K. Chakrabarti

Dissociative recombination and vibrational excitation of CO+: model calculations and comparison with experiment

The latest molecular data—potential energy curves and Rydberg/valence interactions—characterizing the super-excited electronic states of CO are reviewed, in order to provide inputs for the study of their fragmentation dynamics. Starting from this input, the main paths and mechanisms for CO+ dissociative recombination are analyzed; its cross sections are computed using a method based on multichannel quantum defect theory. Convoluted cross sections, giving both isotropic and anisotropic Maxwellian rate coefficients, are compared with merged-beam and storage-ring xperimental results. The calculated cross sections underestimate the measured ones by a factor of two, but display a very similar resonant shape. These facts confirm the quality of our approach for the dynamics, and call for more accurate and more extensive molecular structure calculations. Keywords: dissociative recombination, electron impact vibrational excitation, vibrationally excited, multichannel quantum defect theory (Some figures may appear in colour only in the online journal)

Electron collisions with BF+: bound and continuum states of BF

Rydberg and continuum states of the BF molecule are studied as a function of geometry using an electron collision formalism in the framework of the R-matrix method. Up to 14 BF+ target states are used in a close-coupling expansion and bound states are searched for as negative energy solutions of the scattering calculation. Potential energy curves and quantum defects are obtained for the excited states of BF. Resonance positions and widths are also calculated for Feshbach resonances in the system. The data obtained can be used to model dissociative recombination of the BF+ molecular ion.

R-matrix calculation of electron collisions with the BF+ molecular ion

Electron collisions with the BF+ molecular ion are studied using the framework of the diatomic version of the UK molecular R-matrix codes. A configuration-interaction calculation is performed for BF+ to obtain potential energy curves and target properties for 14 lowest doublet and quartet states. Scattering calculations are performed which yield resonance parameters and excitation cross sections in the energy range 0–20 eV. Cross sections for rotational excitations and an approximate calculation for the electron impact dissociation cross section for BF+ are also presented.

R-matrix calculation of the continuum states of carbon monoxide

Calculations on electron collisions with CO + molecular ion are presented as a function of electron energy and CO + geometry. Resonance positions and widths are obtained for the low-lying Feshbach resonances in the system. Plots of resonance curves suggest that low-energy dissociative recombination can occur via resonances of 1 �, 1 �, 3 � + and 3 � symmetries. Electron impact electronic excitation is considered both for excitation to the first excited A 2 � state and to yield a dissociation cross section. The latter calculations suggest that the lower of two experimental measurements of this process is likely to be correct. Finally results are presented for bound states in the continuum for both 1 � − and 3 � − symmetries. (Some figures in this article are in colour only in the electronic version)

Hyperspherical partial-wave theory applied to electron-hydrogen-atom ionization calculation for equal-energy-sharing kinematics

Hyperspherical partial-wave theory has been applied here in a new way in the calculation of the triple differential cross sections for the ionization of hydrogen atoms by electron impact at low energies for various equal-energy-sharing kinematic conditions. The agreement of the cross section results with the recent absolute measurements of [J. Roeder, M. Baertschy, and I. Bray, Phys. Rev. A 45, 2951 (2002)] and with the latest theoretical results of the ECS and CCC calculations [J. Roeder, M. Baertschy, and I. Bray, Phys. Rev. A (to be published)] for different kinematic conditions at 17.6 eV is very encouraging. The other calculated results, for relatively higher energies, are also generally satisfactory, particularly for large {theta}{sub ab} geometries. In view of the present results, together with the fact that it is capable of describing unequal-energy-sharing kinematics [J. N. Das, J. Phys. B 35, 1165 (2002)], it may be said that the hyperspherical partial-wave theory is quite appropriate for the description of ionization events of electron-hydrogen-type systems. It is also clear that the present approach in the implementation of the hyperspherical partial-wave theory is very appropriate.

Hyperspherical partial wave calculation for double photoionization of the helium atom at 20 eV excess energy

The hyperspherical partial wave approach has been applied here in the study of double photoionization of the helium atom for equal-energy-sharing geometries at 20 eV excess energy. Calculations have been done both in length and velocity gauges and are found to agree with each other, with the CCC results and with experiments and to exhibit some advantages for the corresponding three-particle wavefunction over other wavefunctions in use.

A theoretical study of the dissociative recombination of SH+ with electrons through the 2Π states of SH

A quantitative theoretical study of the dissociative recombination of SH+ with electrons has been carried out. Multireference, configuration interaction calculations were used to determine accurate potential energy curves for SH+ and SH. The block diagonalization method was used to disentangle strongly interacting SH valence and Rydberg states and to construct a diabatic Hamiltonian whose diagonal matrix elements provide the diabatic potential energy curves. The off-diagonal elements are related to the electronic valence-Rydberg couplings. Cross sections and rate coefficients for the dissociative recombination reaction were calculated with a stepwise version of the multichannel quantum defect theory, using the molecular data provided by the block diagonalization method. The calculated rates are compared with the most recent measurements performed on the ion Test Storage Ring (TSR) in Heidelberg, Germany.

Dissociative recombination and vibrational excitation of BF+ in low energy electron collisions

The latest molecular data - potential energy curves and Rydberg-valence interactions - characterising the super-excited electronic states of BF are reviewed in order to provide the input for the study of their fragmentation dynamics. Starting from this input, the main paths and mechanisms of BF+ dissociative recombination and vibrational excitation are analysed. Their cross sections are computed for the first time using a method based on the multichannel quantum defect theory (MQDT), and Maxwellian rate-coefficients are calculated and displayed in ready-to-be-used format for low temperature plasma kinetics simulations.

Higher lying resonances in low-energy electron scattering with carbon monoxide

AbstractR-matrix calculations on electron collisions with CO are reported whose aim is to identify any higher-lying resonances above the well-reported and lowest 2Π resonance at about 1.6 eV. Extensive tests with respect to basis sets, target models and scattering models are performed. The final results are reported for the larger cc-pVTZ basis set using a 50 state close-coupling (CC) calculation. The Breit-Wigner eigenphase sum and the time-delay methods are used to detect and fit any resonances. Both these methods find a very narrow 2Σ+ symmetry Feshbach-type resonance very close to the target excitation threshold of the b 3Σ+ state which lies at 12.9 eV in the calculations. This resonance is seen in the CC calculation using cc-pVTZ basis set while a CC calculation using the cc-pVDZ basis set does not produce this feature. The electronic structure of CO− is analysed in the asymptotic region; 45 molecular states are found to correlate with states dissociating to an anion and an atom. Electronic structure calculations are used to study the behaviour of these states at large internuclear separation. Quantitative results for the total, elastic and electronic excitation cross sections are also presented. The significance of these results for models of the observed dissociative electron attachment of CO in the 10 eV region is discussed. Graphical abstract

R-matrix calculation of the potential energy curves for Rydberg states of carbon monoxide

An electron collision formalism is used to consider Rydberg states of the CO molecule as a function of internuclear separation. Up to 14 target states of CO+ are used in close-coupled expansion and bound states are characterized by searching for negative energy solutions of the scattering calculations. These calculations give potential energy curves for excited states of CO as well as quantum defects as a function of internuclear separation, R. The quantum defects are found to depend only weakly on R except in the region of perturbations caused by intruder states.