Rotational dynamics of the methyl radical in superfluid ⁴He nanodroplets

Abstract

We report the ro-vibrational spectrum of the ν₃(e′) band of the methyl radical (CH₃) solvated in superfluid ⁴He nanodroplets. Five allowed transitions produce population in the NK = 0₀, 1₁, 1₀, 2₂ and 2₀ rotational levels. The observed transitions exhibit variable Lorentzian line shapes, consistent with state specific homogeneous broadening effects. Population relaxation of the 0₀ and 1₁ levels is only allowed through vibrationally inelastic decay channels, and the PP₁(1) and RR₀(0) transitions accessing these levels have 4.12(1) and 4.66(1) GHz full-width at half-maximum line widths, respectively. The line widths of the PR₁(1) and RR₁(1) transitions are comparatively broader (8.6(1) and 57.0(6) GHz, respectively), consistent with rotational relaxation of the 2₀ and 2₂ levels within the vibrationally excited manifold. The nuclear spin symmetry allowed rotational relaxation channel for the excited 1₀ level has an energy difference similar to those associated with the 2₀ and 2₂ levels. However, the PQ₁(1) transition that accesses the 10 level is 2.3 and 15.1 times narrower than the PR₁(1) and RR₁(1) lines, respectively. The relative line widths of these transitions are rationalized in terms of the anisotropy in the He-CH₃ potential energy surface, which couples the molecule rotation to the collective modes of the droplet.