been investigated in the 77,500–81,500 cm−1 region using a combination of Resonance Enhanced Multi-Photon Ionization (REMPI) spectroscopy and Velocity Map Imaging (VMI). By monitoring REMPI and photodissociative product channels across the spectrum, several novel excited states, product channels, excited state symmetries and lifetimes, as well as highly localized perturbations were observed and characterized, including the first VMI study of Ar* dissociating from XeAr Rydberg states accessed by two-photon excitation. In this work we have analyzed 38 vibronic bands representing nine different electronic transitions, and we provide new assignments for two 0+←0+ electronic transitions dissociating to the Xe* 5d [3/2]02 (ca. 80,323 cm−1) and Xe* 5d [7/2]03 (ca. 80,970 cm−1) limits. Several new predissociation product channels were identified at the two- and three-photon levels, including production of Xe* 5p[5/2]3, Xe* 6s'[1/2]o1, Xe* 6p[1/2]1, Ar* 4p[1/2]0, Ar* 4p'[3/2]1, Ar* 4p'[1/2]1, and Ar* 4p[5/2]3. Using the multidimensional analysis offered by VMI, we explore interesting photophysics whereby a resonant state that is reached after absorbing two photons can predissociate to yield Xe*, but which also can absorb a 3rd photon, yielding super-excited Ar*Xe that predissociates to Ar* limits. The ground state dissociation energy for XeAr was determined to be D0 = 114.4 ± 2.7 cm−1, in excellent agreement with previous measurements.

Interfacial Nanostructure and Hydrogen Bond Networks of Choline Chloride and Glycerol Mixtures Probed with X-ray and Vibrational Spectroscopies.

The molecular distribution at the liquid-vapor interface and evolution of the hydrogen bond interactions in mixtures of glycerol and choline chloride are investigated using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Nanoscale depth profiles of supersaturated deep eutectic solvent (DES) mixtures up to ∼2 nm measured by ambient-pressure XPS show the enhancement of choline cation (Ch+) concentration by a factor of 2 at the liquid-vapor interface compared to the bulk. In addition, Raman spectral analysis of a wide range of DES mixtures reveals the conversion of gauche-conformer Ch+ into the anti-conformer in relatively lower ChCl concentrations. Finally, the depletion of Ch+ from the interface (probing depth = 0.4 nm) is demonstrated by aerosol-based velocity map imaging XPS measurements of glyceline and water mixtures. The nanostructure of liquid-vapor interfaces and structural rearrangement by hydration can provide critical insight into the molecular origin of the deep eutectic behavior and gas-capturing application of DESs.

Spatial and Momentum Mapping Modes for Velocity Map Imaging Spectrometer

Spatial and Momentum Mapping Modes for Velocity Map Imaging Spectrometer

N-H and N-C Bond Dissociation Pathways in Ultraviolet Photodissociation of Dimethylamine.

We investigated the interlinked N-H and N-C photochemistry of primary and secondary amines via the state-resolved detection of vibrationally excited CH3 product and H atom product by 200-235 nm dimethylamine photodissociation using resonance-enhanced multiphoton ionization (REMPI) and velocity map imaging (VMI) techniques. The out-of-plane bending (ν2) vibrationally excited CH3 showed a bimodal translational energy distribution that became unimodal with a near-zero product yield at longer photolysis wavelengths (λphotolysis). In contrast, a unimodal distribution was observed for the C-H stretching (νCH) vibrationally excited CH3 products with an almost constant product yield in the examined λphotolysis region. We ascribed the state-specific energy releases of the CH3 products to two reaction pathways based on calculations of the potential energy surface (PES): the direct N-CH3 dissociation pathway and the indirect N-CH3 dissociation pathway via the N-H bond conical intersection. Meanwhile, the H atom product showed a bimodal energy distribution similar to the ammonia photodissociation model, with an excited-state counterproduct channel that became accessible at a shorter λphotolysis. These results suggest that the N-H and N-C bond dissociations are connected, and these dissociations cause different photochemistry between primary/secondary amines and tertiary amines.

Spectroscopic Characterization of Thermal Methane Activation by Lewis-Acid-Base Pair in a Gas-Phase Metal Nitride Anion Ta2N3.

Activation and transformation of methane is one of the "holy grails" in catalysis. Understanding the nature of active sites and mechanistic details via spectroscopic characterization of the reactive sites and key intermediates is of great challenge but crucial for the development of novel strategies for methane transformation. Herein, by employing photoelectron velocity-map imaging (PEVMI) spectroscopy in conjunction with quantum chemistry calculations, the Lewis acid-base pair (LABP) of [Taδ+-Nδ-] unit in Ta2N3 - acting as an active center to accomplish the heterolytic cleavage of C-H bond in CH4 has been confirmed by direct characterization of the reactant ion Ta2N3 - and the CH4-adduct intermediate Ta2N3CH4 -. Two active vibrational modes for the reactant (Ta2N3 -) and four active vibrational modes for the intermediate (Ta2N3CH4 -) were observed from the vibrationally resolved PEVMI spectra, which unequivocally determined the structure of Ta2N3 - and Ta2N3CH4 -. Upon heating, the LABP intermediate (Ta2N3CH4 -) containing the NH and Ta-CH3 unit can undergo the processes of C-N coupling and dehydrogenation to form the product with an adsorbed HCN molecule.

Vacuum Ultraviolet Single Photon Ionization and Decomposition of 2-Aminopropionitrile in Astrophysical Objects

The molecule 2-aminopropionitrile (2-APN; H3CCH(NH2)CN) is a chiral precursor of the amino acid alanine and could play an important role in the emergence of the homochirality of life. To date, 2-APN has not yet been detected in the interstellar medium. To address the question of its absence, possibly through destruction by vacuum ultraviolet radiation in astrophysical media, we studied its photoionization and dissociative photoionization in the 9–14 eV (89–137 nm) energy range using photoelectron–photoion coincidence spectroscopy with velocity map imaging. Density functional theory calculations were performed at the PBE0/aug-cc-pVTZ level to determine structures for the various cationic fragments identified experimentally. Their appearance energies were calculated. The adiabatic ionization energy (AIE) of 2-APN is AIE = (9.85±0.01) eV and the appearance energy (AE) of the major fragment ion N≡CC(H)NH2 + is AE = (10.57±0.01) eV. Both ions, 2-APN+ and N≡CC(H)NH2 +, make up more than 90% of the formed ions. N≡CC(H)NH2 +, an isomer of the protonated HCN dimer, is also formed with high yield in the dissociative photoionization of aminoacetonitrile (NH2CH2CN), another α-aminonitrile observed in Sgr B2(N). Photoion yield spectra have been calibrated to absolute ionization cross sections. From these, we derive photoionization rates in several typical radiation fields relevant to different astrophysical objects. The rates show that, under almost all explored radiation fields, dissociative ionization is the dominant pathway for photoionization, leading to the loss of a methyl group (CH3) to form the planar thermodynamically stable amino cyano methylidynium ion N≡CC(H)NH2 +. The observed absence of 2-APN in Sgr B2 could thus be due to dominant dissociative photoionization.

Structural and Chemical Bonding Properties of AuS2H0/-: A Photoelectron Velocity-Map Imaging Spectroscopic and Theoretical Study.

Mass-selected photoelectron velocity-map imaging spectroscopy was employed to investigate the geometrical and electronic properties of AuS2H-/0. The comprehensive comparison between the experiment and theoretical calculations establishes that the ground-state AuS2H- anion has a mixed-ligand structure [SAuSH]- with an unsymmetrical S-Au-S unit. Further chemical bonding analyses on AuS2H and comparison with the isoelectronic AuS2- suggest that the unique S-Au-S unit in these species features two three-center, three-electron π-bonding, and one three-center, two-electron σ-bonding. The isoelectronic replacement of the extra electron in AuS2- by the H atom can lead to σ bonding evolution from the electron-sharing bond to the dative bond. These findings are conducive to the fundamental understanding of the intrinsic stability of thiolate-protected gold nanoclusters and their delicate ligand design to achieve desirable properties.

Imaging the state-to-state charge-transfer dynamics between the spin-orbit excited Ar+(2P1/2) ion and N2

Ar++N2 → Ar+N2+ has served as a paradigm for charge-transfer dynamics studies during the last several decades. Despite significant experimental and theoretical efforts on this model system, state-resolved experimental investigations on the microscopic charge-transfer mechanism between the spin-orbit excited Ar+(2P1/2) ion and N2 have been rare. Here, we measure the first quantum state-to-state differential cross sections for Ar++N2 → Ar+N2+ with the Ar+ ion prepared exclusively in the spin-orbit excited state 2P1/2 on a crossed-beam setup with three-dimensional velocity-map imaging. Trajectory surface-hopping calculations qualitatively reproduce the vibrationally dependent rotational and angular distributions of the N2+ product. Both the scattering images and theoretical calculations show that the charge-transfer dynamics of the spin-orbit excited Ar+(2P1/2) ion differs significantly from that of the spin-orbit ground Ar+(2P3/2) when colliding with N2. Such state-to-state information makes quantitative understanding of this benchmark charge-transfer reaction within reach.

Contrasting Dynamics in Isoelectronic Anions Formed by Electron Attachment.

Cyanogen NCCN and cyanoacetylene HCCCN are isoelectronic molecules, and as such, they have many similar properties. We focus on the bond cleavage in these induced by the dissociative electron attachment. In both molecules, resonant electron attachment produces CN- with very similar energy dependence. We investigate the very different dissociation dynamics, in each of the two molecules, revealed by velocity map imaging of this common fragment. Different dynamics are manifested both in the excess energy partitioning and in the angular distributions of fragments. Based on the comparison with electron energy loss spectra, which provide information about possible parent states of the resonances (both optically allowed and forbidden excited states of the neutral target), we ascribe the observed effect to the distortion of the nuclear frame during the formation of core-excited resonance in cyanoacetylene. The proposed mechanism also explains a puzzling difference in the magnitude of the CN- cross section in the two molecules which has been so far unexplained.

Bond dissociation energy of N2 measured by state-to-state resolved threshold fragment yield spectra.

The precise determination of the bond dissociation energy of N2 is crucial for thermochemistry database and theoretical calculations. However, there has been ongoing debate regarding its exact value. In this study, we used the velocity map imaging method combined with an extreme ultraviolet laser to measure the threshold fragment yield (TFY) spectra of N2 in the N(2D) + N(2D) photodissociation channels. By integrating the signals within a small circular area on the fragment velocity map images, we were able to obtain TFY spectra at nine different dissociation thresholds. These spectra are rotational state-resolved for the N2(J″) molecules and spin-orbit state-resolved for the dissociation channels involving N(2D) fragments. By employing the Wigner threshold law to simulate the TFY spectra and conducting statistical analysis on the comprehensive dataset, we determined the N2 bond dissociation energy to be 78 691.09 ± 0.15cm-1. This work now places N2 among the few diatomic molecules with bond dissociation energies measured at sub-wavenumber precision.

Photodissociation of CO2 via the 1Πg state: Wavelength-dependent imaging studies of O(1D2) photoproducts.

Photodissociation of CO2 via the 1Πg state is investigated using a time-sliced velocity-mapped ion imaging apparatus combined with a tunable vacuum ultraviolet photolysis source. The main O(1D2) + CO(X1Σ+) channel is directly observed from the measured images of O(1D2) photoproducts at 129.08-134.76nm. The total kinetic energy release spectra determined based on these images show that the energetic thresholds for the O(1D2) + CO(X1Σ+) photoproducts correspond to the thermochemical thresholds for the photodissociation of CO2(v2 = 0) and CO2(v2 = 1). One significant difference among the CO(X1Σ+, v) vibrational distributions for the predominant CO2(v2 = 0) dissociation is that the population of CO(v = 0) becomes favorable at 130.23-133.45nm compared to the Boltzmann-like component (v > 0) that always exists at 129.08-134.76nm. The wavelength dependences of the overall β are found to follow the variation trend of the CO(v = 0) abnormal intensity. The vibrational state-specific β values present a roughly decreasing trend with an increase in v, whereas β(v = 0) appears to be significantly larger than β(v = 1) at 130.23-133.45nm compared to 134.76 and 129.08nm. The non-statistical CO(v = 0) with larger β values at 130.23-133.45nm implies that an additional pathway may open through the conical intersection coupling to the dissociative 21A'  state, except for the ever-existing pathway that yields the Boltzmann-like component. In contrast, at 129.08nm, the restoration of the statistical equilibrium in the CO(X1Σ+, v) vibrational distribution may be caused by the emergence of novel dissociation pathways arising from the participation of the 31A″ state.

Dissociative electron attachment to carbon tetrachloride probed by velocity map imaging.

Bond-breaking in CCl4via dissociative electron attachment (DEA) has been studied using a velocity map imaging (VMI) spectrometer. A number of effects related to the dissociation dynamics have been revealed. The near-zero eV s-wave electron attachment, which leads to the production of Cl- anions, is accompanied by a very efficient intramolecular vibrational redistribution. This is manifested by a small fraction of the excess energy being released in the form of the fragments' translation energy. A similar effect is observed for higher-lying electronic resonances with one exception: the resonance centered around 6.2 eV leads to the production of fast Cl2- fragments and their angular distribution is forward peaking. This behavior could not be explained with a single-electronic-state model in the axial recoil approximation and is most probably caused by bending dynamics initiated by a Jahn-Teller distortion of the transient anion. The CCl2- fragment has a reverse backward-peaking angular distribution, suggesting the presence of a long-distance electron hopping mechanism between the fragments.