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, ν6, is very inefficient, suggesting that the ν6 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.