Strong-field ionization of aligned molecules

Theoretical studies of strong-field ionization of molecules are impeded by the complexity of the molecular electronic structure. Up till now, full ab initio calculations of the alignmentdependent ionization are available only for H2 and H2. For larger molecules, despite a tremendous amount of experiments, no ab initio calculations are available, and the most widely used approaches to explain strong-field processes are the molecular tunneling theory and strongfield approximation. Calculations of alignmentdependent ionization yields based on these theories fail to explain recent experiments: Tunneling theory and strong-field approximation predict the ionization yield to follow the electron density of the initial electronic state, in contrast with observations for the CO2 molecule. [1] In [2], we use ab initio theory within the single-active electron approximation to investigate the response of polyatomic molecules to intense femtosecond laser pulses. Our approach is grid based, which is the most widely used approach in strong-field physics, and takes input potentials from standard quantum chemistry codes. We only consider the dynamics of the outermost electron (the HOMO orbital): the remaining electrons are accounted for by an effective potential. The computed ionization yields are shown in Fig. 1 for CO2 and CS2 as a function of the angle β. The orientation-dependencies of ionization are generally similar for the two molecules. For CO2 and CS2, the ionization yields are largest at 54±3◦ and 48±3◦, respectively. These results are in unprecedented agreement with recent experiments [1], which predict the ionization yields to peak at about 46◦. Our approach is clearly superior to the tunneling theory (cf. the figure) and the strong-field approximation (results not shown here) which predict the ionization yields for CO2 to peak at about 25◦.