The dynamics of evaporation from a liquid surface

We explore the collisional energy transfer dynamics of benzene molecules spontaneously evaporating from an in vacuo water -ethanol liquid beam. We find that rotations are cooled significantly more than the lowest-energy vibrational modes, while the rotational energy distributions are Boltzmann. Within experimental uncertainty, the rotational temperatures of vibrationally-excited evaporating molecules are the same as the ground state. Collision-induced gas phase energy transfer measurements reveal that benzene undergoes fast rotational relaxation, from which we deduce that the rotational temperature measured in the evaporation experiments (200-230 K) is an indication of the translational energy of the evaporate. Conversely, vibrational relaxation of the high frequency mode, m6, is very inefficient, suggesting that the m6 temperature (260-270 K) is an indication of the liquid surface temperature. Modelling of the relaxation dynamics by both 'temperature gap' and 'Master Equation' approaches indicates that the equivalent of 150-260 hard-sphere collisions occur during the transition from liquid to vacuum.