Analysis of RABITT time delays using the stationary multi-photon molecular R-matrix approach

We employ the recently developed multi-photon R-matrix method for molecular above-threshold photoionization to obtain second-order ionization amplitudes that govern the interference in RABITT experiments. This allows us to extract RABITT time delays that are in better agreement with non-perturbative time-dependent simulations of this process than the typically used combination of first-order (Wigner) delays and asymptotic corrections. We calculate molecular-frame as well as orientation-averaged RABITT delays for H$_2$, N$_2$, CO$_2$, H$_2$O and N$_2$O and analyze the origin of various structures in the time delays including the effects of partial wave interference, shape resonances and orientation-averaging. Time-delays for B and C states of CO$_2^{+}$ are strongly affected by absorption of the second (IR) photon in the ion. This effect corresponds to an additional contribution, $\tau_{\text{coupl}}$, to the asymptotic approximation for the RABITT delays $\tau \approx \tau_{\text{mol}} +\tau_{cc} + \tau_{\text{coupl}}$. Applicability of the asymptotic theory depends on the target and IR photon energy but typically starts at approximately 30 -- 35 eV of XUV photon energy.