Detection of atomic hydrogen by two-color laser-induced grating spectroscopy

We report a novel multiphoton scheme for the detection of atomic hydrogen. Interference of two laser beams near 243 nm crossed at a small angle induces a spatially modulated two-photon excitation (i.e., a grating of excited atoms), which diffracts a third laser beam tuned to either 486 or 656 nm to generate a coherent signal beam. We demonstrate the technique by making H-atom concentration measurements that compare quantitatively with those made using laser-induced fluorescence in low-pressure H(2)/O(2) flames.