Helium Nanodroplets: Formation, Physical Properties and Superfluidity

Abstract

In this introductory chapter, we begin by informing the reader about the fascinating history of superfluidity in bulk liquid helium. This is followed by relating attempts in using liquid helium as a low temperature matrix for spectroscopy. After a brief review of the thermodynamic properties of helium in Sect. 1.2, the different types of free jet expansions used in experiments to produce clusters and nanodroplets of different sizes are described in Sect. 1.3. First it is shown how they depend on the nature and location in the phase diagram of the isentropes which determine the course of the expansion. Depending on the four regimes of isentropes, different number sizes and distributions are expected. Next in Sect. 1.4, the results of theoretical and, where available, experimental results on the total energies, excited states, radial density distributions, and temperatures of clusters and droplets are discussed. Finally, in Sect. 1.5 the theoretical and experimental evidence for the superfluidity of nanodroplets is briefly reviewed. For more information on the production and characteristics of nanodroplets, the reader is referred to the chapters in this book and to the reviews in Appendix.

On the thermodynamic stability of polycations

“…The energy expression (Equation 26) was taken with parameters from the literature, namely ε ∞ = 0.621 [meV], ε 2/3 = 1.526 [meV], and ε 1/3 = 0.513 [meV]. [44,45] Due to numerical instabilities of the fit, the smallest possible ξ = 0.4 was taken. The fitting resulted in a curve reproducing the observations, as can be seen from Figure 6.…”

On the thermodynamic stability of polycations

“…In the next step, we apply the same principles to the thermodynamic stability of the weakly bound clusters in the most straightforward example, i.e., in the case of helium clusters, for which experimental data on the critical sizes of the clusters with different charges is available from the literature. [42,43] We also can find the expression of the helium clusters' (He n ) energies as a function of the number of atoms n: [44,45]…”

On the thermodynamic stability of polycations

“…In the next step, we apply the same principles to the thermodynamic stability of the weakly bound clusters in the most straightforward example, i.e., in the case of helium clusters, for which experimental data on the critical sizes of the clusters with different charges is available from the literature. [33,34] We also can find the parametrization of the helium clusters' energies as a function of the number of atoms n: [35,36]…”