Ion capture in helium droplets: Formation of cold ion-neutral clusters.

摘要

Superfluid helium nanodroplets are used to cool ions and form ion-neutral clusters at a temperature of 0.37 K. A desolvation technique was developed that allowed for the study of captured ions by mass spectrometry. From the mass spectrometry results it was determined that helium droplets may successfully capture sodium cations with kinetic energy of ∼200 eV. Clusters of the neutral molecules H2O, N2, and HCN with Na+ were observed. Based on binding strength considerations, it is argued that the desolvation process imparts little energy into the ion-neutral clusters, avoiding dissociation. This result leads to the conclusion that ion-neutral clusters are formed within the droplet prior to desolvation, indicating that the helium "snowball" that is assumed to form around Na + does not prevent ion-neutral cluster formation. This conclusion is supported by ab initio calculations, the results of which indicate the presence of barrierless pathways for neutral molecule insertion into the helium snowball surrounding Na+.;The process of ion capture by helium droplets was studied by comparison of measured ion-doped droplet size distributions to known pre-ion capture droplet size distributions. The measured ion-doped droplet size distributions were affected by nozzle temperature, ion kinetic energy, and ion mass. These factors primarily affect two parameters of the ion-doped droplet size distribution, the minimum droplet size threshold, Nthr, and the droplet size at maximum signal intensity, Nmax. The effects of the studied factors on the measured distributions cannot be explained in terms of currently accepted droplet cooling mechanisms.;Analysis of the results suggests that droplet doping efficiency can be improved in several ways, including a higher flux of lower-energy ions. An apparatus for the production and focusing of alkali cations at high fluxes and low energies is described. This apparatus was able to produce higher ion currents at lower kinetic energies than previous ion sources.