Nonclassical rotations of single molecules in small helium and hydrogen clusters: Manifestation of “microscopic superfluidity”

The studies of small helium clusters (up to 100 atoms) and molecular-hydrogen clusters (up to 20 molecules) that are formed in a supersonic gas jet and are coupled by the weak van der Waals interaction with a linear chromophore molecule are reviewed The shift of the frequency of the fundamental vibration of the chromophore, as well as a change in the moment of inertia of a cluster with its growth, has been detected by their rotational and vibrational-rotational spectra. A nonclassical behavior of the moment of inertia manifested in its decrease beginning with a certain number of attached He atoms (H$_2$ molecules) has been revealed. This behavior indicates that a part of a cluster is decoupled from the rotational motion of a molecule. The key question of these studies is whether such behavior of the moment of inertia is the manifestation of the super-fluidity of helium and hydrogen at microscopic level. The results are compared to the spectroscopy of molecules and hydrogen clusters in liquid-helium nanodroplets.