RESUMEN
The carbonyl sulfide (OCS) dimer serves as a prototype system for studying intermolecular forces between nonsymmetrical linear polyatomic molecules. Here, we performed a laser spectroscopic investigation of OCS dimers embedded in helium nanodroplets and found rovibrational bands corresponding to the non-polar "sulfur-in" and parallel polar dimers that have been extensively characterized in the gas phase, as well as a new non-polar "oxygen-in" dimer that has long been predicted by theory. Frequency alternations in the rotational branches along with the absence of a Stark effect provided important clues as to its assignment.
RESUMEN
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.
RESUMEN
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.