Orientation-dependent phenomena in photoelectron angular distributions due to strong-field ionization of laser-irradiated diatomic molecules

We report about orientation-dependent effects arising in molecular {\it photoelectron angular distributions} (PAD) due to well pronounced contribution from ionization of inner molecular valence shell(s) to strong-field above-threshold ionization of laser-irradiated homonuclear diatomic molecules ($N_{2}$, $O_{2}$ and $F_{2}$). In particular, our calculation results, obtained within the Density-Functional-Theory based Strong-Field-Approximation [V. I. Usachenko, P. E. Pyak and V. V. Kim {\em Phys. Rev.} A {\bf 79} 116901 (2009)], suggest that within the high-intensity field domain ($I \ge 3\cdot 10^{14}$ $W/cm^{2}$) the molecular ionization dynamics for internuclear axis orientation angles $\pi/3 \le \Theta \le 2\pi/3$ (with respect to the incident laser field polarization) does become very pronounced and manifested by the well predominant contribution rather from the $1\pi_{u}$ inner shell than from the {\it highest occupied molecular orbital} (HOMO) ($3\sigma_{g}$ in $N_{2}$ or $1\pi_{g}$ in $O_{2}$ and $F_{2}$, normally predominantly contributing under standard cases).