REMPI spectroscopy and predissociation of the 1B1(v = 0) rotational levels of H2O, HOD and D2O

Rotational analysis of the (2 + 1) resonance enhanced multiphoton ionization (REMPI) spectrum of the 1B1 Rydberg state of the water isotopomers H2O, HOD and D2O is reported. Spectroscopic parameters for the v = 0 vibrational level of the 1B1 state of the mixed isotopomer HOD are derived and its spectra are accurately simulated for the first time using the PGOPHER program. Simulation of two photon spectra of the 1B1–1A1 transition of HOD requires two transition moments, and the ratio of these is determined and explained by a simple geometrical model. Optimal transitions for state-selective detection of low energy rotational states are identified for all three molecules. Analysis of the linewidths in the present work, combined with previous work [H. H. Kuge and K. Kleinermanns, J. Chem. Phys., 1989, 90, 46–52; K. J. Yuan et al., Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 19148–19153; M. N. R. Ashfold et al., Chem. Phys., 1984, 84, 35–50; G. Meijer et al., J. Chem. Phys., 1986, 85, 6914–6922.], suggests that while a simple 〈Ja′2〉-dependent model for heterogeneous predissociation of the 1B1 Rydberg state accounts for much of the quantum number dependence, it is not sufficient for describing the predissociation in any of the three isotopomers. The component of the linewidth due to the homogeneous predissociation attributed to predissociation of the 1B1 by the Ã1B1 state was found to be significantly narrower than in previous work, indicating a longer lifetime of the 1B1 Rydberg state. The current work provides the basis for on-going studies of rotational energy transfer in the mixed isotopomers of water using the velocity map imaging technique.