High-resolution infrared and microwave spectra of He(N)-carbonyl sulfide (He(N)-OCS) clusters with N ranging from 2 to 8 have been detected and unambiguously assigned. The spectra show the formation of a solvation layer beginning with an equatorial "donut" of five helium atoms around the OCS molecule. The cluster moment of inertia increases as a function of N and overshoots the liquid droplet limit for N > 5, implying that even atoms in the first solvation shell are decoupled from the OCS rotation in helium nanodroplets. To the extent that this is due to superfluidity, the results directly explore the microscopic evolution of a phenomenon that is formally macroscopic in nature.

Infrared spectra of HeN–CO complexes with N up to about 20 have been observed in the 2145 cm−1 region of the C–O stretch vibration using a tunable diode laser spectrometer to probe pulsed supersonic expansions from moderately high pressure (⩽40 atm) cooled (>−150 °C) jet sources. Cooler (⩽0.2 K) or warmer (⩽0.5 K) effective rotational temperatures were obtained using pinhole or slit jet nozzles, respectively. Two series of R(0) transitions were observed, each correlating smoothly with the known a-type (K=0←0) and b-type (K=1←0) R(0) lines of the binary complex, He–CO. Although the b-type series starts off about 7 times stronger for N=1, it was observed to lose intensity to the a series with increasing N-value. The numbering of cluster size was reliably established up to N=14 for the a-type and N=6 for the b-type series. Some warmer lines due to higher J-value transitions [e.g., R(1)] were observed and tentatively assigned, but these were not sufficient to enable rotational analysis. Thus it has not yet been possible to separate the effects of vibrational shifts and rotational dynamics on the line positions. Two critical regions were observed in the cluster size evolution around N=7 and 15, and these may be related to the theoretically calculated maximum and minimum, respectively, in the incremental binding energy per helium atom.

High resolution microwave and infrared spectra of He(N)-N2O clusters were studied in the range N=3 to 12. The apparent cluster moments of inertia increase from N=3 to 6, but then decrease, showing oscillatory behavior for N=7 to 12. This provides direct experimental evidence for the decoupling of helium atoms from the rotation of the dopant molecule in this size regime, signaling the transition from a molecular complex to a quantum solvated system and directly exploring the microscopic evolution of "molecular superfluidity."

Rovibrational spectroscopy of molecules boasts extremely high precision, but its usefulness relies on the assignment of spectral features to corresponding quantum mechanical transitions. In the case of ortho-H(2)-CO, a weakly bound complex abundant in the interstellar medium (although not yet observed there), the rather complex spectrum has been unexplained for more than a decade. We assigned this spectrum by comparison with a purely ab initio calculation. For most lines, agreement to within 0.01 centimeter(-1) between experiment and theory was achieved. Our results show that the applicability of rovibrational spectroscopy can be extended with the assistance of high-accuracy quantum mechanical computations.

We present high resolution spectra of He(N)-OCS clusters with N up to 39 in the microwave and 72 in the infrared regions, observed with apparatus-limited line widths of about 15 kHz and 0.001 cm(-1), respectively. The derived rotational constant, B (proportional to the inverse moment of inertia), passes through a minimum at N=9, then rises due to onset of superfluid effects, and exhibits broad oscillations with maxima at N=24, 47 and minima at 36, 62. We interpret these unexpected oscillations as a manifestation of the aufbau of a nonclassical helium solvation shell structure. These results bridge an important part of the gap between individual molecules and bulk matter with atom by atom resolution, providing new insight into microscopic superfluidity and a critical challenge for theory.