(2+1) Resonance-enhanced multiphoton ionization (REMPI) and (2+1 ') mass-analyzed threshold ionization (MATI) spectroscopy of H₂O

Rotationally resolved MATI spectra of H₂O, recorded using two-colour (2 + 1′) multiphoton excitation via the C̃ state, are reported. The spectroscopy provides a possible route to clean state-selection of rovibronic levels of the H₂O⁺ ion. The rotational line intensities observed in the spectra are discussed in relation to previous photoelectron spectra and theoretical calculations. The observation of intense ΔK_a = even transitions is attributed to a large contribution to the transition moment from an nf←nd component. Room temperature and jet-cooled (2 + 1) REMPI spectra of H₂O are also reported for the region 80000–92000 cm−1. These involve two-photon transitions from the ground X̃ ¹A₁ state to various vibrational levels of the C̃ ¹B₁, D̃̃′ ¹B₁ and D̃̃″ ¹A₂ states as well as the first linear ¹B₂ state.