Nonequilibrium Vibrational Distribution of PolyatomicMolecules Desorbed from an Ice Surface

Broadband rotational spectroscopy is coupled with buffer gas cooling to examine the population in low-frequency vibrational modes of isopropyl cyanide following temperature-programmed desorption from a neat ice. The measurements permit determination of the nascent vibrational temperature which is validated using analogous measurements of a room temperature sample. It is found that the two lowest frequency vibrational modes yield distinct vibrational temperatures which are significantly lower than the temperature of desorption. This is evidence of nonequilibrium dynamics in sublimation. Although long predicted, this is the first such evidence of this behavior for polyatomic molecules.