Collisionally Assisted Spectroscopy of Water from 27 000 to 34 000 cm⁻¹

We report here an experimental approach that enables measurement of weak transitions to a wide range of rovibrational levels of water in the energy region 27 000−34 200 cm−1. We have previously demonstrated the use of laser double-resonance overtone excitation to access highly excited vibrational levels from single rovibrational states. Although this approach simplifies the assignment of the spectra, it strongly reduces the number of observed transitions and hence our ability to test theoretical predictions. Here, we increase significantly the number of observed transitions by allowing rotational relaxation of H2O at intermediate levels of the double-resonance excitation scheme to the levels of the same nuclear spin (ortho or para). Our recently developed semiempirical potential energy surface PES12 enables assignment of the resulting complex spectra and reproduction of the measured transitions with accuracy better than 1 cm−1.