Cold metal clusters: Helium droplets as a nanoscale cryostat

In this Letter we study the formation of metal clusters of silver, indium, and europium embedded in helium droplets and present spectroscopic (UV) evidence that, contrary to common belief, metal atoms can be stable inside helium droplets. Our method provides a beam of metal clusters up to 2000 amu with a precisely known temperature of 0.37 K, 2 orders of magnitude lower than previously achieved for free metal clusters. This dramatic decrease in temperature is important for high resolution spectroscopic experiments and opens the door to beam studies of superconducting metal clusters.