On the possibility of selecting molecules embedded in superfluid helium nanodroplets (clusters)

Methods for producing beams of nanometer-sized superfluid helium droplets and techniques for embedding single molecules and clusters in them open up many possibilities for spectroscopy, as well as providing insight into many physical and chemical processes occurring on the atomic and molecular level at extremely low temperatures ($T\le 0.4$ K). In this paper, results of investigations into the possibility of selecting molecules embedded in superfluid helium nanodroplets (clusters) are reviewed. The method proposed starts with the selective vibrational excitation of cluster-embedded molecules by intense IR laser radiation (which greatly reduces the size of the excited clusters), followed by size-separating the clusters via scattering the cluster beam from a crossing molecular (atomic) beam. It is shown that molecules of a particular isotope (component) composition can be selected with this method. The advantages and disadvantages of the method are discussed. Methods for creating and doping helium nanodroplets and some other examples of their applications are also outlined.