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

We employ the recently developed multiphoton 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 nonperturbative 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, CO2, H2O, and N2O 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 CO2+ are strongly affected by absorption of the second (IR) photon in the ion. This effect corresponds to an additional contribution tau(coupl) to the asymptotic approximation for the RABITT delays tau approximate to tau(mol) + tau(c) + tau(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.