MJ and collision energy dependent attenuation of Rg (3P2) by CF3Br

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 M_J of Rg (³P₂) and the collision energy. For Ar (³P₂), the attenuation process indicates a M_J dependence, in particular, the cross-section of the M_J = 0 state is lower compared with that for other states. On the other hand, Kr (³P₂) shows no M_J dependent attenuation.     

Elastic scattering of low-energy electrons by NO

We report elastic integral, momentum transfer and differential cross sections for electron scattering by N₂O for energies up to 50 eV. These results were obtained at the static-exchange approximation with the Schwinger Multichannel Method with Pseudopotentials [M.H.F. Bettega, L.G. Ferreira and M.A.P. Lima, Phys. Rev. A 47, 1111 (1993)]. In general our results show good agreement with experimental data and with other theoretical results but some discrepancies are found. We have also found a shape resonance around 4 eV in agreement with previous calculations using the R-matrix Method of Sarpal et al. [J. Phys. B 29, 857 (1996)]. On the other hand, the existence of a resonance at about 13 eV, clearly seen by the Schwinger Variational Iterative Method [Michelin et al., J. Phys. B 29, 2115 (1996)], can not be confirmed by our calculations. At this energy, our cross sections show a broad bump with no clear resonant behavior given by the eigenphase sum. Received: 13 November 1997 / Revised: 13 March 1998 / Accepted: 9 April 1998     

The viscosity cross-section for elastic electron-xenon collisions including electron spin polarization

The results of the evaluation of the viscosity cross-section for elastic electron-xenon collisions, taking into account the spin-orbit interaction of the continuum electron, in the energy interval from 0.1 eV to 50 eV are presented and discussed. The calculations are performed on the basis of three theoretically derived sets of phase shift data obtained by different authors and on the deduced relativistic expression for the viscosity cross-section in terms of phase shifts discerning the spin-up and spin-down states of the scattered electrons. Comparison with viscosity cross-sections, as evaluated from non-relativistic phase shifts extracted from experiments, strongly favours the relativistic results. The assumption of isotropic scattering is critically examined and the error induced by its use is shown to persist to the same extent as in non-relativistic calculations, at least in the energy region considered. Received: 22 April 1998 / Received in final form: 16 December 1998     

Studies on vibrational excitation differential cross-sections of low-energy electron scattering from N2 molecule

The vibrational excitation differential cross-sections (DCS) of low-energy electron-N₂ scattering are studied using vibrational close-coupling (VCC) method and vibrational scattering potentials which include static, exchange and polarization contributions. By including the contributions of 18 partial waves, 20 vibrational states, and 16 molecular symmetries (up to Λ=7), the converged vibrational excitation (0↦2, 0↦3, 0↦4) DCS agree well with experimental results. Also obtained are converged vibrational (1↦0, 1↦1, 1↦2, 1↦3) DCS, with the impact energies being those of the main resonant peaks (1.92 eV, 1.90 eV, 1.62 eV, 1.63 eV).     

Steric effect in O+/H2 and H+/H2O collisions

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.     

Calculation of absolute electron-impact ionization cross-sections of dimers and trimers

We report calculations of the electron-impact ionization cross-sections of selected dimers (homonuclear diatomic molecules) and trimers (homonuclear triatomic molecules) using a method which relies only on macroscopic quantities in conjunction with a “defect concept”. The empirically determined defect describes the deviation of the cluster (dimer, trimer) cross-sections from a simple linear dependence on the cluster size. We compare the calculated cross-sections to experimental data for the dimers S₂ and F₂ and the trimer O₃ and we present predictions for the ionization cross-sections of Br₂, I₂, C₂ and C₃ for which no experimental data are available. Lastly, we extend the method to the calculation of ionization cross-sections for the fullerenes C₆₀ and C₇₀. Received 6 December 1999 and Received in final form 10 April 2000     

Application of R-matrix method to electron-H<Subscript>2</Subscript>S collisions in the low energy range

Electron-H₂S collision process is studied using the R-matrix method. Nine low-lying states of H₂S molecule are considered in the R-matrix formalism to obtain elastic integral, differential, momentum transfer and excitation cross sections for this scattering system. We have represented our target states using configuration interaction (CI) wavefunctions. We obtained adequate representation of vertical spectrum of the target states included in the scattering calculations. The cross sections are compared with the experiment and other theoretical results. We have obtained good agreement for elastic and momentum transfer cross sections with experiment for entire energy range considered. The differential cross sections are in excellent agreement with experiment in the range 3–15 eV. A prominent feature of this calculation is the detection of a shape resonance in ²B₂ symmetry which decays via dissociative electron attachment (DEA). Born correction is applied for the elastic and dipole allowed transition to account for higher partial waves excluded in the R-matrix calculation. The electron energy range is 0.025–15 eV.     

Scattering of electrons from gas-phase N2O(1Σ): Computed cross-sections and angular distributions in comparison with experiments

Ab initio calculations are reported for the quantum scattering of electrons from N₂O molecules in the gas phase and for energies which range from near threshold up to about 100 eV. Elastic integral cross-sections and angular distributions are examined in detail and an extensive comparison is made with existing experiments. The agreement found with the latter data is fairly good and results are further discussed to explain the physical mechanisms at work for this polar target.     

Geometric shielding corrections for total cross section calculations of electron scattering by CH4, C2H6, C2H3F3, C2H4, C2F4, C2Cl4 and C2Cl2F2 from 30–5000?eV

To quantify the changes in the geometric shielding effect in a molecule as the incident electron energy varies, an empirical fraction, which represents the total cross section contributions of shielded atoms in a molecule at different energies, is presented. Using this empirical fraction, the total cross sections for electron scattering by CH₄, C₂H₆, C₂H₃F₃, C₂H₄, C₂F₄, C₂Cl₄ and C₂Cl₂F₂ are calculated over a wide energy range from 30 to 5000 eV by the additivity rule model at the Hartree-Fock level. The quantitative total cross sections are compared with those obtained by experiment and other theories where available. Good agreement is attained above 100 eV.     

Kinetic energy release for the collision-induced dissociation of CO<Superscript>+</Superscript>

The kinetic energy release distributions (KERDs) of C⁺ and O⁺ fragments arising from 5 keV collision-induced dissociation (CID) of CO⁺ ions with helium have been measured. The KERDs of C⁺ and O⁺ exhibit different features corresponding to the states that participate in CID processes. We have identified groups of dissociative and predissociative states, and compare them with theoretical and experimental values.     

Electron scattering by formic acid in the gas phase: comparing measured and computed angular distributions

Elastic differential cross-sections are measured over a broad range of energy values and the results are compared with quantum calculations for the same process. The agreement between measured and computed data turns out to be fairly good and calculations further allow the separation of the cis and trans conformer behavior and to compare it with the angular distributions for scattering from the dimer at the same energies.     

Electron-impact dissociation and transient properties of a stored LiH2 - beam

The fragmentation of LiH₂ ^- anions after electron impact was investigated at the heavy-ion storage ring TSR. The main reaction channel was found to be electron detachment followed by a breakup into LiH + H. In the first ms after production of the molecular ions in a cesium sputtering ion source, additional contributions were observed in the Li + H₂ and Li^- + H₂ channels, hinting at an initial population of a short-lived state of the anion. To gain a better understanding of the mechanisms underlying the observed behavior of the system, ab initio calculations of relevant potential energy surfaces were performed at selected geometries. The experimental findings are discussed in the light of these calculations.     

Low-energy electron scattering from gaseous CS<Subscript>2</Subscript>: angular distributions and effect of exchange forces

Ab initio calculations are reported for the quantum scattering of electrons from CS₂ molecules in the gas phase and for energies which range from near threshold up to about 100 eV. Angular distributions are examined in detail and an extensive comparison is made with existing experiments and earlier calculations. The agreement found with the latter data is fairly good and results are further discussed in terms of a physical mechanism of “exchange level shifting" to explain the disappearance of a Π_u resonance suggested by earlier studies.     

Energy dependence of the Penning ionization electron spectrum of Ne* (3P2,0)+Kr

A crossed beam experiment is carried out to measure the energy of electrons emitted in Penning ionization processes by Ne^*(³P_2,0)–Kr collisions. The electron energy spectra have been measured at four different collision energies: 0.050, 0.140, 0.190, 0.460 eV. The analysis of the results allows the separation of spin orbit contributions both in the entrance and in the exit channels providing the related cross-section ratios. Some theoretical considerations have been made to clarify nature and role of interatomic potentials driving the collisions and some general features about the role of atomic fine structure in the Penning ionization processes.     

Multi-electron transitions in -Ne and -Ne collisions

The ratios of the multiple ionization cross-section to those of the single ionization of neon are measured for 2.0-8.0 MeV and ( q = 2-5) ions bombardment. By means of the coincidence beween the charge state-selected scattered projectiles and recoil ions the contribution of the electron capture is separated from the total multiple ionization. A theoretical method is proposed to exclude the Auger transitions from the considered multiple ionization processes in the present work. The (q / v) dependence of the obtained ratios of the “pure” direct multiple ionization to those of the single ionization is discussed. The electronic structure dependence of the electron transition occurring in ion-neon collisions is studied and discussed for the symmetrical collision systems - and -. Received: 4 June 1998 / Received in final form: 24 August 1998     

Note on electron impact excitation for transition of Be-like isoelectronic sequence

Relativistic distorted-wave method was used to calculate the electron impact excitation collision strengths for transitions 2 s2 ¹ S0-2 s 2 p ³ P1 of Be-like isoelectronic ions. The target states were described, respectively, by 10-level, 46-level and 133-level MCDF configuration-expansion. The relativistic continuum orbitals were calculated in the potential field of frozen target-ion charge distribution with semi-classical exchange potential. The influence of the target states on this collision process along the isoelectronic sequence was investigated in the above three MCDF configuration-expansion modes. It was found that the configurations in the n =3 and the n =4 complexes have great influence on both the high and the low Z ions but the influence is relatively small for intermediate Z ions. The latter phenomenon was attributed to competition between opposing correlation and relativistic effects on the collision strengths. Received: 25 February 1998 / Received in final form: 18 June 1998     

Resonance enhanced multiphoton ionization (REMPI) and REMPI-photoelectron spectroscopy of ammonia

Resonance enhanced multiphoton ionization (REMPI) spectroscopy, preferably linked with kinetic energy analysis of the resulting photoelectrons (REMPI-photoelectron spectroscopy (PES)), continues to make enormous contributions to our understanding of the spectroscopy and, in many cases, the decay dynamics of small molecules in excited (normally Rydberg) electronic states. Here we present results of recent REMPI and REMPI-PES studies involving the ammonia molecule which provide further illustration of some of the many opportunities offered by these techniques. Received: 28 January 1998 / Revised: 3 April 1998 / Accepted: 15 April 1998     

Collision-energy-resolved Penning ionization electron spectroscopy of styrene, 2-vinylpyridine, and 4-vinylpyridine with He*(23S) metastable atoms

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.