A study into the internal energy distributions of molecules liberated from an in vacuo liquid surface.

Abstract

We use the liquid micro-jet technique coupled with laser spectroscopy to measure the rotational and vibrational energy content of benzene liberated from an in vacuo water-ethanol solution. A comparison is made between the internal temperatures of benzene molecules that spontaneously evaporate from the liquid surface and those that are laser desorbed by resonant IR light. In both cases it was found that rotations are cooled significantly more than the lowest vibrational modes and that the rotational energy distributions were Boltzmann. Within error, the rotational temperatures of the vibrationally excited molecules were the same as the vibration-less ground state in both systems. Independent, 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) are an indication of the translational energy of the evaporate. Conversely, relaxation of ν₆ is found to be very inefficient, suggesting that the ν₆ temperature (260-270 K) is an indication of the surface temperature of the liquid. Modelling the relaxation of ν₁₆ indicates that >10² collisions are occurring during the transition from liquid to vacuum, an order of magnitude more than have been reported to occur in the gas phase immediately above the liquid surface. The temporal distribution of internal temperatures within the plume of laser desorbed molecules reveals that the coldest molecules are found close to the front of the expansion, and reach temperatures 60 to 100 K lower than the liquid surface temperature. Spectroscopic analysis of the mass spectra of desorbed products revealed that clusters were formed post benzene ionisation, in ion-dipole association reactions. The position of maximum cluster formation within the desorption plume was found to be a compromise between the point of maximum internal cooling and the highest translational velocities. Best estimates of the translational temperature of the desorption plume were achieved through 3D Monte Carlo simulations of the gas expansion. The simulation revealed the plume can be described with a temperature of ~200 K and travelling with a bulk velocity of ~100 m.s⁻ ¹.