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

Abstract

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 〈J_a′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 ùB₁ 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.