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 Is S-2(1/2)-2s S-2(1/2) 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 x 10(-28) cm(4) W-1. This compares well with the ab initio prediction of 5 +/- 1 x 10(-28) cm(4) W-1 under these experimental conditions. Crown Copyright (C) 2000 Published by Elsevier Science B.V. All rights reserved.