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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.
