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

Abstract

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 ${\mathrm{H}}_{2}, {\mathrm{N}}_{2}, {\mathrm{CO}}_{2}, {\mathrm{H}}_{2}\mathrm{O}$, and ${\mathrm{N}}_{2}\mathrm{O}$ 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 ${\mathrm{CO}}_{2}{}^{+}$ are strongly affected by absorption of the second (IR) photon in the ion. This effect corresponds to an additional contribution ${\ensuremath{\tau}}_{\text{coupl}}$ to the asymptotic approximation for the RABITT delays $\ensuremath{\tau}\ensuremath{\approx}{\ensuremath{\tau}}_{\text{mol}}+{\ensuremath{\tau}}_{cc}+{\ensuremath{\tau}}_{\text{coupl}}$. 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.