Electron impact ionization of CCl 4 and SF 6 embedded in superfluid helium droplets

Abstract

Electron impact ionization of helium nano-droplets containing several 10 4 He atoms and doped with CCl 4 or SF 6 molecules is studied with high-mass resolution. The mass spectra show significant clustering of CCl 4 molecules, less so for SF 6 under our experimental conditions. Positive ion efficiency curves as a function of electron energy indicate complete immersion of the molecules inside the helium droplets in both cases. For CCl 4 we observe the molecular parent cation CCl 4 + that preferentially is formed via Penning ionization upon collisions with He*. In contrast, no parent cation SF 6 + is seen for He droplets doped with SF 6. The fragmentation patterns for both molecules embedded in He are compared with gas phase studies. Ionization via electron transfer to He + forms highly excited ions that cannot be stabilized by the surrounding He droplet. Besides the atomic fragments F + and Cl + several molecular fragment cations are observed with He atoms attached.