Electronic spectroscopy in He droplets

Recent experiments on the electronic spectroscopy of atoms, clusters, and organic molecules embedded in helium nanodroplets are reviewed. Electronic transitions imply a larger degree of distortion of the helium environment as compared to vibrational and rotational excitations. Thus new phenomena arise such as the appearance of side bands in the spectra, which are due to the excitation of helium collective vibrations, large changes of the effective molecular rotational constants and even the expulsion of an atom (or molecule) from the cluster upon excitation. These features make it possible to probe the helium environment and its interactions with molecular chromophores on the atomic scale. Real-time studies of the manifestations of superfluidity and of chemical processes in the droplets via femtosecond excitation techniques, provide a new perspective to this field. The considerable amount of data available so far shows the large potential of helium droplets for isolation and spectroscopy of large molecules and clusters. The low temperature and the high spectral resolution achievable because of the relative homogeneity of this medium, are instrumental for separating solvation effects that are obscured by the presence of much larger fluctuations in more classical environments. Hence, electronic and geometrical structures of even large entities become accessible. (C) 2001 American Institute of Physics.