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

Abstract

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 RA-BITT 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 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 CO + 2 are strongly affected by absorption of the second (IR) photon in the ion. This effect corresponds to an additional contribution, τ coupl , to the asymptotic approximation for the RABITT delays τ ≈ τ mol + τcc + τ 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.

A polarization-consistent R-matrix model: application to the photoionization of molecules

“…Figure 6 shows the total photoionization cross-section for N 2 O. Across the energy range considered, there is good agreement between the PC-CHF(N) calculation and the CC model of (Benda et al 2022). Furthermore, both theories are in good agreement with the experimental results of (Brion and Tan 1979).…”

“…However, once we shift the first ionization potential to the experimental value we can see better agreement between the thresholds obtained from the PC-CHF(N) and CC calculations and the experimental data of Truesdale et al (1983). Each of the four SE thresholds are obtained from independent SE calculations and they are all shifted separately to match Benda et al (2022) and the experimental data of Truesdale et al (1983). Energies obtained by shifting the first ionization threshold to that of experiment are shown in parentheses.…”

“…The ionization thresholds obtained with the PC-CHF(N) model are shown in table 2. Comparing the unaltered thresholds with the accurate CC calculations of Benda et al (2022) we can see that the HF description is relatively poor. However, once we shift the first ionization potential to the experimental value we can see better agreement between the thresholds obtained from the PC-CHF(N) and CC calculations and the experimental data of Truesdale et al (1983).…”

“…The CC calculation of Benda et al (2022) uses the cc-pVTZ atomic basis set. For the continuum a BTO only basis set was used consisting of 30 B-splines.…”

“…N 2 O photoionization cross-section (left panels) and asymmetry parameters (right panels) for transitions into the first four ionic states, X 2 Π, A 2 Σ + , B 2 Π and C 2 Σ + . The smoothed (black solid) and raw (light grey) results of the PC-CHF(N) model are compared with SE (red dashed) calculations, the CC (blue dot-dash) calculations of Benda et al (2022) and the experimental data of Brion and Tan (1979) (red triangle), Carlson et al (1983) (orange circle) and Truesdale et al (1983) (green bar). obtained using the PC-CHF(N) and CC R-matrix models with the ePolyScat-D calculations of Fujimoto et al (2020) and experimental data of Fujimoto et al (2020), Leach et al (2002) and Schwell et al (2002).…”