Rotational cooling and translational trapping during the thermal evaporation of free nitrogen clusters

Thermal evaporation of free nitrogen clusters is investigated using molecular dynamics simulations. Energy distributions of molecules evaporating from (N2)64 clusters at initial temperatures between 50 and 190 K are reported. Rotational energy distributions always follow single temperature Boltzmann distributions. The resulting rotational temperatures first increase with cluster temperature and then saturate at about 87 K. For the translation, Maxwell–Boltzmann distributions permit an adequate description for initial cluster temperatures above 90 K where the resulting translational temperatures roughly correspond to the initial cluster temperatures. For lower cluster temperatures, however, non-thermal behavior is observed and attributed to a velocity-dependent recapture of the slowest evaporating molecules.