Line broadening of excimers bound to the surface of 4 He clusters investigated by comparison with corona discharge excitation spectra

A new method for assessing the site-specific emission from electronically excited helium droplets is presented. The fluorescence features of helium droplets show sharp rotationally resolved lines indicating desorption of excimers and emission far outside the droplets as well as blue-shifted and strongly broadened features due to emission of excimers confined in cavities within the droplets. A third feature is identified: slightly broadened rotational lines that we attribute to emission from excimers bound to the droplet surface. The line broadening arises from collisions with the helium gas within the surface layer of the helium droplets. These conditions are simulated using a high pressure gas cell in which helium gas is electronically excited using a corona discharge. Rotational line broadening of similar magnitude to that of large droplets (N ∼ 10 atoms) is observed for gas pressures at about 5 bar and 80 K, corresponding to a number density of 4.52 × 10−4 Å−3. We conclude that the excimers are located within a shell separated by 6 to 7 Å from the radius where the density has dropped to 50% of its centre value. Helium droplets that are smaller (N ∼ 10 atoms) exhibit rotational lines that are less broadened, which we attribute to the superposition of features originating from desorbed and from surface-bound excimers. A fit of the linewidths reveals that around 50% of the excimers are bound to the surface of the smaller droplets.