Quantum Solvation of Carbonyl Sulfide with Helium Atoms

Abstract

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.

Bridging the Gap between Small Clusters and Nanodroplets: Spectroscopic Study and Computer Simulation of Carbon Dioxide Solvated with Helium Atoms

“…For 4 He N -OCS, B decreases in the range N < 8, undershooting its nanodroplet value, which implies a subsequent turnaround at larger cluster sizes [10]. Quantum Monte Carlo (QMC) calculations of rotational excitations of the cluster confirm the observed undershoot, locating the minimum of B at N = 6 [14] or 8-9 [15].…”

“…1(b), and it appears that this value will be approached from above as N goes from 17 to 5000. For He N -OCS, B decreases steadily from N = 0 to 8, passing through the nanodroplet value at about N = 5, and its eventual turnaround has not yet been observed experimentally [10]. For He N -N 2 O, the variation of B is rather similar to that of He N -CO 2 , but with the significant difference that B always remains above its nanodroplet value for He N -N 2 O [11].…”

“…Recently, measurements of the evolution of rotational constants with the size of the cluster have been reported [10,11]. For 4 He N -OCS, B decreases in the range N < 8, undershooting its nanodroplet value, which implies a subsequent turnaround at larger cluster sizes [10].…”

“…The apparatus was similar to that used previously in our work on He clusters [10,16]. A tunable infrared diode laser is used to probe a pulsed supersonic jet expansion.…”

Excimers in the Lowest Rotational Quantum State in Liquid Helium

“…However, a great deal of information has been derived from experiments with helium clusters [1][2][3][4] and droplets in beams, [5][6][7] where single foreign molecules can be embedded using the pick-up method. 5,8 Infrared spectroscopy of these molecules shows sharp lines, indicating free rotation and apparent superfluidity of 4 He droplets.…”