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Between an ion and a hydrogen ( H) atom in an excited state, there exists an effectively isotropic interaction potential which behaves as 1/ R at large separations R. The abnormal long-range part, stemming from the energy degeneracy peculiar to the H system, is expected to have a drastic influence on the low-energy collision processes. The present paper addresses this issue by considering protonium ( p̄ p) formation in antiproton ( p̄) collisions with excited H atoms. The calculation of the p̄ p formation process is carried out by means of a semiclassical (quantum-classical hybrid) approach. At low collision energies E, the formation cross section is found to be far above the so-called Langevin cross section and reveals the E dependence. This result is in clear contrast to that for the H atoms in the ground state with no degeneracy. Graphical abstract: [ABSTRACT FROM AUTHOR]

In this work, the dynamics of reaction S(P) + SH( X∏) → S( X∑) + H(S) has been investigated using quasi-classical trajectories method on a newly constructed accurate double many-body expansion potential energy surface of Song et al. [J. Phys. Chem. A 115, 5274 (2011)]. The integral cross section and the rate constant are obtained. Furthermore, the distribution reflecting vector correlation and the polarization-dependent differential cross section are investigated. The results show a decreasing behavior of the integral cross section as the increasing of the collision energy, and dominate complex forming mechanism at lower energies, as well as the stripping mechanism at higher energies. Those results indicate that the product S is sensitively affected by the collision energy. Graphical abstract: [ABSTRACT FROM AUTHOR]

Water, its dimer and their dissociative products (OH, HO, HO) play an important role in several diverse processes including atmospheric chemistry, radiation induced damage within cellular systems and atmospheric plasmas used in industry. The interaction of electrons with these species is therefore an important collision process but since OH, HO and HO are difficult to prepare as isolated experimental targets to date, electron scattering cross sections from such species are lacking in the literature. In this paper we report the results of a semi-empirical method to estimate such cross sections, benchmarking these cross sections against our knowledge of electron scattering from the water monomer. Calculations on HO, HO and (HO) are performed with improved Additivity Rules. Graphical abstract: [ABSTRACT FROM AUTHOR]

Forces on a small plane measurement target caused by sputtered atomic particles originated at plane sputter targets by a beam of argon particles are investigated. The rotatable sputter targets are centrally mounted in front of the ion source and allow a variation of the angle of incidence. The measurement surface of the force probe is arranged laterally beside the sputter target; it collects the particles released perpendicularly to the beam direction. Sputter targets of silver, copper, aluminum, titanium, and carbon are studied. The measurements are compared with a model that traces the trajectories of the argon ions and the sputtered target atoms between the ion source, the sputter target, and the force probe. The sputtering is calculated with the popular SRIM code based on binary collision cascades. The measured forces are found to be in general greater than the simulated forces. The dependence on the angle of incidence is similar in case of the elements heavier than the ions. In case of the lighter elements, only the simulation exhibits a pronounced maximum at angles between 65° and 70°. Graphical abstract: [ABSTRACT FROM AUTHOR]

The combinatorial frequency generation by periodic and quasiperiodic stacks of nonlinear semiconductor layers has been studied in the self-consistent problem formulation, taking into account mobility of carriers. The three-wave mixing technique is applied to study the nonlinear processes. It is shown that the mixing processes in passive semiconductor structures are driven by the competitive effects of the collision of charges and resonance interactions of carriers with pump waves. The effects of the pump wave dissipation, layer parameters and stack arrangements on the efficiency of the combinatorial frequency generation are discussed. [ABSTRACT FROM AUTHOR]

We try to present a theoretical evolutionary model leading to the excitations of nonlinear pulsational eigenmodes in a planar (1D) collisional dust molecular cloud (DMC) on the Jeans scale. The basis of the adopted model is the Jeans assumption of self-gravitating homogeneous uniform medium for simplification. It is a self-gravitating multi-fluid consisting of the Boltzmann distributed warm electrons and ions, and the inertial cold dust grains with partial ionization. Dust-charge fluctuations, convections and all the possible collisions are included. The grain-charge behaves as a dynamical variable owing mainly to the attachment of the electrons and ions to the grain-surfaces randomly. The adopted technique is centered around a mathematical model based on new solitary spectral patterns within the hydrodynamic framework. The collective dynamics of the patterns is governed by driven Korteweg-de Vries ( d-KdV) and Korteweg-de Vries (KdV) equations obtained by a standard multiscale analysis. Then, simplified analytical and numerical solutions are presented. The grain-charge fluctuation and collision processes play a key role in the DMC stability. The sensitive dependence of the eigenmode amplitudes on diverse relevant plasma parameters is discussed. The significance of the main results in astrophysical, laboratory and space environments are concisely summarized. [ABSTRACT FROM AUTHOR]

The growth of Fe clusters by collisions of Fe atoms with Ar atoms flowing in a supersonic beam was investigated by Fe mass flux measurements and transmission electron (TEM) microscopy. Moderate Ar densities of the order of 1 × 10 m were sufficient to cause cluster growth which was attributed to the low temperature of the Ar beam. TEM imaging of deposited clusters revealed diameter distributions from 2 to 10 nm depending on the deposition time. Extrapolation to zero deposition time revealed a cluster size of 2.4 nm grown in the gas phase. Growth on the surface was attributed to diffusion of single Fe atoms which are co-deposited with the clusters in the process and which agglomerate when they hit a cluster. [ABSTRACT FROM AUTHOR]

We present an analysis of Na-He collisional profiles at high density and very low temperature in a unified line shape semi-classical theory which contains the impact as well the quasistatic limits. Clearly understanding the variation of shape of the two fine-structure components of the 3 s−3 p line with increasing helium density allows us to understand experimental spectra of a Na atom attached to He nanodroplets. We compare our collisional approach to path integral Monte Carlo calculations using the same ab initio Na-He molecular potentials. [ABSTRACT FROM AUTHOR]

We report calculated electron impact ionization cross sections (EICSs) for beryllium (Be) and some of its hydrides from the ionization threshold to 1 keV using the Deutsch-Märk (DM) and the Binary-Encounter-Bethe (BEB) formalisms. The positions of the maxima of the DM and BEB cross sections are very close in each case while the DM cross section values at the maxima are consistently higher. Our calculations for Be are in qualitative agreement with results from earlier calculations (convergent close-coupling, R matrix, distorted-wave and plane-wave Born approximation) in the low energy region. For the various beryllium hydrides, we know of no other available data. The maximum cross section values for the various compounds range from 4.0 × 10 to 9.4 × 10 cm at energies of 44 to 56 eV for the DM cross sections and 3.0 × 10 to 5.4 × 10 cm at energies of 40.5 to 60 eV for the BEB cross sections. [ABSTRACT FROM AUTHOR]

Relevant phenomena occurring in the projectile-solid interaction and the subsequent emission of secondary ions from the sample surface will be theoretically treated in this work. The goal is to determine the velocity and energy distributions of the secondary ions emitted from a target of solid ammonia, through the description of secondary electron emission by the nuclear track and the establishment of secondary ion trajectories. Calculations were made with the computer code SEID. Theoretical distributions, valid for secondary ions that no longer interact with the solid, were compared with the experimental distributions obtained for ammonia ice bombarded by Cf fission fragments. It was observed that ionic species of mass ~86 u, located within 10 Å from the projectile impact point, are desorbed without being neutralized, unlike lighter ions (~18 u) that leave the surface from more distant regions. Considering the ionization energy ( I = 35 eV) of NH, the experimental value of 6 km/s was reproduced. These comparisons also show that the model partially describes the behavior observed in the energy distribution of the emitted ions. As a result, new processes are proposed to be included in the model. [ABSTRACT FROM AUTHOR]

A mechanism of double strand breaking in DNA due to the action of two electrons is considered. These are the electrons produced in the vicinity of DNA molecules due to ionization of water molecules with a consecutive emission of two electrons, making such a mechanism possible. The transport of secondary electrons, including the additional electrons, is studied in relation to the assessment of radiation damage due to incident ions. This work is a stage in the inclusion of double ionization events into the multiscale approach to ion-beam cancer therapy. [ABSTRACT FROM AUTHOR]

Cross sections of total single electron loss (SL), single electron loss with single ionization (SLSI) for Li + H, He collisions and cross sections of single electron loss with double ionization (SLDI) for Li + He collisions in the low- to intermediate-energy range have been calculated using a simple method based on the Bohr-Lindhard (B-L) model. The distributions of both the projectile and target electrons are considered. The calculated results are compared with the available experimental data. [ABSTRACT FROM AUTHOR]

Theoretical studies of electron impact double ionisation cross sections of Mg ions have been performed in the binary encounter approximation (BEA). Direct double ionisation has been investigated in the modified double binary encounter model. Ionization cross sections of different shells have been also calculated in order to analyse the contributions to double ionisation from ionisation-autoionization. The effect of the Coulombic field of the target ion on the incident electron has been considered in the present work. Accurate expression of σ (cross-section for energy transfer ΔE) and Hartree-Fock velocity distributions for the target electrons have been used throughout the calculations. The theoretical results show satisfactory agreement with the experimental observations. [ABSTRACT FROM AUTHOR]

The cross sections for rotational, fine-structure and projection-change transitions in collision of MgH (Σ) molecule with He and He atoms were computed at cold and ultracold temperatures using the ab initio potential energy surface. The significant suppression feature and resonance pattern caused by spin-rotation interaction were found and discussed. The collisions exhibit a strong isotope effect in the ultracold regime. The rotational relaxations of the fine-structure excited energy levels tendentiously preserve the F index. The ratio of the cross sections for elastic and spin-flipping collision with He partner is always greater than seven orders of magnitude for the energy range 10−1cm and suggests the MgH molecule is a good candidate for He buffer gas cooling. [ABSTRACT FROM AUTHOR]

We report the measurements of relative cross sections for multielectron processes in collisions of Xe ions with argon atoms in the velocity range of 0.65-1.32 a.u. By means of the coincidence time-of-flight (TOF) technique, the final charge states of both the projectile and target ions for each collision event are determined. The present experimental data are compared with the scaling law by Selberg et al. [Phys. Rev. A 54, 4127 (1996)] and the extended classical over-barrier (ECB) model. [ABSTRACT FROM AUTHOR]

Experimental differential cross sections for the optical collision process Na(3 s)S + Xe + hν → Na(3 p)P + Xe are presented. By comparison with the results of accurate quantum calculations, the observed oscillatory structure is used to investigate the XΣ and BΣ potential curves of the NaXe van der Waals molecule. The repulsive branches of both curves are probed in addition to the binding minimum of the ground state. A value of 92 cm is obtained for the dissociation energy. [ABSTRACT FROM AUTHOR]

The effects due to d-orbital polarization in collision processes between a single rhodium atom and a 12 atom rhodium cluster are investigated by means of ab initio molecular dynamics. For the initial configuration of the 12 atom rhodium targets we adopt two different low energy structures. The kinetic energy and impact parameter of the projectile are chosen in such a way that fusion, scattering and fragmentation of the cluster do occur. The collision is treated by means of density functional theory molecular dynamics (DFT-MD). Both spin unpolarized and polarized treatments are implemented in order to clearly distinguish the effects that are due to d-orbital polarization. We find a novel block dynamics, of parts of the cluster, which is due to the directional nature of d-bonds. In addition, the treatment of the collisions by means of high temperature DFT dynamics yields promising minimal energy configurations, which are target dependent but are difficult to obtain otherwise. [ABSTRACT FROM AUTHOR]

The first-order Coulomb-Born approximation has been applied to the study of positronium formation through K-shell electron capture in the collision of positron with multi-electron atomic targets. The single-zeta Roothaan-Hartree-Fock wave functions are used to describe the electron initial bound states. The differential and total cross sections are computed for the impact of positron on helium, carbon, neon, sodium and argon atoms, with the formation of positronium in its ground state. For helium atoms, the calculated total cross sections are compared with the available experimental data and other theoretical calculations. The comparison shows a good agreement between the present calculations and the measurements. [ABSTRACT FROM AUTHOR]

Positronium (Ps) formation during positron-hydrogen collisions in Debye plasma has been studied in detail by using the second-order distorted-wave approximation with the inclusion of the adiabatic dipole polarization potential. Reliable results have been reported at several incident positron energies in the range from the Ps formation threshold to 500 eV for different values of the Debye screening parameter μ. Resonances for S-, P- and D-wave partial Ps formation cross sections appear to exist for 0.03 ≤ μ ≤ 0.3. Interesting structures in the differential cross sections are displayed in the surface plots. It is reported here for the first time that there is a huge probability of Ps formation in dense plasma corresponding to Debye screening parameter μ ≥ 0.3. [ABSTRACT FROM AUTHOR]

By using a multiple-scale method, we study the formation of the temporal ultraslow vector optical solitons (USVOS) in a cold lifetime-broadened five-level V type atomic system via electromagnetically induced transparency. And we find that the two orthogonally polarized components of the weak pulsed probe field can evolve into a pair of temporal USVOS. Subsequently, by numerical simulating the collision between two temporal USVOS, we obtain that their collision properties are correlated with their incident angle and initial phase shift. Whether two solitons are in phase or out of phase, their collisions are almost elastic. Especially, when two solitons are parallel input, the collisions between them are periodical. Our results may have potential application in optical information processing and engineering. [ABSTRACT FROM AUTHOR]

n accurate and rapid kinetic model describing the collisional transport of particles is presented. It is derived from the Fokker-Planck equation and involves an angular closure in the phase space leading to a set of hyperbolic equations for the moments of the distribution function evolving in time, space and energy. This method provides an alternative to the prohibitive cost of a direct solution to the full kinetic equation. Moreover, it is exact for the cases of collimated beams and the quasi-isotropic distributions. It can be approximated with the usual numerical schemes of the non-linear hyperbolic analysis. This model has features that are required to simulate the electron or ion transport for the inertial confinement fusion and the dose deposition for radiation therapy of cancers. [ABSTRACT FROM AUTHOR]

The Bohm sheath criterion is modified for collisional plasma containing Boltzmann electrons and cold fluid ions at the presence of external magnetic field. Based on fluid model, the effects of the strength and the orientation of an external magnetic field on the upper and lower limits of Bohm sheath criterion have been studied by considering the collision frequency between ions and neutrals. The results show that the sheath criterion depends on the orientation and magnitude of magnetic field and the ion flow velocity at the sheath boundary. [ABSTRACT FROM AUTHOR]

We develop a theoretical model to describe the radio-frequency (rf) induced coupling of a pair of colliding atoms to a Feshbach molecule when a magnetic field arbitrarily far from the Feshbach resonance is modulated in time. We use the dressed atom picture, and show that the coupling strength in presence of rf is equal to the Feshbach coupling strength multiplied by the square of a Bessel function. The argument of this function is equal to the ratio of the atomic rf Rabi frequency to the rf frequency. We experimentally demonstrate this law by measuring the rate of rf-association of molecules using a Feshbach resonance in d wave collisions between ultra-cold chromium atoms. [ABSTRACT FROM AUTHOR]

Using an interferometric method, detailed analysis of the self-broadened 748.8 nm Ne line shapes emitted from the glow discharge of neon was performed, where departures from the ordinary Voigt profile were observed. These departures are shown to be consistent with fits of the speed-dependent asymmetric Voigt profile to experimental profiles. It was shown that neglecting the correlation between Doppler and collisional broadening may cause errors in determined line shape parameter values. Coefficients of the pressure broadening, shift and collision-time asymmetry are determined and compared with those calculated in the adiabatic approximation for the van der Waals and Lennard-Jones potentials. [ABSTRACT FROM AUTHOR]

The electron phase space evolution in a non-relativistic and homogeneous laser plasma generated by a nanosecond laser in a near infrared region in the presence of stimulated Raman scattering is investigated by a numerical simulation. The mechanism of electron acceleration in the potential wells of the plasma wave accompanying the Raman back-scattering is analyzed in a 1D Vlasov-Maxwell model. The dominant wave modes are both the backward and the forward propagating Raman waves, each accompanied by a daughter electrostatic wave. In addition to a strong interaction of plasma electrons with the primary electrostatic wave in the case of back-scattering, a cascading is observed consisting in a secondary scattering of the primary Raman back-scattered wave. This phenomenon reduces the Raman reflectivity and causes an acceleration of electrons against the direction of the heating laser beam. Moreover, the strong trapping in the primary electrostatic wave generated by the Raman back-scattering leads due to the trapped particle instability to a significant spectral broadening of the original plasma wave and a subsequent intermittent behaviour of the scattering process. The high phase velocity electrostatic daughter wave of the forward Raman scattering cannot trap the electrons directly, but there is an indication of non-resonant quasi-modes combined of this wave and of the simultaneously existing electrostatic daughter wave accompanying the Raman back-scattering. The transform method is used for a solution of the set of partial differential equations, which consists of the Vlasov equation and of the full set of Maxwell equations in a 1D approximation. A simplified Fokker-Planck collision term is added to overcome the numerical instabilities during the simulation. The model has relevance to a long scale plasma geometry, such as occurring in the indirect drive experiments near the light entrance holes of target hohlraum. [ABSTRACT FROM AUTHOR]

The four-photon annihilation of electron-positron pairs is considered. In our treatment the energies of the colliding particles can be arbitrary. We also discuss the lower-energy limit of the cross-section of three-photon annihilation of the (e-, e+)-pair. [ABSTRACT FROM AUTHOR]

In the present research paper, the study of the head-on collision between two ion thermal solitary waves is investigated in a two-fluid (i.e., a pair-ion) plasma consisting of positive and negative ions as well as a fraction of stationary (positively/negatively) charged dust impurities, using the extended Poincaré-Lighthill-Kuo method. The effects of the concentration of charged dust impurities and the positive-to-negative ion temperature ratio on the solitary waves collisions are investigated. It is found that the phase shift is significantly affected by the presence of the positive-to-negative ion temperature ratio and positively/negatively charged dust grains. [ABSTRACT FROM AUTHOR]

A systematic study on the Weibel instability induced filamentation of a relativistic electron beam is made by including the effects of collisions and temperature. The stabilization and de-stabilization of the beam filamentation due to beam temperature and plasma collisions, respectively, is explored through an analytical model and PIC simulations. The de-stabilization of the beam filamentation is attributed to the collision driven negative energy wave generation in the beam plasma system. [ABSTRACT FROM AUTHOR]

A detailed and large-scaled calculation is performed on the total electron impact excitation rate coefficients from the ground state to the 106 fine-structure levels in 3l-14l′ of Ni-like Gd36+ employing the relativistic configuration-interaction distorted-wave approximation. The resonance contributions from 3l174l′n′′l′′ and 3l175l′n′′l′′ doubly-excited states of Cu-like Gd35+ are taken into account using the isolated process and isolated resonances approximation. The effects of the radiative decays from the resonances are investigated carefully and are found to be significant. The present rate coefficients, as well as the collision strength, are compared extensively with the previously published results. We believe our results should be more accurate and reliable. [ABSTRACT FROM AUTHOR]

The transit times are obtained for a symmetrized (two identical bosons) and an antisymmetrized (two identical fermions) quantum colliding configuration. Considering two identical particles symmetrically impinging on a one-dimensional barrier, we demonstrate that the phase time and the dwell time give connected results where, however, the exact position of the scattered particles is explicitly determined by the phase time (group delay). For the antisymmetrized wave function configuration, an unusual effect of accelerated transmission is clearly identified in a simultaneous tunneling of two identical fermions. [ABSTRACT FROM AUTHOR]

The importance of the excited states of Positronium (Ps) in the formation cross sections (both differential and total) of the negative hydrogen ion (H-) are investigated theoretically for the charge transfer reaction, Ps (n = 1, 2) + H ↦ e+ + H- for a wide range of incident energies (e.g., threshold – 500 eV). The calculations are performed in the frame work of a qualitative model, the post collisional Coulomb modified eikonal approximation (CMEA). A comparative study is also made between the capture from ground and excited states of the Ps. The present CMEA model takes account of higher order effects which is essential for a rearrangement process where the first Born type approximation (Coulomb Born for the ionic case) is not supposed to be adequate. At low incident energies, the excited states of Ps (2s, 2p) are found to play a dominant role in the H- formation cross sections. Significant deviations are noted between the present CMEA and the Coulomb Born (CBA) results even at very high incident energies (e.g., Ei = 500 eV), indicating the importance of higher order effects. At high incident energies the present CMEA differential cross section (DCS) exhibits a double peak structure which is totally absent in the CBA and could again be attributed to higher order effects. [ABSTRACT FROM AUTHOR]

Electron capture processes in collision between slow X7+ (X = N, O and Ne) ions and C60 fullerene have been investigated using coincident measurements of the number n of ejected electrons, the mass and charge of the multicharged C60 r+ recoil ions and their fragments Cm i+ and the final charge state of the outgoing projectiles X(q-s)+ ( $1 \leqslant s \leqslant q-1$ ). The collision velocity is about 0.4 a.u. The partial cross-sections σr s , corresponding to r electrons transferred to the projectile with only s electrons stabilized, have been measured. Cross-sections for collisions “inside” and those “outside” the C60 cage have been separated by analyzing the kinetic energy of the outgoing projectile. The mean final charge state for frontal collisions has been measured to 3.1, 2.6 and 2.5 for N7+, O7+ and Ne7+ respectively. These results show the importance of the core effect on the stabilisation processes of captured electrons. [ABSTRACT FROM AUTHOR]

The dynamics of collisional processes between Mg atoms and caesium ions is studied using the hemiquantal (HQ) approach with special attention to the collisional channels leading to Mg(3 1P) and Cs(6 2P) states, for which the corresponding emission excitation functions have been previously measured in our laboratory. The radial and angular non-adiabatic couplings between the manifold of quasimolecular states have been determined using an ab initio configuration interaction calculation. The cross-sections for the different channels, as a function of the laboratory collisional energy, are compared with experimental values. The dynamical calculations indicate that, for the inelastic processes considered, the range of relevant impact parameters is small, active collisions being of the head-on type. . [ABSTRACT FROM AUTHOR]

Electron capture processes in collision between slow X7+ (X = N, O and Ne) ions and C60 fullerene have been investigated using coincident measurements of the number n of ejected electrons, the mass and charge of the multicharged C60 r+ recoil ions and their fragments Cm i+ and the final charge state of the outgoing projectiles X(q-s)+ ( $1 \leqslant s \leqslant q-1$ ). The collision velocity is about 0.4 a.u. The partial cross-sections σr s , corresponding to r electrons transferred to the projectile with only s electrons stabilized, have been measured. Cross-sections for collisions “inside” and those “outside” the C60 cage have been separated by analyzing the kinetic energy of the outgoing projectile. The mean final charge state for frontal collisions has been measured to 3.1, 2.6 and 2.5 for N7+, O7+ and Ne7+ respectively. These results show the importance of the core effect on the stabilisation processes of captured electrons. [ABSTRACT FROM AUTHOR]

e+ + H(ns) ↦e+ + H(n′s) transitions for arbitrary n and n′ have been studied using the distorted-wave formalism in the momentum space [Ghoshal and Mandal, Phys. Rev. A 72, 032714 (2005)]. The distorted-wave scattering amplitudes have been written in a simple closed analytical form. A detailed study has been made on differential and total cross sections in the energy range 20–300 eV. Resonance-like behaviour of the differential cross section has been observed in the the region of lower scattering angles for high Rydberg transitions. To the best of our knowledge the distorted-wave results for differential and total cross sections for such arbitrary transitions are reported for the first time in the literature. [ABSTRACT FROM AUTHOR]

Electron excitation collision strengths for the transitions from the ground state to the fine-structure levels of the 3s23p53d and 3s23p63d configurations in Ni XI are calculated using the Breit-Pauli R-matrix method. Configuration interaction wavefunctions have been used to represent the target states. The relativistic effects are incorporated in the Breit-Pauli approximation by including one-body mass correction, Darwin, and spin-orbit interaction terms in the scattering equations. Collision strengths are tabulated at selected energies in the range 10 to 75 Rydberg. Effective collision strengths are determined by integrating collision strengths by assuming a Maxwellian distribution of electron energies. The effective collision strengths are listed over a wide temperature range (2×104-1×107 K) applicable to astrophysical plasmas. Our results are the only collision strengths and effective collision strengths available for this ion. We believe that the data calculated in this work will be useful in solar, astrophysical and laser applications. [ABSTRACT FROM AUTHOR]

Radiative lifetimes for 2≤v′≤44 rovibronic C1Σ+ state levels of NaRb and quenching collision cross-sections with Rb atoms have been directly measured in a thermal cell by detecting time resolved laser induced fluorescence after pulsed excitation. Many body multipartitioning theory was applied to calculate C1Σ+-X1Σ+ and C1Σ+-A1Σ+ transition dipole moments. The relevant ab initio matrix elements were converted to the C1Σ+ state radiative lifetimes. The strong spin-orbit A1Σ+∼ b3Π coupling effect on the total C → A transition probabilities and lifetimes of the C1Σ+ state is discussed. The measured radiative lifetimes show a decrease from 61 to 34 ns as the v′ values increase, the results being in good agreement with calculations. The averaged collisional quenching cross-section value σ=(3±1)×10-14 cm2 was determined for NaRb (C1Σ+) + Rb collisions from the Stern-Volmer plots. [ABSTRACT FROM AUTHOR]

Vibrationally inelastic quantum calculations are carried out at low collision energies for the scattering of a beam of positrons off acetylene gaseous molecules. The normal mode analysis is assumed to be valid and the relative fluxes into the C–C and C–H symmetric vibrational modes are computed within a Body-Fixed (BF) formulation of the dynamics by solving the relevant vibrational Coupled Channels (VCC) equations. The clear dominance of the C–C mode is observed near threshold and the implications of this finding are briefly considered in relation to more global indicators like the average vibrational energy transfer indices as obtained in the present work. [ABSTRACT FROM AUTHOR]

Using molecular-dynamics simulation, we study the processes occurring after impact of clusters on a rigid wall. Comparing the impact of model clusters consisting of 13 atoms, or of 13 diatomic molecules with varied bond strength, the systematics in the results of the collision process are investigated. Four regimes of impact-induced cluster fragmentation are identified: intact reflection, shattering into large fragments, complete fragmentation, and molecule dissociation. The effect of the number of degrees of freedom activated in the collision on the translational and internal energies of the reflected fragments is discussed in detail. As a rule, with increasing number of degrees of freedom which can be activated in the collision, the translational energy sinks. On the other hand, for weak intramolecular bonding, intramolecular vibrations are easily excited at small impact energies, reducing the resulting translational energy. The presence of even a very weak attractive well epsilonw at the surface has a major influence on the sticking behavior of the clusters — and hence also on the absolute reflected energies — even at impact energies E0 ≫ epsilonw. [ABSTRACT FROM AUTHOR]

Stereodynamics in ionization of toluene and o-, p-chlorotoluenes by collision with He*(23S) metastable atoms were investigated by two-dimensional collision-energy/electron-energy-resolved Penning ionization electron spectroscopy. Anisotropic interactions around the molecule were studied by the collision energy dependence of partial ionization cross-sections (CEDPICS) as well as model potential calculations, and shielding effect by the methyl group was observed in CEDPICS for ionization from Cl lone-pair orbitals of o- chlorotoluene. Attractive interaction with He*(23S) around the π orbital region was found to be larger for toluene rather than o-, p- chlorotoluenes. Exterior electron density (EED) calculation of partial ionization cross-sections in Penning ionization and negative CEDPICS for ionization band observed in ca. 4 eV in electron energy indicated that π-2π+1 shake-up state was observed in Penning ionization electron spectroscopy of toluene. [ABSTRACT FROM AUTHOR]

In charge-transfer collisions of C4+(1s2 1S) with He (1s2 1S), the process of double electron capture into the ground state C2+ is well-known to dominate other channels by an order of magnitude for projectile energies below 10 keV. This work presents a calculation of differential cross-sections resolved in the angle and energy gain variables, based on an ab initio treatment of electronic states, and compares with the measurements published in the literature (projectile energy E=270, 400, and 470 eV). We also briefly discuss the semi-empirical two-state models developed by experimentalists for this process. [ABSTRACT FROM AUTHOR]

Collisional ionization of styrene (phenylethylene), 2-vinylpyridine, and 4-vinylpyridine with metastable He*(23S) atoms were studied by means of collision-energy/electron-energy resolved two-dimensional Penning ionization electron spectroscopy. Collision energy dependence of partial ionization cross-sections, which reflects the anisotropic interactions between a He*(23S) atom and the target molecules, indicates that attractive interaction for the out-of-plane access of a He*(23S) atom to phenyl group is stronger than that for the out-of-plane access to vinyl group. Moreover, it was found for vinylpyridines that the attractive interaction around π electrons became weaker than that for styrene, and that the attractive interaction for the in-plane access to the nitrogen atom is stronger than that for out-of-plane π-directions. However, in 2-vinylpyridine, the hydrogen atom of vinyl group prevents a He*(23S) atom from approaching to the nitrogen atom along in-plane directions, and thus the attractive interactions around the nitrogen atom were shielded by the vinyl group. The experimentally observed anisotropic interactions were qualitatively supported with ab initio model interaction potential calculations between a Li (He*(23S)) atom and the target molecule. Concerning with electronic structures of investigated molecules, the assignment of Penning ionization electron spectrum for 4-vinylpyridine was discussed on the basis of different behavior of collision-energy dependence of partial ionization cross-sections, and the satellite ionization band in Penning ionization electron spectra was also reported for styrene. [ABSTRACT FROM AUTHOR]

The positive water and hydronium ions are of interest in a variety of chemical and biological applications. Here we study the steric effect in charge transfer collisions, i.e. the spatial dependence of single electron capture, in collisions mediated by these ions. In particular, the steric effect is demonstrated in the O+(2D)/H2 and H+/H2O charge transfer collisions in the energy range of 100 eV/amu to 10 keV/amu. [ABSTRACT FROM AUTHOR]

In our previous work we investigated electronically non-adiabatic effects in $\rm Cl~(^{2}P_{3/2,1/2})+D_{2}$ using crossed molecular beam scattering coupled with velocity mapped ion imaging. The prior experiments placed limits on the cross-section for electronically non-adiabatic spin-orbit excitation $\rm Cl~(^{2}P_{3/2})+D_{2} \to Cl^*~(^{2}P_{1/2})+D_{2}$ and electronically non-adiabatic spin-orbit quenching $\rm Cl^*~(^{2}P_{1/2})+D_{2}\to Cl~(^{2}P_{3/2})+D_{2}$ . In the present work, we investigate electronically non-adiabatic spin-orbit quenching for $\rm Cl^*~(^{2}P_{1/2})+H_{2}$ which is the required first step for the reaction of Cl* to produce ground state HCl+H products. In these experiments we collide Cl (2P) with H2 at a series of fixed collision energies using a crossed molecular beam machine with velocity mapped ion imaging detection. Through an analysis of our ion images, we determine the fraction of electronically adiabatic scattering in Cl* +H2, which allows us to place limits on the cross-section for electronically non-adiabatic scattering or quenching. We determine the following quenching cross-sections σ quench(2.1 kcal/mol) = 26 ± 21 Å2, σ quench(4.0 kcal/mol) = 21 ± 49 Å2, and σ quench(5.6 kcal/mol) = 14 ± 41 Å2. [ABSTRACT FROM AUTHOR]

The total attenuation cross-section of Rg (3P2) (Rg = Ar, Kr) by the collision with CF3Br is measured as a function of the magnetic sub level MJ of Rg (3P2) and the collision energy. For Ar (3P2), the attenuation process indicates a MJ dependence, in particular, the cross-section of the MJ = 0 state is lower compared with that for other states. On the other hand, Kr (3P2) shows no MJ dependent attenuation. [ABSTRACT FROM AUTHOR]

The crossed molecular beam technique is used for producing C60 - species through potassium-buckyball collision processes, and studying the collision dynamics in an energy ranging from 10 eV up to 500 eV. At low collision energies only the negative parent ion is formed. As long as the collision energy is increased the fragmentation pattern of the fullerene negative ion could be identified and relative total cross-sections could be measured. Surprisingly, some satellite contributions have been observed in the band structure of C60 - parent ion time-of-flight spectrum, which points to the existence of distinct conformational C60 - isomers, which are likely to be formed during the collision, favoured by the strong polarization of the C60 - in the presence of the K+ projectile ion. Such presumed detection of different spheroidal metastable C60 - conformers was made possible due to an electric effect operating at the nanoscale. [ABSTRACT FROM AUTHOR]

The differential cross-section for electron-hydrogen atom collisions in the presence of a linearly polarized laser field is studied as a function of a scattering angle of low energy electron by employing second-Born approximation (SBA). Detailed analysis is performed in no forward scattering angle. [ABSTRACT FROM AUTHOR]

Collision induced absorption (CIA) spectra of pure D2 in the first overtone region from 5250 to 7250 $\mbox{cm}^{-1}$ , recorded at 77, 201 and 298 K, have been analyzed. The observed spectra at 77, 201 and 298 K were modelled by a total of 92, 214 and 267 components respectively of double vibrational transitions at room temperature of the type X2(J)+X0(J) and X1(J)+X1(J), where X is O, Q or S transitions. Profile analyses of the spectra were carried out using the Birnbaum-Cohen line-shape function for the individual components of the band, and characteristic line shape parameters were determined from the analysis. The observed and calculated profiles agree well over the whole overtone band, and the agreement is better than 97% in the three cases studied. Binary and ternary absorption coefficients were determined from the integrated absorption of the band. [ABSTRACT FROM AUTHOR]

Using two variants of the Laser Photoelectron Attachment (LPA) method involving a differentially-pumped, seeded supersonic beam (0.05% and 12.5% of SF6 molecules in helium carrier gas, nozzle temperatures T0= 300–600 K, stagnation pressures p0= 1–5 bar) and mass spectrometric ion detection, we have investigated the energy dependence of anion formation in low-energy electron collisions with SF6 molecules at high energy resolution. Using the standard LPA method, the yield for SF6- as well as SF5- and F- anions was studied with an energy width around 1 meV over the electron energy range 0–200 meV. In addition, a variant of the LPA method with extended energy range (denoted as EXLPA) was developed and applied to measure the yield for SF6- and SF5- formation over the energy range 0–1.5 eV with an energy width of about 20 meV. The cross-section for formation of SF6- decreases by five orders of magnitude over the range 1–500 meV and is only weakly dependent on nozzle temperature. The yield for SF5- formation shows — apart from a weak zero energy peak which grows strongly with rising temperature — a broad maximum (located around 0.6 eV for T0= 300 K and shifting to lower energies with rising T0) and a monotonical decrease towards higher energies. SF5- attachment spectra taken at elevated temperatures exhibit changes with rising stagnation pressure which directly reflect rovibrational cooling of the SF6 molecules with rising pressure. The SF5-/SF6- intensity ratio at near-zero energy and the low-energy shape of the broad peak in the SF5- spectra are used as thermometers for the internal temperature of the SF6 molecules in the seeded supersonic beam which (at p0= 1 bar) are found to be 50–100 K lower than the nozzle temperature. The energy dependence of the yield for F- formation is similar to that for SF6-, but the F- signals are three to four orders of magnitude lower than those for SF6-; in view of the rather high endothermicity of F- formation the origin of the F- signals is discussed in some detail. [ABSTRACT FROM AUTHOR]

We report a quantum dynamical treatment of the vibrational excitation of the bending mode of water molecules by collision with low energy positrons in the energy regions close to threshold openings. The exact vibrationally coupled-channel equations derived for the total e+-H2O system are solved in a Body-Fixed-Vibrational-Coupled-Channels (BF-VCC) reference frame, using a single-center expansion of the total wavefunction and of the interaction potential. The vibrationally inelastic cross-sections for transitions from the ground to the lowest excited state of the bending mode clearly show the bending excitation channel to be the dominant inelastic process at low collision energies. Comparisons with our earlier calculations for the other modes and for the excited processes induced by electron impact are also presented and analysed. [ABSTRACT FROM AUTHOR]