High-resolution anionic velocity map imaging apparatus for dissociative electron attachment dynamics study.

Dissociative electron attachment (DEA) of a molecular target XY, e- + XY → XY- → X + Y-, is an important process in plasma, atmosphere, interstellar space, and ionizing radiation. DEA dynamics, i.e.

Inverse Isotope Kinetic Effect of the Charge Transfer Reactions of Ar with HO and DO.

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 (HO/DO) 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 DO, by the velocity map imaging measurements of HO/DO products.

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.

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

The reactive collision between He and CO plays an important role in substance evolutions of the planetary CO-rich atmosphere. Using a three-dimensional ion velocity map imaging technique, we investigate the low-energy ion-molecule reactions He + CO → He + CO/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 CO.

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.

Direct production of molecular oxygen from carbon dioxide and helium ion collisions.

The prebiotic mechanism to produce molecular oxygen (O) in carbon dioxide (CO)-rich planetary atmospheres is of great importance in understanding astrochemical reactions and is potentially relevant to the origin of life on Earth. Here, we demonstrate that, aside from the direct productions of O by photodissociation and dissociative electron attachment, the low-energy ion-molecule reaction between cationic helium in solar winds and molecular CO is a noticeable mechanism. Branching ratios of the reaction channels are determined, and their absolute cross-sections are estimated accordingly.

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).

Dissociation Dynamics of Anionic Nitrogen Dioxide in the Low-Lying Resonant States.

Experimental studies of the dynamics near the molecular dissociation threshold are frequently frustrated, due to the small cross sections and the demand for identification of close-lying electronic states or nuclear motions with multiple degrees of freedom. Using the high-resolution anion velocity map imaging technique, here we report a dynamics study of the dissociations of anionic nitrogen dioxide in low-lying resonant states formed by electron attachment [e + NO (XA) → NO → NO (XΠ) + O (P)]. The long-term puzzling issues about the near-threshold dissociations of NO are settled.

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.

Electron-Induced Synthesis of Dimethyl Ether in the Liquid-Vapor Interface of Methanol.

Ether synthesis from alcohol is known to be acid-catalyzed. Such a process could happen in the acidified liquid of alcohol, but hitherto lacking the experimental evidence. Here we demonstrate that dimethyl ether is spontaneously synthesized in the liquid-vapor interface of pure methanol after ionizing radiation with electrons.

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

Dissociative electron attachments via the lowest shape resonant state Π of CO, e + CO → 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 CO(Π) proceeds through a combinational motion of bond stretching and bending.

Collision-Energy Dependent Stereodynamics of Dissociative Charge Exchange Reaction between Ar and CO.

Long-distance charge-dipole attraction between atomic ion and randomly oriented polar molecule potentially makes the molecular orientation, which profoundly influences the products' kinetics of collisional reaction. Using the three-dimensional ion velocity map imaging technique, here we report a collision-energy dependent stereodynamics of dissociative charge exchange reaction Ar + CO → Ar + O + C in a range of 7.46-9.

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

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 N yields in the large impact-parameter charge transfer from low-energy Ar to N. Besides the ground-state (XΣ ) N produced in the energy-resonant charge transfer, a few slower N ions are proposed to be in the superpositions of the XΣ -AΠ and AΠ-BΣ states on the basis of the accidental degeneracy or energetic closeness of the vibrational states around the XΣ -AΠ and AΠ-BΣ crossings in the non-Franck-Condon region.

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

We report an ion velocity imaging study of the charge exchange reactions between Ar ion and /-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.

Direct observation of long-lived cyanide anions in superexcited states.

The cyanide anion (CN) has been identified in cometary coma, interstellar medium, planetary atmosphere and circumstellar envelopes, but its origin and abundance are still disputed. An isolated CN is stabilized in the vibrational states up to ν = 17 of the electronic ground-state Σ, but it is not thought to survive in the electronic or vibrational states above the electron autodetachment threshold, namely, in superexcited states. Here we report the direct observation of long-lived CN yields of the dissociative electron attachment to cyanogen bromide (BrCN), and confirm that some of the CN yields are distributed in the superexcited vibrational states ν ≥ 18 (Σ) or the superexcited electronic states Σ and Π.

Stereodynamics Observed in the Reactive Collisions of Low-Energy Ar with Randomly Oriented O.

Stereodynamics of the collisional reaction between mutually aligned or oriented reactants has been a striking topic of chemical dynamics for decades. However, the stereodynamic aspects are scarcely revealed for the low-energy collision with a randomly oriented target. Here in the dissociative charge-exchange reaction between randomly oriented O and low-energy Ar, we, using the three-dimensional ion velocity map imaging technique, clearly observe a linear alignment and a nearly isotropic distribution of the O yields along the collision axis.

Probing the Potential Energy Surfaces of BrCN by Dissociative Electron Attachment.

State coupling certainly determines the topologic features of the molecular potential energy surface (PES) and potentially diversifies chemical reaction pathways. Here we report the new PESs of BrCN in the low-lying electronic states that are distinctly different from the previous predictions in the short Br-CN bond region but validated by the high-resolution ion velocity imaging measurements of low-energy dissociative electron attachment (DEA) to BrCN. Besides the vibrating CN ions produced in the fast Br-CN bond stretching motions, we confirm that the ro-vibrating CN ions with a nearly isotropic angular distribution are produced by receiving a torque in the combinational motion of Br-CN bond bending and stretching.

Identifying the Molecular Orientation and Clusters in the Liquid-Vapor Interface of 1-Propanol by Time-Delayed Mass Spectrometry.

Structural inhomogeneity of the liquid-vapor interface, such as the spatial orientation of molecular specific groups and the non-uniform distribution of hydrogen-bonded (HB) clusters, is crucial for understanding the physicochemical processes therein. Although the molecular orientation at the outermost layer was authenticated, to date, direct experimental evidence of the existence of different-sized HB clusters, as a long-standing theoretical argument, is still lacking. Here we report time-delayed electron-impact tandem mass spectrometry, and its powerful ability to identify the local structures of the liquid-vapor interface of 1-propanol is demonstrated not only by mapping the molecular orientations both in the outermost layer and in the subsurface but also by validating the existence of the HB molecular dimers in the subsurface by detecting their protonated ions.

Vibrationally resolved photoemissions of N (CΠ → BΠ) and CO (bΣ → aΠ) by low-energy electron impacts.

Vibrationally resolved photoemission spectra of the electronic-state transitions CΠ → BΠ of N and bΣ → aΠ of CO following low-energy electron impacts are measured with a crossed-beam experimental arrangement. The absolute cross sections of CΠ (ν') → BΠ (ν″) of N 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 CΠ of N show the maxima at the electron-impact energies 14.

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.

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

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 CHF at an electron-molecule resonant state formed by electron attachment.

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

Charge exchange reactions between Ar(P) and O (XΣ) 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.

Dissociative Electron Attachment to Molecular Acetonitrile.

Low-energy dissociative electron attachment to molecular acetonitrile (CHCN) is investigated by recording the efficiency curves of CHCN, 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 + CHCN → H + CHCN and CH + CN are respectively about 1.51 and 1.

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

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 CO. In a center-of-mass collision energy range of 7.23-15.

Time-delayed mass spectrometry of the low-energy electron impact with a liquid beam surface.

We set up an experimental apparatus to investigate the low-energy electron impact with a liquid beam surface, in which a cylindrical liquid beam with a diameter of 25 m emits as the laminar flow from a microjet and the positively charged ions produced by the electron-impact ionizations are detected with a linear time-of-flight mass spectrometer. We propose a time-delayed mass spectrometry for this apparatus to identify the cationic fragments produced on the liquid surface, in which the application of the ion extracting pulse is delayed with different time intervals after the electron beam pulse. Sensitivity and specificity of the present methodology are demonstrated by the combinational experiments of the gas-phase and liquid ethanol.