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 more atoms, is produced by fragmentation of a low entropy supersonic expansion. It subsequently is excited by electron impact, producing various charged and metastable droplet states depending on the electron energy. We will describe experiments with negatively charged cluster ions, which are observed for low energy impacts when N > 2 x 10(5). In these experiments, after a flight time in high vacuum of several milliseconds the droplets pass through a weak transverse field above an electron multiplier. A signal from spontaneously detached electrons is observed, which suggests that the ion, while long lived, is inherently metastable. Furthermore, when the beam is crossed with an infrared light beam above the detector, the detachment rate is significantly increased. 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 spontaneous 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 of detached electrons have energies below 1eV, with photo detached electrons the more energetic.