Interplay between theory and experiment in investigations of molecules embedded in superfluid helium nanodroplets†

Abstract

Helium is the only substance that has been observed on macroscopic scale to form the fourth state of matter, the superfluid state. However, until recently superfluid helium had not found any practical applications, mainly because it expels all other atoms or molecules. Only in the 1990s was it discovered that it is possible to mix in other substances with superfluid helium if helium is prepared as small droplets, called nanodroplets, containing only a few thousand atoms. This discovery led to the development of a new and very powerful experimental technique, called helium-nanodroplet spectroscopy. Superfluid helium creates a gentle matrix around the impurities and – due to superfluidity and to very weak interactions of helium atoms with other atoms or molecules – allows measurements of the spectra with precision not much lower than in the gas phase. Consequently, helium-nanodroplet spectroscopy enables very accurate probing of molecules or clusters which cannot be investigated in the gas phase due to their instability. This category includes ‘fragile’ molecules, isomers, radicals, and clusters in secondary minima. The major experimental developments will be described, emphasizing their importance for understanding basic principles of physics and new insights into chemically relevant processes. The experiments have been assisted by theoretical work on impurity–He n clusters. Most such work involves first-principles quantum simulations. Although the number of helium atoms that can be included in such simulations is significantly smaller than in a typical nanodroplet, theory explains most of the observed trends reasonably well. Theoretical results can also be compared directly and much more precisely than in the case of the droplets with the results of molecular beam experiments on clusters of controllable size, with the number of helium atoms ranging from 1 to almost 100. Most of the simulations published to date will be discussed and the level of agreement with experiment will be critically evaluated. The results of the simulations are very sensitive to details of the He–He and impurity–He interaction potentials used, and most of the current discrepancies between theory and experiment can be traced down to the uncertainties of the potentials. Thus, an important component of this review will be an analysis of various sources of errors in potential energy surfaces. †Dedicated to Roger E. Miller.