Orientation-Dependent Multiphoton Ionization of Diatomic Molecules: Many-Electron Time-Dependent Density Functional Approach

We present time-dependent density functional calculations of multiphoton ionization (MPI) of diatomic molecules N$_{2}$, O$_{2}$, and F$_{2}$ with arbitrary orientation of the molecular axis by intense linearly-polarized laser pulses with the wavelength 800~nm. For the peak intensity $1\times 10^{14}$~W/cm$^{2}$, the inner shells contributions to the MPI probability are quite significant for N$_{2}$ molecule, particularly at intermediate angles, while for O$_{2}$ the highest-occupied molecular orbital contribution is still dominant. Our calculations performed on F$_{2}$ molecule at the peak intensities $2\times 10^{14}$~W/cm$^{2}$ and $1\times 10^{15}$~W/cm$^{2}$ show that the effect of the inner shell electrons on MPI becomes more important with increasing intensity of the laser field, and orbital switching may occur: the contributions of spin orbitals with larger ionization potentials exceed those of spin orbitals with smaller ionization potentials. The orientation dependence of the total MPI probability becomes more isotropic for stronger laser field.