Inverse Isotope Kinetic Effect of the Charge Transfer Reactions of Ar+ with H2O and D2O.

Hydrogen isotopic effect, as the key to revealing the origin of Earth's water, arises from the H/D mass difference and quantum dynamics at the transition state of reaction. The ion-molecule charge-exchange reaction between water (H2O/D2O) and argon ion (Ar+) proceeds spontaneously and promptly, where there is no transition-state or intermediate complex. In this energetically resonant process, we find an inverse kinetic isotope effect (KIE) leading to the higher charge transfer rate for D2O, by the velocity map imaging measurements of H2O+/D2O+ products. Using the average dipole orientation capture model, we estimate the orientation angles of C2v axis of H2O/D2O relative to the Ar+ approaching direction and attribute to the difference of stereodynamics. According to the long-distance Landau-Zener charge transfer model, this inverse KIE could be also attributed to the density-of-state difference of molecular bending motion between H2O+ and D2O+ around the resonant charge transfer.

Ion Velocity Map Imaging Study of the Reactive Collisions between Carbon Dioxide and Helium Ion.

The reactive collision between He+ and CO2 plays an important role in substance evolutions of the planetary CO2-rich atmosphere. Using a three-dimensional ion velocity map imaging technique, we investigate the low-energy ion-molecule reactions He+ + CO2 → He + CO2+/He + CO+ + O/He + CO + O+. The velocity images of the products CO+ and O+ of dissociative charge-exchange reactions are distinctly different from those of charge-exchange product CO2+. The remarkable features of stereodynamics are observed in the dissociative charge-exchange reaction and are attributed to the spatial alignment of the initially random target CO2 during the He+ approach. Branching ratios of different channels of dissociative charge exchange are further obtained with the Doppler kinematics model, indicating a high preference for the energy-resonant channel.

Evaporative cooling and reaction of carbon dioxide clusters by low-energy electron attachment.

Anionic carbonate CO3- has been found in interstellar space and the Martian atmosphere, but its production mechanism is in debate so far. To mimic the irradiation-induced reactions on icy micrograins in the Martian atmosphere and the icy shell of interstellar dust, here we report a laboratory investigation on the dissociative electron attachments to the molecular clusters of CO2. We find that anionic species (CO2)n-1O- and (CO2)n- (n = 2, 3, 4) are produced in the concerted reaction and further stabilized by the evaporative cooling after the electron attachment. We further propose a dynamics model to elucidate their competitive productions: the (CO2)n- yields survive substantially in the molecular evaporative cooling at the lower electron attachment energy, while the reactions leading to (CO2)n-1O- are favored at the higher attachment energy. This work provides new insights into physicochemical processes in CO2-rich atmospheres and interstellar space.

Electron Impact with the Liquid-Vapor Interface.

Reaction dynamics in the liquid-vapor interface is one of the crucial physical sciences but is still starving for in-depth exploration. It is challenging to selectively detect the interfacial species or the yields of chemical reaction therein, meanwhile shielding or reducing the interference from the vapor and liquid bulk. Mass spectrometry is a straightforward method but is also frustrated in such a selective detection. Using a liquid microjet in combination with a pulsed electron beam, a linear time-of-flight mass spectrometer, and a quadrupole mass filter, we recently innovated time-delayed mass spectrometry for investigations of the liquid-vapor interface. In this Account, we illustrate how this unique method succeeds in disentangling different sources, i.e., the vapor and liquid-vapor interface, of the ionic yields of the electron impacts with a liquid beam of alcohol in vacuum. These achievements are basically attributed to the application of an onion-peeling strategy in the ion detection. Concretely, the microsecond time scale of molecular volatilization can be resolved well by tuning the delay time between the nanosecond pulses of incident electron bunch and ion attractor. First, the specific orientation of the interfacial molecule, i.e., a well-known fact about the hydrophobic hydrocarbon groups pointing outside the liquid surface of alcohol, is validated again. More importantly, the dynamic features of time-delayed mass spectra, in particular, for the ionic yields from the liquid-vapor interface, are rationalized explicitly. Moreover, we demonstrate evidence of in situ molecular dimers in the liquid-vapor interface of 1-propanol. As the first example of electron-induced reaction in the liquid-vapor interface, dimethyl ether can be synthesized in the liquid methanol interface due to local interfacial acidification by high-energy electron impacts. On the contrary, the low energy electron can lead to local basicity through dissociative electron attachment (DEA). Besides the primary low-energy electrons, the low-energy secondary and inelastically scattered electrons in the higher-energy impacts of the primary electrons can also participate in the DEA process. In contrast to the gas- or solid-phase DEAs, that in the liquid-vapor interface shows distinct differences in both the types and efficiencies of anionic products. With these and efforts in the future, we develop a molecular-level understanding of how the chemical reactions happen in the liquid-vapor interface.

Dissociation dynamics of anionic carbon monoxide in dark states.

A resonant system consisting of an excess electron and a closed-shell atom or molecule, as a temporary negative ion, is usually in doublet-spin states that are analogous to bright states of photoexcitation of the neutral. However, anionic higher-spin states, noted as dark states, are scarcely accessed. Here, we report the dissociation dynamics of CO- in dark quartet resonant states that are formed by electron attachments to electronically excited CO (a3Π). Among the dissociations to O-(2P) + C(3P), O-(2P) + C(1D), and O-(2P) + C(1S), the latter two are spin-forbidden in the quartet-spin resonant states of CO-, while the first process is preferred in 4Σ- and 4Π states. The present finding sheds new light on anionic dark states.

Dissociation Dynamics of Anionic Carbon Dioxide in the Shape Resonant State 2Πu.

Dissociative electron attachments via the lowest shape resonant state 2Πu of CO2-, e- + CO2 → O- + CO, are investigated with our high-resolution anion velocity map imaging apparatus. The production efficiency curve of O- obtained in this work is consistent with those reported previously. The forward-backward asymmetric distribution superimposed on the isotopic background is observed in the time-sliced velocity image of O- yield, implying that the dissociation of CO2-(2Πu) proceeds through a combinational motion of bond stretching and bending. Thereby, the coproduct CO is proposed to be in the rovibrational states. The long-standing arguments about the dissociation dynamics of CO2-(2Πu) are settled.

Ion-Molecule Charge Exchange Reactions between Ar+ and trans-/cis-Dichloroethylene.

We report an ion velocity imaging study of the charge exchange reactions between Ar+ ion and trans-/cis-dichloroethylene in the collision energy range of 2.1-9.5 eV, and we find that the energy-resonant charge transfer plays a dominant role in the large impact-parameter reaction. The parent yields C2H2Cl2+ in the high-lying excited states are directly produced in the charge exchange reactions, while they prefer spontaneous fragmentations in photoionization. This significant difference indicates that the present charge exchange reactions are much slower than the photoelectron detachment. The structural relaxations of the target molecule are allowed in multiple dimensions of freedom during the charge transfer, which should be frequently observed for the charge exchange reactions with large molecules.

Superposition-state N2 + produced in the intermolecular charge transfer from low-energy Ar+ to N2.

Molecular electronic or vibrational states can be superimposed temporarily in an extremely short laser pulse, and the superposition-state transients formed therein receive much attention, owing to the extensive interest in molecular fundamentals and the potential applications in quantum information processing. Using the crossed-beam ion velocity map imaging technique, we disentangle two distinctly different pathways leading to the forward-scattered N2 + yields in the large impact-parameter charge transfer from low-energy Ar+ to N2. Besides the ground-state (X2Σg +) N2 + produced in the energy-resonant charge transfer, a few slower N2 + ions are proposed to be in the superpositions of the X2Σg +-A2Πu and A2Πu-B2Σu + states on the basis of the accidental degeneracy or energetic closeness of the vibrational states around the X2Σg +-A2Πu and A2Πu-B2Σu + crossings in the non-Franck-Condon region. This finding potentially shows a brand-new way to prepare the superposition-state molecular ion.

Ion momentum imaging study of the ion-molecule reaction Ar+ + O2→ Ar + O2.

Charge exchange reactions between Ar+(2P) and O2 (X3Σ) are investigated in the collision energy range of 3.40-9.24 eV within the center-of-mass coordinate, by using the ion momentum imaging technique. The internal energy of the product O2+ is enhanced gradually with the increase of collision energy, and the forward-scattered O2+ ions are distributed in the broader range of scattering angle at higher collision energies. At the low collision energy of 3.40 eV, the resonant charge transfer, similar to a photon ionization process, leads to the Franck-Condon-like vibrational state population of O2+ at the a4Πu state. At the higher collision energies, besides a4Πu and the high-lying states that are visible in the photoionization process, the O2+ products could be populated at some electronically bound states in the non-Franck-Condon region. The present observations indicate again the strong collision-energy dependences of the charge exchange reactions, but distinctly different from our previous findings for Ar+ + NO → Ar + NO+.

Collision-Energy Dependence of the Ion-Molecule Charge-Exchange Reaction Ar+ + CO → Ar + CO.

Ion-molecule charge-exchange reactions Ar+ + CO → Ar + CO+ at the center-of-mass collision energies of 4.40, 6.40, and 8.39 eV are investigated using ion velocity map imaging technique. Although multiple electronically excited states of CO+ are accessed, the population of CO+ at the A2Π state is predominant in the present collision-energy range. In contrast to our previous study for NO, but similar to the case of O2, the forward-scattered CO+ yields show a broader angular distribution at the higher collision energy. Typically, the Franck-Condon-region charge transfer, energy resonant charge transfer, and intimate collision are three different mechanisms in which the intimate collision experiences an intermediate complex, and this mechanism usually plays an essential role in the thermal-energy reactions. However, the present observations indicate that this mechanism, concerning the intermediate (Ar-CO)+, is still of utmost importance in a relatively high collision-energy range.

Synchronous and asynchronous dynamics of the concerted three-body dissociations of temporary negative ion CH2F2.

Molecular concerted three-body dissociation is a fast process, but still can be classified into synchronous and asynchronous pathways. It is challenging in experiments to evaluate different contributions of the aforementioned mechanisms. Here, we report an experimental identification of the synchronous and asynchronous concerted three-body dissociations of temporary negative ion CH2F2 - at an electron-molecule resonant state formed by electron attachment. The synchronous-asynchronous branching ratios indicate that the asynchronous process is predominant although the synchronous contribution is slightly enhanced with the increase in the electron attachment energy. This study provides two intuitive pictures of the concerted three-body dissociations, in particular for the nonequivalent-bond cleavages of a polyatomic molecule.

Vibrationally resolved photoemissions of N2 (C3Πu → B3Πg) and CO (b3Σ+ → a3Π) by low-energy electron impacts.

Vibrationally resolved photoemission spectra of the electronic-state transitions C3Πu → B3Πg of N2 and b3Σ+ → a3Π of CO following low-energy electron impacts are measured with a crossed-beam experimental arrangement. The absolute cross sections of C3Πu (ν') → B3Πg (ν″) of N2 are presented for the vibrational state-to-state transitions (ν',ν″) = (0,0), (0,1), (1,0), (1,2), and (2,1). The excitation cross sections of the metastable state C3Πu of N2 show the maxima at the electron-impact energies 14.10 (ν' = 0) eV and 14.50 (ν' = 1) eV, which are potentially related to the core-excited vibrational Feshbach resonant state 2Σu + of N2 - formed by electron attachment. The absolute cross sections of b3Σ+ (ν' = 0) → a3Π (ν″ = 0, 1, 2, 3, 4) of CO are given by the calibrations with those of N2 measured in this work. Besides the maximum excitation cross section 5.85 × 10-18 cm2 at 10.74 eV of the CO b3Σ+ (ν' = 0) state, some fine structures on the excitation function profile are attributed to different shapes and Feshbach resonant states of CO- formed by electron attachment, while the others arise from the direct electron-impact excitation. Some discrepancies, particularly for N2, between the present data and the results available in the literature studies arise from different experimental techniques and data-processing procedures. Furthermore, contributions of physical processes such as wave-packet evolution and non-Franck-Condon dynamics are highlighted here.

Ion Momentum Imaging Study of the Ion-Molecule Charge Exchange Reaction of Ar+ + CO2.

Three-dimensional ion momentum imaging is developed in a combination of ion velocity map imaging technique and delay-line anode ion detection, and it is applied for the ion-molecule charge exchange reaction between Ar+ and CO2. In a center-of-mass collision energy range of 7.23-15.96 eV, CO2+ products are primarily populated at the ground state X2Πg and the single-electron excited states A2Πu, B2Σu+, and C2Σg+; the multielectron excited states of CO2+ are also found at the higher collision energies. The production efficiency profiles of CO2+ are distinctly different from the photoionization electron spectrum of CO2, implying that the charge transfer from Ar+ would be not fast as expected. The strong electron correlations in the short-lived intermediate (Ar-CO2)+ should be responsible for the CO2+ yields at the multielectron excited states.

Dissociative Electron Attachment to Molecular Acetonitrile.

Low-energy dissociative electron attachment to molecular acetonitrile (CH3CN) is investigated by recording the efficiency curves of CH2CN-, CHCN-, and CN- products, but the present curves are distinctly different from those in the previous reports. Now it is recommended that the reaction thresholds of e- + CH3CN → H2 + CHCN- and CH3 + CN- are respectively about 1.51 and 1.52 eV, and four shape-resonant states, one 2Π and three 2∑, of CH3CN- are involved in the fragmentations at the low and high electron-attachment energies. By using a high-resolution anion velocity-map imaging spectrometer, we obtain the CN- momentum images at the electron attachment energies of 7.10, 7.60, and 8.10 eV and interpret them with four pathways concerning two- and three-body dissociations.

Collision-Energy Dependence of the Ion-Molecule Charge Exchange Reaction Ar+ + NO.

High-lying quantum states of molecule are apt to be populated by translational-to-internal energy transfer in the collisions with atom, which usually becomes more significant with the increase of collision energy. However, in the charge exchange reaction Ar+ + NO → Ar + NO+, the products NO+ prefer a dominant population at the lowest triplet state a3Σ+, in particular, in the higher energy collisions; the higher states b3Π and w3Δ of NO+ are accessed only at the lower collision energies. Such a striking collision-energy dependence is attributed to two distinctly different processes: the former is controlled with an energetically resonant charge-transfer mechanism; while the latter experiences an intermediate complex (Ar-NO)+ which permits a more efficient translational-to-internal energy transfer.

Ab initio molecular dynamics simulation study of dissociative electron attachment to dialanine conformers.

Dissociative electron attachment (DEA) processes of six low-lying conformers (1-6) of dialanine in the gas phase are investigated by using ab initio molecular dynamics simulations. The incoming electron is captured and primarily occupies the virtual molecular orbital π*, which is followed by the different dissociation processes. The electron attachments to conformers 1 and 2 having the stronger N-H···N and O-H···O intramolecular hydrogen bonds do not lead to fragmentations, but two different backbone bonds are broken in the DEAs to conformers 3 (or 4) and 6, respectively. It is interesting that the hydrogen abstraction of -NH from the terminal methyl group -CH3 is found in the roaming dissociation of the temporary anion of conformer 3. The present simulations enable us to have more insights into the peptide backbone bond breaks in the DEA process and demonstrate a promising way toward understanding of the radiation damages of complicated biological system.

Note: Coherent resonances observed in the dissociative electron attachment to carbon monoxide.

Succeeding our previous finding about coherent interference of the resonant states of CO(-) formed by the low-energy electron attachment [Tian et al. Phys. Rev. A 88, 012708 (2013)], here we provide further evidence of the coherent interference. The completely backward distributions of the O(-) fragment of the temporary CO(-) are observed with anion velocity map imaging technique in an electron energy range of 11.3-12.6 eV and explained as the results of the coherent interferences of three resonant states. Furthermore, the state configuration of the interference is changed with the increase of electron attachment energy.

Dissociative electron attachments to ethanol and acetaldehyde: A combined experimental and simulation study.

Dissociation dynamics of the temporary negative ions of ethanol and acetaldehyde formed by the low-energy electron attachments is investigated by using the anion velocity map imaging technique and ab initio molecular dynamics simulations. The momentum images of the dominant fragments O(-)/OH(-) and CH3 (-) are recorded, indicating the low kinetic energies of O(-)/OH(-) for ethanol while the low and high kinetic energy distributions of O(-) ions for acetaldehyde. The CH3 (-) image for acetaldehyde also shows the low kinetic energy. With help of the dynamics simulations, the fragmentation processes are qualitatively clarified. A new cascade dissociation pathway to produce the slow O(-) ion via the dehydrogenated intermediate, CH3CHO(-) (acetaldehyde anion), is proposed for the dissociative electron attachment to ethanol. After the electron attachment to acetaldehyde molecule, the slow CH3 (-) is produced quickly in the two-body dissociation with the internal energy redistributions in different aspects before bond cleavages.

Different aggregation dynamics of benzene-water mixtures.

All-atom molecular dynamics simulations for benzene-water mixtures are performed, aiming to explore the relationship between the microscopic structures and the thermodynamic properties, in particular, the transformation dynamics from the mutually soluble state to the phase-separated state. We find that the molecular aggregation of benzene in the water-rich mixture is distinctly different from that of water in the benzene-rich mixture. This aggregation difference is attributed to the different intermolecular interactions: the clustering of benzene molecules in the water-rich mixture is primarily driven by weak short-distance π-π interactions; while the formation of water clusters in the benzene-rich solution is triggered by long-range dipole-dipole electrostatic interactions. Moreover, the molecular aggregations show double-scaled features: firstly assembling in a quasi-plane at a low concentration, then bulking in three dimensions with an increase in concentration.

Direct Observation of Anionic Yields from the Liquid-Vapor Interface by Electron Irradiation.

Dissociative electron attachment (DEA) is widely believed to play a high-profile role in ionizing radiation damages of bioorganic molecules, and its fundamentals are mainly learned from the gas-phase studies. However, the DEA process in aqueous solution is still in debate. Here we provide experimental evidence about the DEA processes of liquid methanol by using electron-impact-time-delayed mass spectrometry. In contrast to the gas- and solid-phase DEAs, methoxide ion CH3O- is the predominant product from the liquid interface. Furthermore, this anion can be produced with both the primary low-energy electrons and the inelastically scattered and secondary low-energy electrons. On the contrary, the primary low-energy electrons in the liquid bulk are more likely to be solvated, rather than directly participating in the DEA process. Our study provides new insights into radiation chemistry, particularly of bioorganic relevance.