Bias in the temperature of helium nanodroplets measured by an embedded rotor

The ro--vibrational spectra of molecules dissolved in liquid $^4$He nanodroplets display rotational structure. Where resolved, this structure has been used to determine a temperature that has been assumed to equal that of the intrinsic excitations of the helium droplets containing the molecules. Consideration of the density of states as a function of energy and total angular momentum demonstrates that there is a small but significant bias of the rotor populations that make the temperature extracted from a fit to its rotational level populations slightly higher than the temperature of the ripplons of the droplet. This bias grows with both the total angular momentum of the droplet and with the moment of inertia of the solute molecule.Comment: 6 pages, 1 figure, to be published in Journal of Chemical Physic

Potential of a neutral impurity in a large 4^4He clusters

This paper presents an analysis of the motion of an neutral impurity species in a nanometer scale $^4$He cluster, extending a previous study of the dynamics of an ionic impurity. It is shown that for realistic neutral impurity-He potentials, such as those of SF$_6$ and OCS, the impurity is kept well away of the the surface of the cluster by long range induction and dispersion interactions with He, but that a large number of `particle in a box' center of mass states are thermally populated. It is explicitly demonstrated how to calculate the spectrum that arises from the coupling of the impurity rotation and the center of mass motion, and it is found that this is a potentially significant source of inhomogeneous broadening in vibration-rotation spectra of anisotropic impurities. Another source of inhomogeneous broadening is the hydrodynamic coupling of the rotation of the impurity with the center of mass velocity. A quantum hamiltonian to describe this effect is derived from the classical hydrodynamic kinetic energy of an ellipsoid. Simple analytic expressions are derived for the resulting spectral line shape for an impurity in bulk He, and the relevant matrix elements derived to allow fully quantum calculations of the coupling of the center of mass motion and rotation for an impurity confined in a spherical He cluster. Lastly, the hydrodynamic contribution to the impurity effective moment of inertia is evaluated and found to produce only a minor fractional increase.Comment: 25 pages, 1 table, 13 figures, to be published in Molecular Physic

Potential of an ionic impurityin a large 4^4He cluster

This paper presents an analysis of the motion of an impurity ion in a nanometer scale $^4$He cluster. Due to induction forces, ions are strongly localized near the center of the cluster, with a root mean squared thermal displacements of only a few \AA. The trapping potential is found to be nearly harmonic, with a frequency of 2.3(1.0) GHz for a positive (negative) ion in a He cluster of radius 5 nm. The anharmonicity is small and positive (energy increases slightly faster than linear with quantum number). It is suggested that by using frequency sweep microwave radiation, it should be possible to drive the ion center of mass motion up to high quantum numbers, allowing the study of the critical velocity as a function of cluster size.Comment: 14 pages, 0 figures, To be published in Molecular Physic

Dynamic features of successive upwelling events in the Baltic Sea - a numerical case study

Coastal upwelling often reveals itself during the thermal stratification season as an abrupt sea surface temperature (SST) drop. Its intensity depends not only on the magnitude of an upwelling-favourable wind impulse but also on the temperature stratification of the water column during the initial stage of the event. When a "chain" of upwelling events is taking place, one event may play a part in forming the initial stratification for the next one; consequently, SST may drop significantly even with a reduced wind impulse. Two upwelling events were simulated on the Polish coast in August 1996 using a three-dimensional, baroclinic prognostic model. The model results proved to be in good agreement with in situ observations and satellite data. Comparison of the simulated upwelling events show that the first one required a wind impulse of 28000 kg m-1 s-1 to reach its mature, full form, whereas an impulse of only 7500 kg m-1 s-1 was sufficient to bring about a significant drop in SST at the end of the second event. In practical applications like operational modelling, the initial stratification conditions prior to an upwelling event should be described with care in order to be able to simulate the coming event with very good accuracy

Quantum Hydrodynamic Model for the enhanced moments of Inertia of molecules in Helium Nanodroplets: Application to SF6_6

The increase in moment of inertia of SF$_6$ in helium nanodroplets is calculated using the quantum hydrodynamic approach. This required an extension of the numerical solution to the hydrodynamic equation to three explicit dimensions. Based upon an expansion of the density in terms of the lowest four Octahedral spherical harmonics, the predicted increase in moment of inertia is $170 {\rm u \AA^2}$, compared to an experimentally determined value of $310(10) {\rm u \AA^2}$, i.e., 55% of the observed value. The difference is likely in at least part due to lack of convergence with respect to the angular expansion, but at present we do not have access to the full densities from which a higher order expansion can be determined. The present results contradict those of Kwon et al., J. Chem. Phys. {\bf 113}, 6469 (2000), who predicted that the hydrodynamic theory predicted less than 10% of the observed increase in moment of inertia.Comment: 10 pages, including 1 figur

KASPAR in the wild - Initial findings from a pilot study

This extended abstract describes the initial pilot work when evaluating the use of the UH Humanoid Robot KASPAR in a specialist nursery for children with social and communication disorders. Staff and volunteers at the nursery were trained in the use of KASPAR and are currently using KASPAR in their day to day activities in the nursery. This paper focuses on the design and results from the initial interviews with the participants. Results high-light the challenges of transferring experimental technologies like KASPAR from a research setting into everyday practice