Capture and ionization of atoms and molecules in liquid helium droplets

A method for helium cluster production is presented utilizing a cryogenic molecular beam source. Pure liquid helium droplets of mean size N = 100 to 15,000 atoms ionized by electron impact show surprising ion fragment distributions. For all cluster sizes He{dollar}sb2sp+{dollar} is the most probable cluster ion fragment, accounting for 40-70% of the total ion yield. The high relative intensity of He{dollar}sb2sp+{dollar} proves that the large liquid droplets dissipate the ionization energy through an impulsive process which ejects He{dollar}sb2sp+{dollar} from the cluster. The other helium ion clusters that have been the focus of previous studies are most likely formed by a similar mechanism.; Capture of atoms and molecules in the helium clusters is accomplished using three very similar techniques. Capture of many different atomic and molecular dopants is presented using electron impact ionization mass spectrometry. The results display the power of the capture technique when applied to helium droplets.; Liquid helium droplets of initial mean cluster size, N, ranging from 600-8,000 atoms are doped with argon using the standard capture technique. The doped clusters are ionized by electron impact, and the resulting fragment ions are monitored as a function of argon capture pressure. Analysis of the pressure dependence ion signals is used to determine: (1) the probability for charge transfer from He{dollar}sp+{dollar} to the argon dopant within the droplet, and (2) the probability for fragmentation of the Ar{dollar}sb{lcub}rm k{rcub}{dollar} parent ions during charge transfer. The conclusion from these results is that the charge transfer and fragmentation processes within the ionized droplet are dependent on the initial size of the helium droplet and the number of argon atoms captured. The detected ion distribution is non-Poissonian. Similar analysis is performed for clusters doped with captured neon, xenon, and NO. The charge transfer probability is shown to have a strong dependence on the dopant species captured by the cluster. A model for charge transfer is presented which uses the positive hole resonant hopping mechanism to determine that the positive charge hops 3-4 times prior to localization with the argon dopant or another He atom forming He{dollar}sb2sp+.{dollar}...