The first observations of the four-photon ionization of H atoms at 364.7 nm via a three-photon resonance on the Lyman alpha transition are reported. The resonance ionization detection of H and D atoms in an H 2/D 2/O 2/Ar flame is demonstrated and H-atom density profiles are measured with good spatial resolution. A broadening of 2.2 cm −1 /GW cm 2 of the four-photon resonance by ac Stark shifting is observed.
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