The role of electron correlation in strong-field ionization of molecules studied using the molecular R-matrix with time approach

We apply the recently developed molecular R-matrix with time (RMT) approach to show explicitly for the case of water that electron correlation in strong field ionization is more important at lower intensities and we observe the transition from below- to above-barrier ionization regime.

The role of electron correlation in single photon ionization has been studied in detail for decades but it is much harder to do the same in the strong field regime. On the theory side the challenges are to represent simultaneously the ionized electron, the electronic structure of the target molecule and the multi-electron interaction; all in the presence of the time-dependent strong field. For that reason a number of theoretical approaches have been developed with different strengths and weaknesses.

Recently, we have extended to molecules the originally atomic R-matrix method with time. The method works in the Close-Coupling representation which allows studying the role of state coupling and electron correlation in detail. We use it to show that at intensities up to 50 TW/cm² state coupling determines the shape of the polarization-orientation-dependent yields: when the ionic states are coupled the yields always resemble the shape of the HOMO even though the involved states are not dipole-coupled. This effect is caused by the field-free and field-assisted electron correlation.