Two-photon excitation of then=3level in H and D atoms

Tunable coherent ultraviolet radiation has been used to excite selectively the $n=3\mathrm{}$ level in hydrogen and deuterium atoms via two-photon absorption from the ground state. The resulting Balmer-$\alpha$ fluorescence at 656 nm was observed as well as resonant three-photon ionization. This work demonstrates several advantages over other techniques for selective detection of neutral H and D atoms, including three-dimensional spatial resolution and a remote monitoring capability afforded by the use of laser-induced-fluorescence detection.