Rotational and vibrational dynamics of methane in helium nanodroplets

High resolution infrared laser spectroscopy has been used to study the rotational and vibrational dynamics of methane in helium nanodroplets. Because of the nuclear spin symmetry, four transitions (P(1),Q(1),R(0) and R(1)) are observed. While the first three have widths of approximately 0.2 cm−1, which we attribute to moderately fast vibrational relaxation, the significantly larger width of the R(1) transition suggests even faster rotational relaxation for the latter. At higher methane pressures in the pick-up cell, three bands are observed that are attributed to clusters of methane. The spectra suggest that nearly free rotation of the monomer sub-units is still occurring in these larger clusters.