The OH stretching spectrum of warm water clusters.

The O-H stretching fundamental spectra of water clusters are well known from cold jet expansions, but their detection in the atmosphere remains elusive. Mild supersonic jet expansions of neat water vapor are used to detect trimers and tetramers with intermediate thermal excitation. Spectra close to the evaporative limit are obtained by interfering neighboring expansion fans in a multi slit nozzle experiment. They are interpreted in terms of ring opening in the water trimer, whereas the tetramer spectra can also be interpreted by conventional hot band contributions and thermal excitation of librational modes.