Quantum cascade laser spectroscopy of OCS isotopologues in 4He nanodroplets: A test of adiabatic following for a heavy rotor.

We report high-resolution infrared spectra of OCS isotopologues embedded in helium nanodroplets that were recorded with a newly built spectrometer. For the normal isotopologue, we observed the relatively weak third bending overtone band, in addition to new high J transitions in the C-O stretching fundamental, which has previously been investigated by diode laser spectroscopy [S. Grebenev et al., J. Chem. Phys. 112, 4485 (2000)]. Similar to the gas phase, the overtone band is (only) 45 cm-1 higher in energy than the fundamental, and this leads to additional broadening due to rapid vibrational relaxation that is accompanied by the creation of real/virtual phonon excitations. We also observed spectra in the C-O stretching fundamental for several minor isotopologues of OCS, including 18OCS, O13CS, and OC33S, in addition to some new peaks for OC34S. A rovibrational analysis allowed for determination of the moment of inertia of helium (ΔIHe) that couples to the rotation of OCS for each isotopologue. In the context of the adiabatic following approximation, the helium density structure that follows the rotation of OCS should essentially remain unchanged between the isotopologues, i.e., there should be no dependence of ΔIHe on the gas phase moment of inertia of OCS (IG). While this behavior was expected for the "heavy" OCS rotor investigated here, we instead found an approximately linear 1:1 relation between ΔIHe and IG, which suggests partial breakdown of the adiabatic following approximation, making OCS the heaviest molecule for which evidence for this effect has been obtained.