Direct Spectroscopic Observation of Elementary Excitations in Superfluid He Droplets

Abstract

The absorption spectra of the electronic ${S}_{1}\ensuremath{\leftarrow}{S}_{0}$ transition of glyoxal molecules (${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$${\mathrm{O}}_{2}$) embedded in He droplets ( $\ensuremath{\approx}5500\mathrm{atoms}$) show well-resolved vibronic bands with a width $<0.5{\mathrm{cm}}^{\ensuremath{-}1}$. The phonon wings at higher frequencies have distinct gaps amounting to $\ensuremath{\Delta}E\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}8.1\mathrm{K}$ followed by a small maximum at 14.8 K. The phonon wing shape agrees with a theoretical simulation based on the dispersion curve of elementary excitations in bulk He II, providing the first evidence for superfluidity in the finite-sized He droplets.