Theoretical study of orientation-dependent multiphoton ionization of polyatomic molecules in intense ultrashort laser fields: A new time-dependent Voronoi-cell finite difference method

We present a new grid-based time-dependent method to investigate multiphoton ionization (MPI) of polyatomic molecules in intense ultrashort laser fields. The electronic structure of polyatomic molecules is treated by the density-functional theory (DFT) with proper long-range potential and the Kohn-Sham equation is accurately solved by means of the Voronoi-cell finite difference method on non-uniform and highly adaptive molecular grids utilizing geometrical flexibility of the Voronoi diagram. This method is generalized to the time-dependent problems with the split-operator time-propagation technique in the energy representation, allowing accurate and efficient non-perturbative treatment of attosecond electronic dynamics in strong fields. The new procedure is applied to the study of MPI of N2 and H2O molecules in intense linearly-polarized and ultrashort laser fields with arbitrary field-molecule orientation. Our results demonstrate that the orientation dependence of MPI is determined not just by the highest-occupied molecular orbital (HOMO) but also by the symmetries and dynamics of other contributing molecular orbitals. In particular, the inner orbitals can show dominant contributions to the ionization processes when the molecule is aligned in some specific directions with respect to the field polarization. This feature suggests a new way to selectively probe individual orbitals in strong-field electronic dynamics.