One- and two-photon ionization cross sections of the laser excited 6s6p^1P_1 state of barium

TL;DR

This paper combines experimental measurements and theoretical calculations to analyze one- and two-photon ionization cross sections of excited barium, revealing detailed resonance structures and validating computational methods for heavy alkaline earth elements.

Contribution

It provides the first combined experimental and theoretical analysis of barium's ionization cross sections, demonstrating the effectiveness of R-matrix and quantum defect methods in complex atomic systems.

Findings

Accurate reproduction of resonance structures in ionization spectra.

Validation of computational methods against experimental data.

Foundation for phase-controlled interference studies in ionization.

Abstract

Stimulated by a recent measurement of coherent control in photoionization of atomic barium, we have calculated one- and two-photon ionization cross sections of the aligned 6s6p^1P_1 state of barium in the energy range between the 5d_{3/2} and 5d_{5/2} states of Ba^+. We have also measured these photionization spectra in the same energy region, driving the one- or two-photon processes with the second or first harmonic of a tunable dye laser, respectively. Our calculations employ the eigenchannel R-matrix method and multichannel quantum defect theory to calculate the rich array of autoionizing resonances in this energy range. The non-resonant two-photon process is described using lowest-order perturbation theory for the photon-atom interactions, with a discretized intermediate state one-electron continuum. The calculations provide an absolute normalization for the experiment, and they accurately reproduce the rich resonance structures in both the one and two-photon cross sections, and confirm other aspects of experimental observations. These results demonstrate the ability of these computationally inexpensive methods to reproduce the experimental observables in one- and two-photon ionization of heavy alkaline earths, and they lay the groundwork for future studies of the phase-controlled interference between one-photon and two-photon ionization processes.