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 2x broader than in the fundamental for E species methanol, and no different to the fundamental for A species methanol. For CH3OD, we observed the CO stretching overtone band for the first time, which was characterized by narrow linewidths 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, ΔIHe, that couples to rotation decreases in going from the heavier (CD3OH) to lighter (CH3OH) isotop...