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 H₂, N₂, 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 τcoupl to the asymptotic approximation for the RABITT delays τ≈τₘₒₗ+τcc+τcoupl. 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.