Ultrasensitive detection of molecular hydrogen is reported by using Doppler-free (2+1) resonance-enhanced multiphoton ionization through the E,F 1Σ+g state. By using an arrangement with two near-transform-limited counterpropagating laser beams, a single-shot detection efficiency of 6.8×103 molecules/cm3 has been demonstrated. Frequency scans of the two-photon transitions show that the detection efficiency is limited by ac Stark effects.
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Vrakking et al. (1993) studied this question.
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