Laser Spectroscopy of Methanol Isotopologues in 4He Nanodroplets: Probing the Inertial Response around a Moderately Light Rotor.

High-resolution, mid-infrared spectra of methanol isotopologues (CH3OH, CH3OD, CD3OH, and CD3OD) embedded in superfluid helium nanodroplets have been obtained. For the normal isotopologue, we observed the CO stretching overtone band, the lines within which are 2× broader than in the fundamental for E species methanol and no different than the fundamental for A species methanol. For CH3OD, we observed the CO stretching overtone band for the first time, which was characterized by narrow line widths for both nuclear spin species. Spectra in the CD3 stretching bands were much broader, which is attributed to rapid relaxation to nearby anharmonically coupled vibrational state(s). Apparently the coupling is much stronger for CD3OD, for which the rotational substructure is completely washed out. Inertial analyses of the rotationally resolved fundamental and overtone bands reveal that the moment of inertia of helium, Δ IHe, that couples to rotation decreases in going from the heavier (CD3OH) to lighter (CH3OH) isotopologues (i.e., with decreasing gas phase moment of inertia, IG). The dependence of Δ IHe on IG is larger than that found for other molecules in regions approaching the heavy and light rotor limits, which suggests a relatively large breakdown in the adiabatic following of helium density for this moderately light rotor.