Could you explain the significance of your article to the non-specialist?

Clusters in a molecular beam are often studied without control of the their internal temperature.  For instance, this has complicated studies on structural isomers and thermodynamic properties for quite a while. The availability of doped helium droplet beams at milliKelvin temperatures enabled for the first time the study of metal clusters under these extremely low temperature conditions. Furthermore, the superfluid properties of helium droplets and their interaction with embedded species have stimulated the field of finite quantum liquids. In this way, helium nanodroplet spectroscopy on metal cluster offer a versatile tool to study the physical and chemical properties of ultracold finite systems including the isolation of e.g. reactive species and generate novel complexes.

What has motivated you to conduct this work?

Since in our labs both the helium nanodroplets technology and experiments on metal clusters have been available, it was straightforward to combine these techniques in order to explore the low temperature regime for metal cluster studies.

Where do you see this work developing in the future?

So far only the electronic structures of the high-spin trimer systems have been unraveled in detail. Many more clusters which cannot be generated by other techniques will certainly be discovered and studied by spectroscopic methods. Although a few basic experiments have been performed, femtosecond laser spectroscopy on isolated clusters will certainly be extended, possibly by applying tailored laser pulses in order to gain control over dynamical processes, e.g. Coulomb explosion of embedded particles including large organic complexes. Chemical reactions might be an issue in future investigations. The ultracold conditions provide the laboratory to look quantum state selectively at chemical reactions. The possibility to isolate large complexes might stimulate work also on relevant biological systems even under micro-solvation conditions.

Are there any particular challenges facing future research in this area?

A main drawback of helium droplet isolation spectroscopy compared to the use of free neutral or charged cluster beams is that the attached helium atoms inhibit mass selection of embedded clusters. Hence size selected measurements can only be performed in rare cases. Furthermore, spectroscopy of charged clusters and complexes would be desirable in order to support the numerous results with these species at higher temperatures.