In the tunneling regime and at sufficiently low field amplitudes, the shape of orientation-dependent molecular ionization rate curves usually resembles the shape of the ionized orbital. As the ionizing field strength increases, the shape of the ionization rate can deviate from this pattern. The oft-cited explanation is that the increasing contribution of excited states relative to the ground state modifies the distribution. In this paper we show that orbital distortion and parabolic channel effects, which are independent of excited-state effects, can also significantly modify the angular dependence of the yields of widely studied molecules where excited state effects are negligible. For example, as the ionizing field increases we find that (1) in CO, orbital distortion is responsible for the switch of the position of the global maximum in the orientation-dependent ionization rate to the opposite orientation, and (2) in ${\mathrm{CH}}_3\mathrm{Br}$, the interplay between orbital distortion and parabolic channel effects transforms a local minimum to a local maximum. To simulate orbital distortion and parabolic channel effects, we use the one-electron weak-field asymptotic theory including the first-order correction [OE-WFAT(1)] in the integral representation. Since OE-WFAT(1) incurs expensive computations when the number of orientation angles is large, we also reformulate the original OE-WFAT(1) algorithm into a partial-wave expansion form, which greatly enhances the efficiency of the method.

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• Electronic excitation & ionization
• Multiphoton or tunneling ionization & excitation
• Strong field ionization & excitation
• Molecules
• Adiabatic approximation