High-resolution mass spectrometric study of pure helium droplets, and droplets doped with krypton

Mass spectra of doped and undoped helium droplets are presented. The high resolution of the time-of-flight spectrometer (m/Delta m a parts per thousand... 5000) makes it possible to fully resolve small helium cluster ions from impurities and to unambiguously identify abundance anomalies in the size distribution of He (n) (+). The yield of He-4 (+) shows the well-known enhancement relative to other small cluster ions when the expansion changes from sub- to supercritical, provided the electron energy exceeds a value of 40 +/- A 1 eV, the threshold for formation of electronically excited ions. Upon doping with krypton, pure Kr (n) (+) cluster ions containing up to 41 Kr atoms are observed. The spectra exhibit abundance anomalies at 13, 16, 19, 22 & 23, 26 and 29, in agreement with spectra obtained by ionization of bare krypton clusters that are formed in neat supersonic beams. Mixed clusters He (m) Kr+ indicate closure of a solvation shell at m = 12.