The Elusive Bound OH-Stretching First Overtone of Water Dimer

Water dimer is a classical example of an OH···O hydrogen bound complex and plays a critical role in the atmosphere. The bound OH-stretching (OH_b) fundamental transition displays the characteristic wavenumber redshift and intensity enhancement, observed repeatedly under many different experimental conditions. In contrast, the first OH_b-stretching overtone is predicted to be several orders of magnitude weaker in intensity. Recent full-dimensional ro-vibrational calculations have revised earlier theoretical predictions, suggesting that the OH_b-stretching first overtone may exhibit an intensity comparable to that of other observed OH-stretching transitions at these higher wavenumbers. Using jet expansion cavity ring-down spectroscopy in the predicted spectral region, we successfully observed this previously elusive transition. The observed transition wavenumber, intensity, and band shape show excellent agreement with the new theoretical predictions. The overlap of many closely lying ro-vibrational transitions, even at the cold temperature of the jet, gives rise to a relatively unstructured broad band. We determine the experimental OH_b-stretching first overtone vibrational band origin to be 7049 cm^-1 and estimate the lifetime to be about 10 ps.