Ismanuel Rabadán

Dissociative recombination of NO + : calculations and comparison with experiment

Multichannel quantum defect calculations for NO + dissociative recombination (DR) for electron energies from threshold to 8 eV are presented. The calculations use electronic energies and autoionization widths of valence states obtained from ab initio R-matrix calculations with the corresponding potential curves calibrated using available spectroscopic data. Six valence states open to dissociation are included in the final calculations. Excellent agreement with the measured cross sections is obtained for the low-energy DR, up to 3 eV and, for the first time, the peak observed in the cross section at high energy is accounted for. The importance of the various dissociative states at different electron energies, as well as the direct and indirect processes, is discussed. Compared to previous theoretical studies, the inclusion of a third dissociative state of 2 � symmetry and the larger autoionization width of the B� 2� state are found to be particularly important for the agreement with experiment. (Some figures in this article are in colour only in the electronic version; see www.iop.org)

Ab initio potential energy curves of Rydberg, valence and continuum states of NO

Bound states and quasibound Rydberg states of NO are studied as a function of internuclear distances in the range . R-matrix calculations which include 12 configuration interaction target states in the close-coupling expansion are performed. Rydberg states of up to n = 10 are studied for the , and symmetries of NO. Intruder states, of both valence and Rydberg character, are followed both above and below the ionization threshold. Where possible these states are classified as Rydberg states of an excited state of the ion. Away from avoided crossings, it is found that the quantum defects of the Rydberg series converging on the state of show little sensitivity to bondlength but show considerable dependence on the number of states included in the close-coupled expansion.

On the calculation of electron-impact rotational excitation cross sections for molecular ions

Methods of computing rotational excitation cross sections for electron collisions with diatomic molecular ions are examined for impact energies up to 5 eV. The HeH and NO ions are used as test cases and calculations are performed at various levels of approximation. Previous studies have all used the Coulomb-Born approximation assuming only dipolar potentials. This approximation is found to be unreliable in a number of aspects: short-range and threshold effects are important, and the widely made assumption that only processes need to be considered is particularly questionable. Conversely, full inclusion of vibrational motion is found to be less important.

ROTIONS : A program for the calculation of rotational excitation cross sections in electron-molecular ion collisions

Abstract ROTIONS is a program for the calculation of cross sections for rotational excitation of linear molecular ions by electron impact. The cross sections can be obtained from input T -matrices and from the Coulomb—Born approximation for dipole (Δ j = 1) and quadrupole (Δ j = 2) transitions. The Coulomb—Born results can be used to augment the low partial—wave T -matrices to an effectively infinite number of partial waves.

R-matrix calculation of the bound and continuum states of the - system

A comprehensive theoretical study of NO Rydberg states and of Feshbach resonances associated with excited states of is made for an internuclear separation of 2.175 . Calculations are performed using the UK R-matrix molecular package. The lowest four target states (, , and ) are considered in the construction of the CI target wavefunction. Quantum defect theory is employed to analyse the multiple Rydberg series obtained. In the bound-states region of the NO spectrum, six Rydberg series are resolved and quantum defects are given up to n = 11. In the continuum spectrum, Rydberg series of resonances converging to the three excited states of considered are identified and compared with available experimental data. Some previous assignments for both bound and continuum states are reinterpreted and the dominant source of partial-wave mixing re-analysed. Data for the resonance widths are presented for the first time.

Possible role of double scattering in electron - atom scattering in a laser field

By considering observations of double-scattering effects in the excitation of the level of He, gas density values are estimated for the laser-assisted elastic scattering experiments of Wallbank and Holmes (1993, 1994a, b) for which the Kroll - Watson approximation appears to fail. Using comparable densities for He and lower densities for Ar, and assuming the Kroll - Watson approximation for single-scattering events, differential cross sections are calculated including double scattering for laser-assisted scattering for a range of energies and scattering angles. Comparison with the observed values shows that double-scattering effects can give a semi-quantitative explanation of the apparent breakdown of the Kroll - Watson approximation in both He and Ar.

An ab initio calculation of electron impact vibrational excitation of NO

An ab initio calculation, using the R-matrix method, of the cross sections for the electron impact vibrational excitation of NO+(X 1+,v = 0) up to v = 5 is presented. Calculations have been carried out in both an adiabatic and a non-adiabatic approximation. It is shown that these two approximations produce dramatically different results. This is due to the effect of low-lying Feshbach resonances which can only be treated correctly using the non-adiabatic approach. Rates for vibrational excitation are also presented.

On the validity of the soft-photon approximation for electron-atom collisions in the presence of a laser field

The validity of the Kroll-Watson approximation to treat electron-atom collisions in the presence of a laser field is discussed. Numerical calculations have been carried out using classical mechanics and a model potential to describe electron-helium scattering for conditions of experimental interest. The effect of an additional polarization term, arising from the interaction between the laser field and that due to the electron, is shown to be negligible.

A non-adiabatic calculation of NO Rydberg states above several ionisation thresholds

An ab initio study of the ns, np, nd, nf and ng Rydberg series of NO above a number of ionisation thresholds is presented. Calculations of the eigenphase sum of electron scattering by NO+, using a non-adiabatic R-matrix method, are used to obtain vibrationally resolved positions and widths of NO autoionizing states. The complex quantum defects of the resonances are used to identify the Rydberg series, and these are separated according to the vibrational state of the target. Comparisons are made with experimental studies of this system

Electron scattering in a Yukawa potential in the presence of a high-frequency laser field

A numerical investigation of electron potential scattering in the presence of a high-frequency, high-intensity electromagnetic field has been carried out using classical mechanics. A Yukawa potential has been considered to simulate the electron-atom interaction. The high-frequency approximation, in which the electron moves in the so-called dressed potential has been employed. Both scattering angles and energy transfer from the field have been calculated. Strong fields lead to marked reductions in the scattering angle. Except at high frequencies large energy transfers are obtained, in the approximation that the energy transfer can be evaluated using trajectories in the dressed potential. Preliminary calculations on the validity of the high-frequency approximation show that it yields, within about 30%, the maximum energy transferred as the field phase is altered, but is poor for the detailed variation with phase.