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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.