Superfluid Helium Droplets: A Uniquely Cold Nanomatrix for Molecules and Molecular Complexes

TLDR

Helium droplets provide high‑resolution spectroscopy at ultralow temperatures (0.15–0.37 K) that combine gas‑phase and matrix‑isolation advantages. The review surveys recent spectroscopy and reaction studies of molecules and complexes in helium droplets. Superfluid helium acts as an isothermal nanoscopic reactor that facilitates binary encounters, absorbs recombination energy, and isolates single molecules or reactive events at ultralow temperatures.

Abstract

Herein, recent experiments on the spectroscopy and chemical reactions of molecules and complexes embedded in helium droplets are reviewed. In the droplets, a high spectroscopic resolution, which is comparable to the gas phase is achieved, while an isothermal low-temperature environment is maintained by evaporative cooling at T =0.37 K (4He droplets) or 0.15 K (3He droplets), lower than possible in most solid matrices. Thus the helium-droplet technique combines the benefits of both the gas phase and the classical matrix-isolation techniques. Most important, the superfluid helium facilitates binary encounters, and absorbs the released binding energy upon recombination. Thus the droplet can be viewed as an isothermal nanoscopic reactor, which isolates single molecules, clusters, or even a single reactive encounter at ultralow temperatures.