Sensitive quantum state selective detection of H2O and D2O by (2+1)-resonance enhanced multiphoton ionization

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Abstract
The first observation of (2+1)‐REMPI of H2O and D2O is reported. With the use of a high power tunable excimer laser radiating at 248 nm, the H2O and D2O molecules are ionized after resonant two‐photon absorption into the predissociated C̃ 1B1 state. The clearly observable peaks in the (2+1)‐REMPI spectra are all identified and can be used for sensitive state selective detection. Parent molecular fluorescence excitation spectra (C̃ 1B1→Ã 1B1) were remeasured over an increased spectral range, and are remarkably the same as the (2+1)‐REMPI spectra. Furthermore the OH/OD (A 2Σ+, v’=0→X 2Π, v‘=0) photofragment fluorescence excitation spectra were measured, and these spectra do not show any nonresonant background as stated before. Additional fluorescence bands starting from the vibrationally excited A 2Σ+, v’=1 state were observed. Simulation of the observed REMPI and fluorescence excitation spectra yields the branching ratios for the predissociation, ionization, and fluorescence processes.