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Hydrogen bonds are ubiquitous in nature and of fundamental importance to the chemical and physical properties of molecular systems in the condensed phase. Nevertheless, our understanding of the structural and dynamical properties of hydrogen-bonded complexes in particular in electronic excited states remains very incomplete. Here, by using formic acid (FA) dimer as a prototype of DNA base pair, we investigate the ultrafast decay process initiated by removal of an electron from the inner-valence shell of the molecule upon electron-beam irradiation. Through fragment-ion and electron coincident momentum measurements and

Non-covalently bound aromatic systems are ubiquitous and govern the physicochemical properties of various organic materials. They are important to many phenomena of biological and technological relevance, such as protein folding, base-pair stacking in nucleic acids, molecular recognition and self-assembly, DNA–drug interactions, crystal engineering and organic electronics. Nevertheless, their molecular dynamics and chemical reactivity, particularly in electronic excited states, are not fully understood. Here, we observe intermolecular Coulombic decay in benzene dimers, (C6H6)2—the simplest prototypes of noncovalent π–π interactions between aromatic systems. Intermolecular Coulombic decay is initiated by a carbon 2s vacancy state produced by electron-impact ionization and proceeds through ultrafast energy transfer between the benzene molecules. As a result, the dimer relaxes with the emission of a further low-energy electron (6H6+ cations undergoing Coulomb explosion. Coincident fragment-ion and electron momentum spectroscopy, accompanied by ab initio calculations, enables us to elucidate the dynamical details of this ultrafast relaxation process.

We report on charge state measurements of laser-accelerated carbon ions in the energy range of several MeV penetrating a dense partially ionized plasma. The plasma was generated by irradiation of a foam target with laser-induced hohlraum radiation in the soft X-ray regime. We used the tri-cellulose acetate (C$_{9}$H$_{16}$O$_{8}$) foam of 2 mg/cm$^{-3}$ density, and $1$-mm interaction length as target material. This kind of plasma is advantageous for high-precision measurements, due to good uniformity and long lifetime compared to the ion pulse length and the interaction duration. The plasma parameters were diagnosed to be T$_{e}$=17 eV and n$_{e}$=4 $\times$ 10$^{20}$ cm$^{-3}$. The average charge states passing through the plasma were observed to be higher than those predicted by the commonly-used semiempirical formula. Through solving the rate equations, we attribute the enhancement to the target density effects which will increase the ionization rates on one hand and reduce the electron capture rates on the other hand. In previsous measurement with partially ionized plasma from gas discharge and z-pinch to laser direct irradiation, no target density effects were ever demonstrated. For the first time, we were able to experimentally prove that target density effects start to play a significant role in plasma near the critical density of Nd-Glass laser radiation. The finding is important for heavy ion beam driven high energy density physics and fast ignitions., Comment: 7 pages, 4 figures, 35 conferences

We investigate the ultrafast energy and charge transfer processes between ammonia molecules following ionization reactions initiated by electron impact. Exploring ionization-induced processes in molecular clusters provides us with a detailed insight into the dynamics using experiments in the energy domain. We ionize the ammonia dimer with 200 eV electrons and apply the fragment ions coincident momentum spectroscopy and nonadiabatic molecular dynamics simulations. We identify two mechanisms leading to the doubly charged ammonia dimer. In the first one, a single molecule is ionized. This initiates an ultrafast proton transfer process, leading to the formation of the NH

The single ionization and dissociation of ethanol molecules induced by low-energy electrons ( E 0 = 90 eV ) are investigated using multiparticle coincident momentum spectroscopy. By detecting two outgoing electrons ( e 1 and e 2 ) and one fragment ion in coincidence, we obtain the energy deposition ( E 0 − E 1 − E 2 ) during electron ionization of the molecule, i.e., the binding energy spectra, for production of the different ionic fragments C2H5OH+, C2H4OH+, COH+, and H3O+. These data allow us to study the ionization channels for different ionic products. In particular, we focus on H3O+ as a product of double hydrogen migration. It is found that this channel mainly originates from the ionization of outer-valance orbitals (3a ″ ,10a ′ , 2a ″ , 9a ′ , 8a ′ , 1a ″ , and 7a ′ ). Additionally, there are minor contributions from the inner-valence orbitals such as 6a ′ , 5a ′ , and 4a ′ . Quantum chemistry calculations show two fragmentation pathways: concerted and sequential processes for formation of H3O+.

Ligand engineering based on a closo-B12-cage has been practiced in this work to design a series of stable gas-phase dianions [B12(ECX)12]2− (E = N, P, and As; X = O, S, and Se) via ab initio calculations. The calculated second electron affinities ranging from 1.2 to 3.4 eV indicate that these dianions are electronically stable against electron auto-detachment. Their electronic structures were analyzed based on molecular orbitals and natural bond orbitals. We found that in these designer dianions, the ECX ligands bind the B12-cage via B–E covalent interactions. We simulated the photoelectron spectra of these dianions and discussed the potential application of [B12(NCX)12]2− (X = O, S, and Se) salts as electrolytes in Li- or Mg-ion batteries.

If a molecular dication is produced on a repulsive potential energy surface (PES), it normally dissociates. Before that, however, ultrafast nuclear dynamics can change the PES and significantly influence the fragmentation pathway. Here, we investigate the electron-impact-induced double ionization and subsequent fragmentation processes of the ethanol molecule using multiparticle coincident momentum spectroscopy and ab initio dynamical simulations. For the electronic ground state of the ethanol dication, we observe several fragmentation channels that cannot be reached by direct Coulomb explosion (CE) but require preceding isomerization. Our simulations show that ultrafast hydrogen or proton transfer (PT) can stabilize the repulsive PES of the dication before the direct CE and form intermediate H2 or H2O. These neutrals stay in the vicinity of the precursor, and roaming mechanisms lead to isomerization and finally PT resulting in emission of H3+ or H3O+. The present findings can help to understand the complex fragmentation dynamics of molecular cations.

The inner-valence ionization and fragmentation dynamics of CH4–C6H6 dimer induced by 200 eV electron impact is studied utilizing a multi-particle coincidence momentum spectroscopy. The three-dimensional momentum vectors and kinetic energy release (KER) of the CH4 ++C6H6 + ion pairs are obtained by coincident momentum measurement. Our analysis on the absolute cross sections indicates that the intermediate dication CH4 +–C6H6 + is preferentially produced by the removal of an inner-valence electron from CH4 or C6H6 and subsequent relaxation of ultrafast intermolecular Coulombic decay followed by two-body Coulomb explosion. Combining with ab initio molecular dynamics (AIMD) simulations, the real-time fragmentation dynamics including translational, vibrational and rotational motions are presented as a function of propagation time. The revealed fragmentation dynamics are expected to have a potential implication for crystal structure imaging with various radiation sources.

Imaging the charge distributions and structures of molecules and clusters will promote the understanding of the dynamics of the quantum system. Here, we report a method by using an Ar atom as a tip to probe the charge distributions of benzene (Bz) cations in gas phase. Remarkably, the measured charge distributions of Bz cation (QH =0.204,QC=-0.037)and dication (QH =0.248,QC=0.0853)agree well with the calculated Mulliken distributions,and the structures of Bz dimer is reconstructed by using the measured charge distributions. The structures of two Bz dimer isomers (T-shaped and PD isomers) can be resolved from the measured inter-molecular potential V(R) between two Bz ions, and the structures of Bz dimer agree well with the theoretical predictions., 7 pages, 3 Figures

An experimental procedure is reported, which provides the absolute triple differential cross sections (ATDCSs) for electron-impact ionization of large (bio)molecules. This type of measurements represents the most stringent tests for new or existing theoretical models. We will use this procedure to test the accuracy of the best currently available theoretical models for the problems of electron-impact (65 eV) ionization of the molecules water $({\mathrm{H}}_{2}\mathrm{O})$, tetrahydrofuran $({\mathrm{C}}_{4}{\mathrm{H}}_{8}\mathrm{O})$, and their hydrogen-bonded dimer ${\mathrm{H}}_{2}\mathrm{O}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{C}}_{4}{\mathrm{H}}_{8}\mathrm{O}$. The cross sections were calculated using the molecular three-body distorted-wave (M3DW) model, the multicenter three-distorted-wave (MCTDW) approach, and the multicenter three-distorted-wave using the Ward-Macek approximation (MCTDW-WM). When compared to the new experimental ATDCS results which cover almost the full solid angle of the ejected electron and a broad range of ejected electron energies and projectile scattering angles, it is found that the data for water are generally well reproduced by the M3DW model, while strong deviations in the absolute magnitude of the cross sections are found for the MCTDW. The MCTDW-WM model provides improved agreement over the MCTDW. These theoretical models, however, become less adequate for the ATDCS of ${\mathrm{C}}_{4}{\mathrm{H}}_{8}\mathrm{O}$, in particular concerning the absolute magnitude. Furthermore, we find that a water environment can play a noticeable role for the ionization dynamics in the case of hydrated molecules.

We report an experimental study of the electron-impact ionization (E 0 = 65 eV) and fragmentation of carbon dioxide clusters for the formation of C O 2 + and ( C O 2 ) 2 + using a reaction microscope. All three charged particles, two outgoing electrons and one cation, are detected in coincidence such that the momentum vectors and, consequently, the kinetic energies for these final-state particles are determined. The reaction mechanisms correlated to ionic species are analyzed using the measured binding energy spectra over a broad range of ionization energy (8–32 eV). We observe that the ( C O 2 ) 2 + cations with small momenta are formed mainly from the direct ionization of the outermost 1π g orbital in the carbon dioxide dimer (CO2)2. While the (1π u )−1, (3σ u )−1 and (4σ g )−1 states in ( C O 2 ) 2 + are unstable and most likely dissociate into C O 2 + and neutral species. The ( C O 2 ) 2 + cations with high momenta are attributed to the ionization of small carbon dioxide clusters (CO2) n (n ⩾ 3) and subsequent dissociation processes. Compared to the appearance energy studies of carbon dioxide clusters where only the outermost 1π g orbital ionization was observed for formation of ( C O 2 ) 2 + , several additional channels are identified in this work.

The authors report the ultrafast molecular rotation of dicationic acetylene dimers during Coulomb explosion using time-resolved pump-probe experiments and ab-initio molecular dynamics simulations.

We investigate the formation mechanisms of covalently bound C

Ionization and subsequent isomerization of organic molecules has been suggested as an important source of trihydrogen ${\mathrm{H}}_{3}^{+}$ cations in outer space. The high interest in such reactions has initiated many experimental and theoretical studies for various molecules. Here, we report measurements as well as ab initio molecular dynamics simulations on the fragmentation of dicationic methanol monomers and clusters ionized by low-energy (90 eV) electrons. Experimentally, for dicationic monomers, a fragmentation channel for the formation of ${\mathrm{H}}_{3}^{+}$ in coincidence with a ${\mathrm{COH}}^{+}$ cation is observed. The simulations show that an intermediate neutral ${\mathrm{H}}_{2}$ is formed in the first step, and its roaming around the dication ends in the formation of ${\mathrm{H}}_{3}^{+}$. The entire reaction takes about 100--500 fs. The calculated kinetic energy release for the ${\mathrm{H}}_{3}^{+}+{\mathrm{COH}}^{+}$ ion pair is in excellent agreement with the experimental result. In contrast, for the dicationic clusters, due to the possibility of distributing the two charges onto different molecules, several fast dissociation channels occur and suppress the roaming of ${\mathrm{H}}_{2}$ and formation of ${\mathrm{H}}_{3}^{+}$. The present Letter suggests that the quenching of ${\mathrm{H}}_{3}^{+}$ formation by the chemical environment is a general phenomenon in dicationic clusters of organic molecules.

We study the triple-differential cross section (TDCS) for the electron-impact ionization of the highest occupied molecular orbital of tetrahydrofuran at a projectile energy ${E}_{0}=91\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$. The experimental data were measured using a reaction microscope, which covers a large part of the full solid angle for the secondary electron emission with energies ranging from 6 to 15 eV, and projectile scattering angles ranging from $\ensuremath{-}{10}^{\ensuremath{\circ}}$ to $\ensuremath{-}{20}^{\ensuremath{\circ}}$. The experimental TDCSs are internormalized across all measured scattering angles and ejected energies. They are compared with predictions from the multicenter distorted-wave (MCDW) approximation and a modified MCDW-Nee method which includes the postcollision interaction (PCI) using the Ward-Macek factor. Additional calculations were obtained using a molecular three-body distorted-wave (M3DW) approach which accounts for PCI in an exact treatment. Generally, the MCDW-Nee and M3DW models show better agreement with experiment than the MCDW calculations. This shows the importance of accounting for PCI for low-energy outgoing electrons in electron-impact ionization processes.

Ionization and subsequent isomerization of organic molecules has been suggested as an important source of trihydrogen H_{3}^{+} cations in outer space. The high interest in such reactions has initiated many experimental and theoretical studies for various molecules. Here, we report measurements as well as ab initio molecular dynamics simulations on the fragmentation of dicationic methanol monomers and clusters ionized by low-energy (90 eV) electrons. Experimentally, for dicationic monomers, a fragmentation channel for the formation of H_{3}^{+} in coincidence with a COH^{+} cation is observed. The simulations show that an intermediate neutral H_{2} is formed in the first step, and its roaming around the dication ends in the formation of H_{3}^{+}. The entire reaction takes about 100-500 fs. The calculated kinetic energy release for the H_{3}^{+}+COH^{+} ion pair is in excellent agreement with the experimental result. In contrast, for the dicationic clusters, due to the possibility of distributing the two charges onto different molecules, several fast dissociation channels occur and suppress the roaming of H_{2} and formation of H_{3}^{+}. The present Letter suggests that the quenching of H_{3}^{+} formation by the chemical environment is a general phenomenon in dicationic clusters of organic molecules.

Electron scattering cross sections for pyridine in the energy range 0–100 eV, which we previously measured or calculated, have been critically compiled and complemented here with new measurements of electron energy loss spectra and double differential ionization cross sections. Experimental techniques employed in this study include a linear transmission apparatus and a reaction microscope system. To fulfill the transport model requirements, theoretical data have been recalculated within our independent atom model with screening corrected additivity rule and interference effects (IAM-SCAR) method for energies above 10 eV. In addition, results from the R-matrix and Schwinger multichannel with pseudopotential methods, for energies below 15 eV and 20 eV, respectively, are presented here. The reliability of this complete data set has been evaluated by comparing the simulated energy distribution of electrons transmitted through pyridine, with that observed in an electron-gas transmission experiment under magnetic confinement conditions. In addition, our representation of the angular distribution of the inelastically scattered electrons is discussed on the basis of the present double differential cross section experimental results., This research was partially funded by the Spanish Ministerio de Ciencia, Innovación y Universidades-MICIU (Projects FIS2016-80440 and PID2019-104727RB-C21) and CSIC (Project LINKA 20085). F.C. acknowledges the Radiation Biology and Biophysics Doctoral Training Programme (RaBBiT, PD/00193/2012); UCIBIO (UIDB/04378/2020) and together with P.L.-V. the Portuguese National Funding Agency FCT through the Research Grants CEFITEC (UIDB/00068/2020) and PTDC/FISAQM/31281/2017. M.J.B. and R.D.W. thank the Australian Research Council for funding through grant DP180101655. L. Álvarez and A. García-Abenza thank the local CAM government and MICIU, respectively, for their corresponding grants within the “Garantía Juvenil” programmes. M.H.F.B. acknowledges support from the Brazilian Agency Conselho Nacional de Desenvolvimento Científico e Tecnollógico (CNPq). A.S.B. and M.H.F.B. acknowlegde computational support from Prof. Carlos de Carvalho at LFTC-DFis-UFPR and at LCPAD-UFPR and from C3SL at DInf-UFPR. A.D. and X.R. thank Igor Bray for providing helium ionization cross sections for calibration of the reaction microscope.

Abstract We report a combined experimental and theoretical study on the electron-impact ionization dynamics of H2 at an impact-energy of 520 eV. The molecular-frame fivefold-differential cross sections were measured for electron emission in the plane perpendicular to the incoming projectile beam. An (e, 2e + ion) triple coincidence method was used covering projectile scattering angles of 6.5°, 10.0° and 20.0° and ejected energies of 20 eV and 30 eV. The experimental cross sections are compared with results from the multi-center distorted-wave (MCDW) as well as the molecular three-body distorted wave (M3DW) approaches. M3DW is in overall better agreement with the measured data in the binary lobes than MCDW, while the intensity of recoil lobes are underestimated by both theories. Furthermore, we examine the presence of two-center interference patterns by comparing the experimental cross section ratios between mutually perpendicular alignment angles of the molecular axis with that predicated by the interference model. Agreement with the interference model is found only for Bethe ridge kinematics, i.e. in the binary peak region and with the ejected electron momentum being roughly equal to the momentum transfer. Finally, we suggest a modified interference formula for the recoil peak which takes into account the backscattering of the ejected electron in the ionic potential. Graphical abstract

We review experimental studies of electron impact ionization of atoms and molecules which became feasible by applying the electron-ion momentum vector coincidence technique. Besides briefly discussing excitation of metastable helium we present studies of single ionization of helium and argon and also mention a pilot experiment for positron impact ionization. The main strength of the method is revealed if three or more particles are to be detected and if the cross section of the reaction is small. Exemplarily helium double ionization close to threshold is shown. Studies of molecular ionization reveal a molecular alignment dependent collision dynamics for the molecular hydrogen target. For larger molecules ionized electronic orbital and ionic fragment can be correlated.

Cell and gene damage caused by ionizing radiation has been studied for many years. It is accepted that DNA lesions (single- and double-strand breaks, for example) are induced by secondary species such as radicals, ions and the abundant low-energy secondary electrons generated by the primary radiation. Particularly harmful are dense ionization clusters of several ionization processes within a volume typical for the biomolecular system. Here we report the observation of a damage mechanism in the form of a non-local autoionizing process called intermolecular Coulombic decay (ICD). It directly involves DNA constituents or other organic molecules in an aqueous environment. The products are two energetic ions and three reactive secondary electrons that can cause further damage in their vicinity. Hydrogen-bonded complexes that consist of one tetrahydrofuran (THF) molecule—a surrogate of deoxyribose in the DNA backbone—and one water molecule are used as a model system. After electron impact ionization of the water molecule in the inner-valence shell the vacancy is filled by an outer-valence electron. The released energy is transferred across the hydrogen bridge and leads to ionization of the neighbouring THF molecule. This energy transfer from water to THF is faster than the otherwise occurring intermolecular proton transfer. The signature of the ICD reaction is identified in triple-coincidence measurements of both ions and one of the final state electrons. These results could improve the understanding of radiation damage in biological tissue. The authors study intermolecular Coulomb decay that occurs in a sample of THF and water in a reaction microscope employing triple-coincidence measurements of two ions and one electron. They find that ICD is a previously unconsidered effect between water and other organic molecules that are hydrogen-bonded, with ICD outpacing proton transfer.

We report experimental investigations of two-body fragmentation of ${\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{2+}$ induced by 1 keV electron collision utilizing an ion momentum imaging spectrometer. With the ion-ion coincidence measurement, dissociation channels ${\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{2+}\ensuremath{\rightarrow}{\mathrm{H}}^{+}+{\mathrm{C}}_{2}{\mathrm{H}}^{+}$ (deprotonation) and ${\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{2+}\ensuremath{\rightarrow}{{\mathrm{H}}_{2}}^{+}+{{\mathrm{C}}_{2}}^{+}$ (${{\mathrm{H}}_{2}}^{+}$ formation) are directly identified, while the symmetric breakup ${\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{2+}\ensuremath{\rightarrow}\mathrm{C}{\mathrm{H}}^{+}+\mathrm{C}{\mathrm{H}}^{+}$ channel and vinylidene decarbonation ${\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{2+}\ensuremath{\rightarrow}{\mathrm{C}}^{+}+\mathrm{C}{{\mathrm{H}}_{2}}^{+}$ channel are not well separated in the measured time-of-flight (TOF) correlation map. In this work, by taking advantage of the independence of kinetic energy release (KER) on the dissociation angle, we are able to disentangle the events from the TOF map. Consequently, KER distributions for all four fragmentation channels are deduced, and the relative branching ratios are precisely determined from the measurements. By comparing the measured KER values with the previous calculated potential energy surfaces, pathways for the fragmentation channels are assigned.

We report a measurement of the valence orbital momentum profiles of tetrabromomethane (${\mathrm{CBr}}_{4}$) using symmetric noncoplanar $(e,2e)$ experiments at the impact energies of about 600 and 1200 eV. The experimental momentum profiles for the individual orbitals $5{t}_{1}, 13{t}_{2}, 5e, 12{t}_{2}$, and $9{a}_{1}$ and the branching ratio of $5{t}_{1}$ to $13{t}_{2}$ and $5e$ to $13{t}_{2}$ are obtained and compared with two kinds of calculations under the plane-wave impulse approximation. One is theoretical momentum profiles that have been calculated at the equilibrium geometry, the other is those that involve vibrational effects using a thermal sampling molecular dynamics method. The calculations considering molecular vibrations are in better agreement with experiment than the equilibrium geometry calculations, indicating the important role of nuclear motions on the valence orbital electronic structures of ${\mathrm{CBr}}_{4}$. The distorted-wave effects are observed in the experimental momentum profiles of $5{t}_{1}, 5e$, and $12{t}_{2}$ which display dynamic dependencies on the impact energies. A multicenter interference or bond oscillation effect has been observed from the momentum profile ratios of $5{t}_{1}$ to $13{t}_{2}$ and $5e$ to $13{t}_{2}$ which has direct information about the bond length of a molecule.

11 pags., 6 figs., 2 tabs. -- This article belongs to the Special Issue Electron and Photon Interactions with Bio(Related) Molecules, Experimental results for the electron impact ionization of benzene, providing double (DDCS) and triple differential cross sections (TDCS) at the incident energy of 90 eV, measured with a multi-particle momentum spectrometer, are reported in this paper. The most intense ionization channel is assigned to the parent ion (C6H6 + ) formation. The DDCS values are presented for three different transferred energies, namely 30, 40 and 50 eV. The present TDCS are given for two fixed values of the ejected electron energy (E2 ), at 5 and 10 eV, and an electron scattering angle (θ1 ) of 10◦ . Different features related to the molecular orbitals of benzene from where the electron is extracted are observed. In addition, a semi-empirical formula to be used as the inelastic angular distribution function in electron transport simulations has been derived from the present DDCS result and compared with other expressions available in the literature., F.C. acknowledges the Radiation Biology and Biophysics Doctoral Training Programme (RaBBiT, PD/00193/2012); UCIBIO (UIDB/04378/2020) and together with P.L.V. and A.I.L. the Portuguese National Funding Agency FCT through the Research Grants CEFITEC (UIDB/00068/2020) and PTDC/FISAQM/31281/2017. L.Á. thanks the local CAM government and MICIU, respectively, for their corresponding grants within the “Garantía Juvenil” programmes.

We investigate the formation mechanisms of covalently bound C4[Formula: see text] cations from direct ionization of hydrogen bonded dimers of acetylene molecules through fragment ion and electron coincident momentum spectroscopy and quantum chemistry calculations. The measurements of momenta and energies of two outgoing electrons and one ion in triple-coincidence allow us to assign the ionization channels associated with different ionic fragments. The measured binding energy spectra show that the formation of C4[Formula: see text] can be attributed to the ionization of the outermost 1 π u orbital of acetylene. The kinetic energy distributions of the ionic fragments indicate that the C4[Formula: see text] ions originate from direct ionization of acetylene dimers while ions resulting from the fragmentation of larger clusters would obtain significantly larger momenta. The formation of C4[Formula: see text] through the evaporation mechanism in larger clusters is not identified in the present experiments. The calculated potential energy curves show a potential well for the electronic ground state of (C2H2)[Formula: see text], supporting that the ionization of (C2H2)2 dimers can form stable [Formula: see text] cations. Further transition state analysis and ab initio molecular dynamics simulations reveal a detailed picture of the formation dynamics. After ionization of (C2H2)2, the system undergoes a significant rearrangement of the structure involving, in particular, C–C bond formation and hydrogen migrations, leading to different C4[Formula: see text] isomers.

In this work, we study the valence orbital momentum profiles of tetrachloromethane (CCl4) performed with binary (e, 2e) measurements at the impact energies of about 600 and 1200 eV with higher energy-resolution of Δ e ∼ 0.7 eV. The experimental momentum profiles for the molecular orbitals 2 t 1 , 7 t 2 , 2 e , 6 t 2 , and 6 a 1 are obtained and compared with calculations under the plane-wave impulse approximation. The calculations at the equilibrium geometry of CCl4 show generally good agreement with the experimental momentum profiles except for the low momentum range of 2 t 1 and 2 e which is considerable lower than the experiment. The experimental momentum profile shows dynamic dependencies on the impact energies which is ascribed to the distorted-wave effects. Further calculation considering molecular vibrations for the 2 t 1 highest occupied molecular orbital (HOMO) shows a better agreement with experiment than the equilibrium calculation, indicating the important role of nuclear motions on the HOMO electronic structure of CCl4.

We report a measurement of the valence orbital-momentum profiles of mercury (Hg) using a high-sensitivity binary $(e,2e)$ electron-momentum spectrometer at incident energies of 600 and 1200 eV plus the binding energy. The $6{s}_{1/2}$ orbital and the spin-orbit components of $5{d}_{5/2}$ and $5{d}_{3/2}$ orbitals are well separated in the measured binding-energy spectra. The experimental momentum distributions for the individual orbitals and the branching ratio of $5{d}_{5/2}$ to $5{d}_{3/2}$ are obtained and compared with predictions from a plane-wave impulse approximation (PWIA) in which the orbital wave functions are calculated using the nonrelativistic (NR) and spin-orbital (SO) relativistic theories. The SO relativistic calculations are in better agreement with experiment than the NR, indicating clearly the importance of relativistic effects in the electronic structure of Hg. We also observe some discrepancies between experiment and PWIA calculations in the high-momentum region of $6{s}_{1/2}$ and the low-momentum region of $5d$ orbitals which display a dynamic dependence on the impact energy. These discrepancies become smaller at a higher energy of about 1200 eV and thus can be qualitatively assigned to the distorted-wave effect in the $(e,2e)$ reaction of Hg.

The dissociation dynamics of CS

We report on a multicenter three-distorted-wave (MCTDW) approach to describe the electron-impact-ionization dynamics of molecules. In MCTDW, both the scattered projectile and the ejected electrons are described by distorted waves. The continuum wave functions of the incident and the two outgoing electrons are solved in the multicenter potential of neutral molecule and molecular ion, respectively. The fully differential cross section is then obtained for a given molecular orientation. Here, we present the spherically averaged triple-differential cross sections of water molecule by 81 eV electron impact. The results show good agreement with experiment in three-dimensional kinematics concerning both the angular dependence and the relative magnitude of the cross sections over a large range of analyzed angle and energy conditions.

Although the biological hazard of alpha-particle radiation is well-recognized, the molecular mechanisms of biodamage are still far from being understood. Irreparable lesions in biomolecules may not only have mechanical origin but also appear due to various electronic and nuclear relaxation processes of ionized states produced by an alpha-particle impact. Two such processes were identified in the present study by considering an acetylene dimer, a biologically relevant system possessing an intermolecular hydrogen bond. The first process is the already well-established intermolecular Coulombic decay of inner-valence-ionized states. The other is a novel relaxation mechanism of dicationic states involving intermolecular proton transfer. Both processes are very fast and trigger Coulomb explosion of the dimer due to creation of charge-separated states. These processes are general and predicted to occur also in alpha-particle-irradiated nucleobase pairs in DNA molecules.

We study ionization and fragmentation of tetrafluoromethane (CF4) molecule induced by electron impact at low energies (E0 = 38 and 67 eV). We use a reaction microscope combined with a pulsed photoemission electron beam for our experimental investigation. The momentum vectors of the two outgoing electrons (energies E1, E2) and one fragment ion are detected in triple coincidence (e, 2e+ ion). After dissociation, the fragment products observed are CF3+, CF2+, CF+, F+ and C+. For CF3+ and CF2+ channels, we measure the ionized orbitals binding energies, the kinetic energy (KE) of the charged fragments and the two-dimensional (2D) correlation map between binding energy (BE) and KE of the fragments. From the BE and KE spectra, we conclude which molecular orbitals contribute to particular fragmentation channels of CF4. We also measure the total ionization cross section for the formation of CF3+ and CF2+ ions as function of projectile energy. We compare our results with earlier experiments and calculations for electron-impact and photoionization. The major contribution to CF3+ formation originates from ionization of the 4t2 orbital while CF2+ is mainly formed after 3t2 orbital ionization. We also observe a weak contribution of the (4a1)−1 state for the channel CF3+.

Fast (${E}_{0}=520\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$) electron-impact ionization of molecular hydrogen (${\mathrm{H}}_{2}$) is studied experimentally and theoretically for a resolved molecular axis angle and 10 eV ejected electron energy. Fully differential cross sections were measured using an electron-electron-ion triple coincidence experiment in which the molecular axis direction is obtained for ionic ground-state dissociation. While predictions from the standard two-wave interference model are in strong disagreement with experiment a multicenter distorted-wave method obtains much better results. This demonstrates the importance of the multicenter ionic potential for the electron emission pattern at low energy and the failure of hitherto widely used simple models which treat molecular ionization as the coherent sum of atomic ionization amplitudes.

We report a combined experimental and theoretical study on the low-energy (E0 = 31.5 eV) electron-impact ionization of molecular hydrogen (H2). Triple differential cross sections are measured for a range of fixed emission angles of one outgoing electron between θ1 =-70° and −130° covering the full 4π solid angle of the second electron. The energy sharing of the outgoing electrons varies from symmetric (E1=E2 =8 eV) to highly asymmetric (E1 = 1 eV and E2 = 15 eV). In addition to the binary and recoil lobes, a structure is observed perpendicular to the incoming beam direction which is due to multiple scattering of the projectile inside the molecular potential. The absolutely normalized experimental cross sections are compared with results from the time-dependent close-coupling (TDCC) calculations. Molecular alignment dependent TDCC results demonstrate that these structures are only present if the molecule axis is lying in the scattering plane.

We investigate the interatomic Coulombic decay (ICD) in argon dimers induced by electron-impact ionization (${E}_{0}=90\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$) using a multiparticle coincidence experiment in which the momentum vectors and, consequently, the kinetic energies for electrons and fragment ions are determined. The signature of the ICD process is obtained from a correlation map between ejected electron energy and kinetic energy release (KER) for ${\mathrm{Ar}}^{+}+{\mathrm{Ar}}^{+}$ fragment ions where low-energy ICD electrons can be identified. Furthermore, two types of ICD processes, termed fast and slow interatomic decay, are separated by the ICD initial-state energies and projectile energy losses. The dependence of the energies of emitted low-energy ICD electrons on the initial-state energy is studied. ICD electron energy spectra and KER spectra are obtained separately for fast and slow decay processes where the KER spectra for the slow decay channel are strongly influenced by nuclear motion. The KER and ICD electron energy spectra are well reproduced by ab initio calculations.

We report an experimental and theoretical study of low-energy electron-impact ionization of tetrahydrofuran, which is a molecule of biological interest. The experiments were performed using an advanced reaction microscope specially built for electron-impact ionization studies. The theoretical calculations were performed within the molecular three-body distorted-wave model. Reasonably good agreement is found between experiment and theory.