Molecular RABITT time delays studied using the time-independent multi-photon R-matrix method

Numerical simulations of multiphoton processes in atoms and molecules typically involve solution of the computationally demanding time-dependent Schrödinger equation. Recently, we developed a computationally efficient time-independent theoretical method for calculation of ionization amplitudes for multiphoton ionization of atoms and molecules, including the above-threshold ionization of arbitrary order. This method is based on the  R-matrix splitting of the configuration space to inner region, where multi-electron methods of complete active space type are used, and the outer region, whose contributions are treated analytically.

In this contribution we apply the new method to calculation of RABITT time delays in molecules. We demonstrate that the access to accurate two-photon ionization amplitudes enables us to obtain the full RABITT time delays, directly comparable with time-dependent methods and with experiment. This is in contrast to methods used so far, which calculate the time delays measured by the two-photon experiment as a sum of the one-photon molecular delay and a so called measurement delay caused by absorption of the probe photon, which is calculated from the asymptotic theory. We illustrate that the accuracy of this asymptotic correction, compared to our two-photon method, decreases dramatically at low energies. Finally, we discuss the origin of several features in the energy dependence of the photoionization time delay and the effect of molecular orientation averaging on them.