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

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.