Effects of orientation and alignment in high-order harmonic generation and above-threshold ionization

When molecules interact with light sources of femtosecond or shorter duration the rotational degrees of freedom are frozen during the response to the strong nonperturbative interaction. We analytically derive how the frozen degrees of freedom affect the measurable signals in high-order harmonic generation and above-threshold ionization. High-order harmonic generation exhibits optical coherence in the signal from different orientations of the molecule. For ionization, the contributions from different orientations are added incoherently. The consequences of these findings are illustrated by numerical results.