Resonance-enhanced multiphoton ionization for diagnosis of a weakly ionized plasma

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Abstract
We describe an experimental setup in which resonance-enhanced multiphoton ionization (REMPI) is used for diagnosis of neutral particles (atoms or molecules) effusing from a plasma. Quantum-state-specific detection, achieved by selective ionization with the help of REMPI, yields the distribution over their internal degrees of freedom. Discrimination against nonlaser-produced ions is performed by means of the combination of an electric and a magnetic field. The applicability of the setup is demonstrated by the determination of the rotational temperature of ground-state hydrogen molecules in a multicusp ion source. Under nondischarge conditions a detection limit is achieved of ≊2×1010 state-selected hydrogen molecules/cm3, corresponding to a value of ≊7×108/cm3 at the analysis region. Under discharge conditions, this detection limit is deteriorated by the presence of metastable hydrogen molecules, which are ionized in a one-photon process.