FERM3D: A finite element R-matrix electron molecule scattering code

FERM3D is a three-dimensional finite element program, for the elastic scattering of a low energy electron from a general polyatomic molecule, which is converted to a potential scattering problem. The code is based on tricubic polynomials in spherical coordinates. The electron-molecule interaction is treated as a sum of three terms: electrostatic, exchange. and polarisation. The electrostatic term can be extracted directly from ab initio codes ({\sc{GAUSSIAN 98}} in the work described here), while the exchange term is approximated using a local density functional. A local polarisation potential based on density functional theory [C. Lee, W. Yang and R. G. Parr, {Phys. Rev. B} {37}, (1988) 785] describes the long range attraction to the molecular target induced by the scattering electron. Photoionisation calculations are also possible and illustrated in the present work. The generality and simplicity of the approach is important in extending electron-scattering calculations to more complex targets than it is possible with other methods.Comment: 30 pages, 4 figures, preprint, Computer Physics Communications (in press

Vibrational interference of Raman and high-harmonic generation pathways

Experiments have shown that the internal vibrational state of a molecule can affect the intensity of high harmonic light generated from that molecule. This paper presents a model which explains this modulation in terms of interference between different vibrational states occurring during the high harmonic process. In addition, a semiclassical model of the continuum electron propagation is developed which connects with rigorous treatments of the electron-ion scattering

Renner-Teller effects in HCO+ dissociative recombination

A theoretical description of the dissociative recombination process for the HCO+ ion suggests that the nonadiabatic Renner-Teller coupling between electronic and vibrational degrees of freedom plays an important role. This finding is consistent with a recent study of this process for another closed-shell molecule, the H3+ ion, where Jahn-Teller coupling was shown to generate a relatively high rate. The cross section obtained here for the dissociative recombination of HCO+exhibits encouraging agreement with a merged-beam experiment.Comment: 11 page

Low energy electron scattering from DNA and RNA bases: shape resonances and radiation damage

Calculations are carried out to determine elastic scattering cross sections and resonance energies for low energy electron impact on uracil and on each of the DNA bases (thymine, cytosine, adenine, guanine), for isolated molecules in their equilibrium geometry. Our calculations are compared with available theory and experiment. We also attempt to correlate this information with experimental dissociation patterns through an analysis of the temporary anion structures that are formed by electron capture in shape resonances.Comment: 20 pages, 12 figures, submitted to J. Chem. Phy

Quantitative Rescattering Theory for high-order harmonic generation from molecules

The Quantitative Rescattering Theory (QRS) for high-order harmonic generation (HHG) by intense laser pulses is presented. According to the QRS, HHG spectra can be expressed as a product of a returning electron wave packet and the photo-recombination differential cross section of the {\em laser-free} continuum electron back to the initial bound state. We show that the shape of the returning electron wave packet is determined mostly by the laser only. The returning electron wave packets can be obtained from the strong-field approximation or from the solution of the time-dependent Schr\"odinger equation (TDSE) for a reference atom. The validity of the QRS is carefully examined by checking against accurate results for both harmonic magnitude and phase from the solution of the TDSE for atomic targets within the single active electron approximation. Combining with accurate transition dipoles obtained from state-of-the-art molecular photoionization calculations, we further show that available experimental measurements for HHG from partially aligned molecules can be explained by the QRS. Our results show that quantitative description of the HHG from aligned molecules has become possible. Since infrared lasers of pulse durations of a few femtoseconds are easily available in the laboratory, they may be used for dynamic imaging of a transient molecule with femtosecond temporal resolutions.Comment: 50 pages, 15 figure

Limits of the Plane Wave Approximation in the Measurement of Molecular Properties

Rescattering electrons offer great potential as probes of molecular properties on ultrafast timescales. The most famous example is molecular tomography, in which high harmonic spectra of oriented molecules are mapped to ``tomographic images'' of the relevant molecular orbitals. The accuracy of such reconstructions may be greatly affected by the distortion of scattering wavefunctions from their asymptotic forms due to interactions with the parent ion. We investigate the validity of the commonly used plane wave approximation in molecular tomography, showing how such distortions affect the resulting orbital reconstructions

High Harmonic Generation in SF6_{6}: Raman-excited Vibrational Quantum Beats

In a recent experiment (N. Wagner et al., PNAS v103, p13279) on SF$_{6}$, a high-harmonic generating laser pulse is preceded by a pump pulse which stimulates Raman-active modes in the molecule. Varying the time delay between the two pulses modulates high harmonic intensity, with frequencies equal to the vibration frequencies of the Raman-active modes. We propose an explanation of this modulation as a quantum interference between competing pathways that occur via adjacent vibrational states of the molecule. The Raman and high harmonic processes act as beamsplitters, producing vibrational quantum beats among the Raman-active vibrational modes that are excited by the first pulse. We introduce a rigorous treatment of the electron-ion recombination process and the effect of the ionic Coulomb field in the electron propagation outside the molecule, improving over the widely-used three-step model.Comment: submitted to PR