{nu}{sub 4} band of CH{sub 4} in helium nanodroplets: A probe of the dynamical response of a superfluid

The triply degenerate {nu}{sub 4} bending mode of methane in superfluid helium nanodroplets was observed at 7.7 {mu}m by high-resolution infrared depletion spectroscopy. Five rotational transitions were detected, with a minimum linewidth of only 65 MHz. The observed lines showed slightly asymmetric line shapes, especially in the case of the P(2) and R(1) rotational lines. Analysis of the line shape indicates that the superfluid environment has a finite response time of a few nanoseconds to equilibrate upon rotational excitation of CH{sub 4}.