SPONTANEOUS AND INFRARED INDUCED ELECTRON DETACHMENT FROM NEGATIVELY CHARGED HELIUM NANODROPLETS

A beam of helium nanodroplets, with sizes ranging up to N = 10(7) or m ore atoms, is produced by fragmentation of a low entropy supersonic ex pansion. It subsequently is excited by electron impact, producing vari ous charged and metastable droplet states depending on the electron en ergy. We will describe experiments with negatively charged cluster ion s, which are observed for low energy impacts when N > 2 x 10(5). In th ese experiments, after a flight time in high vacuum of several millise conds the droplets pass through a weak transverse field above an elect ron multiplier. A signal from spontaneously detached electrons is obse rved, which suggests that the ion, while long lived, is inherently met astable. Furthermore, when the beam is crossed with an infrared light beam above the detector, the detachment rate is significantly increase d. The wavelength dependence of this light induced signal has a broad peak near 1.5 mu m. By deflection measurements it is found that the sp ontaneous detachment signal comes preferentially from smaller clusters , while the light induced signal comes predominantly from larger ones. By stopping potential measurements one can conclude that both kinds o f detached electrons have energies below 1eV, with photo detached elec trons the more energetic.