Perturbative and nonperturbative photoionization of H 2 and H 2 O using the molecular R -matrix-with-time method

The ab initio $R$-matrix with time method has recently been extended to allow simulation of fully nonperturbative multielectron processes in molecules driven by ultrashort arbitrarily polarized strong laser fields. Here we demonstrate the accuracy and capabilities of the current implementation of the method for two targets: We study single-photon and multiphoton ionization of ${\mathrm{H}}_{2}$ and one-photon and strong-field ionization of ${\mathrm{H}}_{2}\mathrm{O}$ and compare the results to available experimental and theoretical data as well as our own time-independent $R$-matrix calculations. We obtain a highly accurate description of total and state-to-state single-photon ionization of ${\mathrm{H}}_{2}\mathrm{O}$ and, using a simplified coupled-channel model, we show that state coupling is essential to obtain qualitatively correct results and that its importance as a function of laser intensity changes. We find that electron correlation plays a more important role at low intensities (up to approximately 50 $\text{TW}/{\mathrm{cm}}^{2}$).