Harvey, A.; (2011) Electron re-scattering from aligned molecules using the R-matrix method. Doctoral thesis , UCL (University College London).

Abstract

Electron re-scattering in a strong laser field provides an important probe of molecular structure and processes, and can allow for time resolved study of nuclear and electronic dynamics at sub-femtosecond timescales with Angstrom spatial resolution. In such experiments; a molecule is ionised in a strong, few cycle, laser field. The changes in sign of the; laser field during the cycle can cause the electron to re-collide with its parent molecular ion. Under these circumstances the electron can either recombine leading to high harmonic generation, or it can be re-scattered. This scattering can be thought of as electron self-diffraction and the process has the potential to act as a detailed probe of the target molecule. It is usual for such experiments to be performed on aligned molecules, as the dynamics of the ionisation and re-collision changes with alignment. This introduces extra physics compared to the standard gas-phase, electron-molecule scattering problem. It is important for the understanding and analysis of such experiments to have a physically sound theoretical model of re-scattering which is capable of treating quantum mechanically the complicated scattering dynamics of an electron-molecular ion collision. This thesis explores the use of sophisticated ab initio quantum mechanical techniques to model this part of the re-scattering process. Previous theoretical models of the re-collision problem have thus greatly simplified this aspect of the problem. An introduction to attosecond physics, and a review of the relevant scattering and R-matrix theory is given. A simple preliminary model not including molecular alignment is described for molecules of experimental interest. Then the formalism for scattering from aligned linear molecules is presented. For linear molecules consisting of more than two atoms we use the polyatomic R-matrix codes. However the polyatomic version of the code only uses Abelian point groups which means that calculations on symmetric or asymmetric linear molecules are performed using the D2h or C2v point groups respectively. A further step is required, involving the reconstruction of T-matrices into the linear molecule symmetry groups D1h or C1v. The formalism for this is also presented. Finally differential and integral cross sections are presented for re-scattering for H2 and CO2.

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