| Summary: | We use the liquid microjet technique coupled with laser spectroscopy to measure the rotational and vibrational energy content of benzene spontaneously evaporating from a water-ethanol solution. We find different temperatures for rotation (206 K) and for the two low-lying vibrational modes, nu(6) (256 K) and nu(16) (229 K). Collision-induced energy-transfer measurements reveal efficient rotational relaxation, from which we deduce that the rotational temperature indicates the translational energy of the evaporate. Conversely, the relaxation of nu(6) is very inefficient, suggesting that the nu(6) temperature indicates the surface temperature of the liquid. Modeling the relaxation of nu(16) indicates that > 10(2) collisions are occurring during the transition from liquid to vacuum, which is an order of magnitude more than has been reported to occur in the gas phase immediately above the liquid surface. Our results reveal that evaporative molecular energy transfer involves many collisions, resulting in moderate collisional cooling as molecules pass from liquid to vapor.
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