Direct Spectroscopic Observation of Elementary Excitations in Superfluid He Droplets

The absorption spectra of the electronic $S_1\leftarrow S_0$ transition of glyoxal molecules (${\mathrm{C}}_2$ ${\mathrm{H}}_2$ ${\mathrm{O}}_2$) embedded in He droplets ( $\approx 5500\mathrm{atoms}$) show well-resolved vibronic bands with a width $<0.5\mathrm{}{\mathrm{cm}}^{-1\mathrm{}}$. The phonon wings at higher frequencies have distinct gaps amounting to $\Delta E=8.1\mathrm{}\mathrm{K}$ 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.