Bubble Formation and Decay in ${}_{}{}^3{}_{}{}^{}He$ and ${}_{}{}^4{}_{}{}^{}He$ Clusters

The energy transfer in ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ clusters electronically excited by monochromatic synchrotron radiation is investigated by luminescence spectroscopy. Depending on the cluster size and the isotopic constitution, either sharp, broadened, or shifted emission bands of single He molecules are observed. The spectral features show that He molecules emit light either within a bubble inside the cluster or in the vacuum after desorption from the cluster. From the luminescence intensity, the cluster diameter, and the radiative lifetime, an average velocity of $\sim 7\mathrm{m}/\mathrm{s}$ of bubbles in ${}^4\mathrm{He}$ clusters could be deduced. In the nonsuperfluid ${}^3\mathrm{He}$ clusters this velocity was observed to be significantly lower.