Alignment-dependent above-threshold ionization of molecules

Within the molecular strong-field approximation, we study the effect of molecular alignment with respect to the linear polarization of an external electromagnetic field in above-threshold ionization rates for some diatomic and polyatomic molecules. We find that the shape of the above-threshold photoelectron ionization spectrum is largely independent of the orientation of the major molecular axis for N-2, meaning that the spectra can be put on top of each other when appropriately scaled. This is also our general finding for electrons emitted along the polarization direction for other molecules, e. g., O-2, C2H4 and C6H6, although at some special geometries ionization is suppressed by symmetry selection rules. For N-2, our calculations support recent experimental findings in N-2 by Zeidler et al (2006 J. Phys. B: At. Mol. Opt. Phys. 39 L159).