Determination of the 1s–2s two-photon excitation cross-section in atomic hydrogen

Hydrogen atoms are ablated from zirconium alloys into the gas phase by a pulsed Nd:YAG laser and photo-ionized with three photons at 243 nm via the two-photon 1s 2S1/2–2s 2S1/2 resonant transition. A determination of the effective 1s–2s two-photon excitation cross-section is necessary to quantify the hydrogen atom density in the ablation plume. A measurement of the ion signal vs. photo-ionization beam energy is fitted to an expression derived from the rate equations. The temporal and spatial properties of the photo-ionization laser beam, transit of the H atoms through the beam, and detector geometry are taken into account. The effective two-photon cross-section for this experimental configuration, derived with the rate equation formalism, is 3.3±0.8×10−28 cm4 W−1. This compares well with the ab initio prediction of 5±1×10−28 cm4 W−1 under these experimental conditions.