Atomically thin superfluid helium films on solid hydrogen

Unexpected Mass Decoupling for 3He-4He Films on a Solid Hydrogen Substrate

Quartz oscillator measurements reveal new behavior in 3He-4He mixture films on a H2 substrate. For mixture films of total coverage greater than one monolayer, in addition to the expected Kosterlitz-Thouless transition, a second mass decoupling event is observed, which behaves differently from the Kosterlitz-Thouless transition.

Spectroscopic Studies of OCS-Doped 4He Clusters with 9−72 Helium Atoms: Observation of Broad Oscillations in the Rotational Moment of Inertia

High-resolution spectra of HeN-OCS clusters with N up to 39 in the microwave region and up to 72 in the infrared region were observed with apparatus-limited line widths of about 15 kHz and 0.001 cm(-1), respectively. The cold (approximately 0.2 K) clusters were produced in pulsed supersonic jet expansions of very dilute OCS + He mixtures and probed using a microwave Fourier transform spectrometer or a tunable infrared diode laser spectrometer. Consistent analyses of the microwave and infrared data yield band origins for the carbonyl stretching vibration, together with rotational parameters for the ground and excited vibrational states. The rotational constant, B, passes through a minimum at N = 9 and then rises as the He atoms uncouple from the OCS rotational motion as a result of superfluid effects. There are broad unexpected oscillations in B, with maxima at N = 24 and 47 and minima at N = 36 and 62. The change in B upon vibrational excitation, which is negative for the OCS molecule, converges to positive values for N > 15. These results help to bridge the gap between individual molecules and bulk matter with atom-by-atom resolution over a significant range of cluster sizes.

Multiple solvation configurations around phthalocyanine in helium droplets

Recent measurements of the emission spectrum of phthalocyanine solvated in superfluid helium nanodroplets exhibit a constant 10.3 cm(-1) splitting of each emission line relative to the absorption spectrum. This splitting has been attributed to two distinct helium environments near the surface of the phthalocyanine molecule. Rigid-body path-integral Monte Carlo provides a means of investigating the origin of the splitting on a detailed microscopic level. Path-integral Monte Carlo simulations of 4He(N)-phthalocyanine at 0.625 K with N ranging from 24 to 150 show two distinct helium configurations. One configuration is commensurate with the molecular substrate and the other is a triangular lattice. We investigate the energetics of these two configurations and use a method for calculating electronic spectral shifts for aromatic molecule-rare-gas clusters due to dispersive interactions to estimate the spectral splitting that would arise from the two helium configurations seen for N=150. The results are in reasonable agreement with the experimentally measured splitting, supporting the existence of two distinct local helium environments near the surface of the molecule in the nanodroplets.

Substrate-tuned boson localization in superfluid (4)He films

We have studied the influence of the substrate on superfluidity in thin 4He films. The inert coverage, the amount of nonsuperfluid 4He localized by the substrate, is found to scale with the well depth of the 4He-substrate interaction potential for six different substrates. Although the inert coverages for these substrates differ by more than a factor of 4, the phase boundary in the temperature-coverage plane is found to have a universal shape.