Search

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. Using time-delayed tandem mass spectrometry measurements in combination with theoretical calculations, we further confirm that the protonated dimethyl ether is produced from the ion-molecule reactions not only in the dense vapor above the interface but also within the molecular clusters of the acidic interface. Our finding provides a convincing piece of evidence about the liquid-vapor interfacial acidification by the electron-impact ionizing radiation, exhibiting a promising way to control the chemical reactions in the liquid surface.

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.

ConspectusReaction 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

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.

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 1Σ+, 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 (1Σ+) or the superexcited electronic states 3Σ+ and 3Π. The triplet state can be accessed directly in the impulsive dissociation of BrCN− or by an intersystem transition from the superexcited vibrational states of CN−. The exceptional stability of CN− in the superexcited states profoundly influences its abundance and is potentially related to the production of other compounds in interstellar space.

Many physical, chemical, and biological processes happen in liquid-vapor interface and are profoundly influenced with the local microstructures. In contrast to the liquid bulk, molecular orientation is the remarkable one of asymmetric structural features of the interface. Here we report an experimental method, namely, electron-impact time-delayed mass spectrometry and give a brief review about our recent progresses. This brand-new method not only enables us to have more insights into the interfacial structures, as done with small-angle X-ray and neutron scatterings and vibrational sum frequency generation spectroscopy, but also provides opportunity to explore the electron-driven chemical reactions therein.

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 O2 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. These observations are rationalized with the Doppler kinetic models in which the O2 bond is assumed to be parallel or unparallel to the collision axis of the large impact parameter collision. The linearly aligned O+, as the predominant yield, is produced in the parallel collision, while a rotating O2+, as the intermediate in the unparallel collision, leads to the isotropic distribution of O+.

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. The latter process is closely related to the potential well of BrCN- at the first excited state A2Π3/2 that arises from the Π-Σ state couplings. Our findings not only suggest that the PESs of other anionic cyanogen halides are in dire need of reexamination but also show that ion velocity imaging of the DEA process is a powerful experimental method for evaluating the theoretical PESs of molecular anions.

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.

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.

Charge exchange reactions between Ar+(2P) and O2 (X3Σ−g) 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+.

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 7.46-9.97 eV. At the lowest collision energy, the most C+ products are forward-scattered and are along the collision axis and are attributed to three different dissociation channels including the predominant one experiencing the rotating intermediate ArC+. At the high collision energies, the remarkably diffusive distribution of C+ arises from the prompt dissociation of the rebounded CO+. The different dynamic processes arising from the nearly collinear collision are elaborated explicitly on the basis of the data analyses using the Doppler kinetics models.

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

Three-dimensional ion momentum imaging is developed in a combination of ion velocity map imaging technique and delay-line anode ion detection, and 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 multi-electron 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 so fast as expected. The strong electron correlations in the short-lived intermediate (Ar-CO2)+ should be responsible for the CO2+ yields at the multi-electron excited states.

Ion-pair dissociation is an important molecular process and frequently happens when the target molecule is pumped to its electronically superexcited states. In contrast to the experimental studies of photoexcitation ion-pair dissociation, there are some experimental challenges in the electron-impact ion-pair dissociation study, in particular, on determination of the energetic threshold. Here we report an experimental development for the ion-pair dissociation study by using the monochromized electron impacts. As an example, the threshold of BrCN→Br-+CN+ is determined as 13.78 eV according to the appearance energy of CN+ signals, meanwhile, the time-sliced ion velocity image of CN+ is recorded at 16.09 eV and indicates an anisotropic distribution of the CN+ momentum.

We report an ion velocity imaging study of the charge exchange reactions between Ar

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

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

Vibrationally resolved photoemission spectra of the electronic-state transitions C

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 CH

Charge exchange reactions between Ar

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.

Shape resonances of electron-molecule system formed in the low-energy electron attachment to four low-lying conformers of serine (serine 1, serine 2, serine 3, and serine 4) in gas phase are investigated using the quantum scattering method with the non-empirical model potentials in single-center expansion. In the attachment energy range of 0–10 eV, three shape resonances for serine 1, serine 2, and serine 4 and four shape resonances for serine 3 are predicted. The one-dimensional potential energy curves of the temporary negative ions of electron-serine are calculated to explore the correlations between the shape resonance and the bond cleavage. The bond-cleavage selectivity of the different resonant states for a certain conformer is demonstrated, and the recent experimental results about the dissociative electron attachment to serine are interpreted on the basis of present calculations.

Does the halogen bonding interaction co-exist in liquid when it competes with the hydrogen bonding interaction? The classical molecular dynamics simulations for the solvation properties of ClF molecule in water are performed with the Lennard-Jones plus Coulomb electrostatic potential parameters that are optimized with ab initio interaction energy calculations for the pre-reactive H2O···ClF complex. We find that the halogen bonding interactions occur between O and Cl atoms and have the comparable strength and population with respect to the hydrogen bonding interactions of Cl···H.

Dissociative electron attachment to molecules in vibrationally excited states is associated with lower threshold energies and higher cross sections than for cold targets. To date, fewer studies have been performed for polyatomic molecules than for diatomic molecules, due to the experimental challenges and theoretical difficulties in describing the complicated dynamics. Here we report an observation of the dissociative electron attachment to $\mathrm{C}{\mathrm{O}}_{2}$ in the vibrationally excited states, where the vibrationally excited $\mathrm{C}{\mathrm{O}}_{2}$ targets are prepared by the electron-impact excitation at energies close to the ${}^{2}{\mathrm{\ensuremath{\Pi}}}_{u}$ resonant state of $\mathrm{C}{{\mathrm{O}}_{2}}^{\ensuremath{-}}$. The high-resolution anion momentum images of the ${\mathrm{O}}^{\ensuremath{-}}$ yield indicate that the rovibrational state distributions of the coproduct CO are dependent on the vibrationally excited states of symmetric bond stretching and bending modes of $\mathrm{C}{\mathrm{O}}_{2}$.

Dissociative electron attachments to OCS with the attachment energies of 4.3, 4.7, 5.0, 6.2, 6.5, 6.8, and 7.0 eV are investigated by measuring the S − momentum distributions with time-sliced anion velocity image mapping technique. In this energy range, the ground-state fragmentation, e − + OCS → CO( X ˜ 1 Σ+) + S − (2Pu), is only one dissociation pathway allowed in energy. All velocity images show the predominant feature of the low-speed S − ions, but the middle- and high-speed S − ions appear at the higher attachment energies (6.2–7.0 eV). The trimodel speed distribution of S − ion should be attributed to the different populations of the rovibrational states of CO fragment, however, the dynamic details of above dissociations are still unclear and demand theoretical calculations.

The C Cl bond cleavages induced by electron attachment to 1-chloroalkyl benzenes [C 6 H 5 (CH 2 ) n Cl, n = 0, 1, 2, 3, 4] are investigated with ab initio molecular dynamics simulations. The fragment Cl − is generated and the C Cl bond cleavage time decreases with the increasing length of the alkyl chain. In the dissociative electron attachment processes, the incoming electron is initially captured to the local π* virtual orbital of the phenyl group and then transfers to the anti-bond σ* orbital of the remote C Cl moiety; the variation of the C Cl bond cleavage times shows a similar trend of the dissociation thresholds of C 6 H 5 (CH 2 ) n Cl + e − → C 6 H 5 (CH 2 ) n + Cl − .

Until recently, it was widely regarded that only one reaction pathway led to the production of molecular oxygen in Earth's prebiotic primitive atmosphere: a three-body recombination reaction of two oxygen atoms and a third body that removes excess energy. However, an additional pathway has recently been observed that involves the photodissociation of CO2 on exposure to ultraviolet light. Here we demonstrate a further pathway to O2 production, again from CO2, but via dissociative electron attachment (DEA). Using anion-velocity image mapping, we provide experimental evidence for a channel of DEA to CO2 that produces O2(X(3)Σ(-)g) + C(-). This observed channel coexists in the same energy range as the competitive three-body dissociation of CO2 to give O + O + C(-). The abundance of low-energy electrons in interstellar space and the upper atmosphere of Earth suggests that the contributions of these pathways are significant and should be incorporated into atmospheric chemistry models.

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

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.

Anion mass spectrometry is developed on the basis of our home-made anion velocity map imaging apparatus. The Cl− product efficiency curve for dissociative electron attachment to 1,2-dichlorobenzene is obtained from 0.2 eV to 8 eV, meanwhile the sliced images of this anion are recorded at 1.2 and 6.0 eV corresponding to two peak positions of the product efficiency curve.

Ammonia (NH3) and ammonium ion (NH4+) are two main pollutants of Earth’s surface water system. Toward an understanding of the microscopic structures and thermodynamic properties of NH3 and NH4+ (along with halogen anion Cl–, Br–, or I–) at the aqueous solution–air interfaces, all-atom molecular dynamics simulations are performed with different solute concentrations and at various temperatures. The surface propensities of NH3, Br–, and I– species are predicted, while the NH4+ is repelled from the interface. The solute concentration strongly influences the spatial distributions of NH3, Cl–, Br–, and I– near the interface; moreover, the specific temperature effects on the surface charge, molecular orientation, and surface tension are demonstrated.

Dissociation dynamics of temporary negative ion NF3(-) formed in the low-energy (0.5 to 4.5 eV) electron attachment is investigated by the anion velocity slice imaging spectroscopy. The kinetic and angular distributions of the F(-) fragment indicate that this fragment is produced via two distinctly different dissociation channels, namely, two-body and three-body fragmentations, at the higher electron attachment energies. The anisotropic distributions of the fast F(-) ions are interpreted as the two-body dissociations relevant to the (2)E resonant state of NF3(-), whereas the slow F(-) can be produced via various three-body dissociations with the products of NF(X (3)Σ(-)) + F + F(-), NF(b (1)Σ(+)) + F + F(-), or N + F2 + F(-), depending on the electron attachment energy.

Photoionization and photodissociation of tetrachloroethylene are investigated using positive and negative photoion mass spectrometry in the photon energy range 8–22 eV. Ionization energy (9.33 ± 0.05 eV) of tetrachloroethylene and appearance energies of fragment ions C2 , C2 , C , C2Cl+ and CCl+ have been determined from the photoionization efficiency (PIE) curves. In addition, the spectral structure assigned as a vibrational progression (v 2) at 13.48 eV is observed in the PIE curve of the parent cation. The preferred dissociation pathways near the threshold energies are proposed according to the present theoretical calculations (W1u) and the relative branch ratio computation. On the other hand, the negative ions 35Cl− and 37Cl− produced in the minor ion-pair dissociation channel are observed within the energy range. The dissociation pathways potentially related to the multi-body ion-pair dissociations are discussed with the help of the thermochemistry data obtained from the PIE spectra.