Photoionization and Photofragmentation of SF₆in Helium Nanodroplets

Abstract

The photoionization of He droplets doped with SF 6 was investigated using tunable vacuum ultraviolet (VUV) synchrotron radiation from the Advanced Light Source (ALS). The resulting ionization and photofragmentation dynamics were characterized using time-of-flight mass spectrometry combined with photofragment and photoelectron imaging. Results are compared to those of gas-phase SF 6 molecules. We find dissociative photoionization to SF 5 + to be the dominant channel, in agreement with previous results. Key new findings are that (a) the photoelectron spectrum of the SF 6 in the droplet is similar but not identical to that of the gas-phase species, (b) the SF 5 + photofragment velocity distribution is considerably slower upon droplet photoionization, and (c) fragmentation to SF 4 + and SF 3 + is much less than in the photoionization of bare SF 6 . From these measurements we obtain new insights into the mechanism of SF 6 photoionization within the droplet and the cooling of the hot photofragment ions produced by dissociative photoionization.

Photoionization Dynamics in Pure Helium Droplets

“…Similar interactions have been noted in experiments on doped droplets. 10,15 C. Slow Electrons in Helium Droplets. In this section, we consider the properties and mechanistic origins of the very slow electrons responsible for the sharp central spots in the images in Figure 3.…”

“…The droplet beam then passed through a 1.5 mm skimmer and entered a differential region, which can also be used as a pickup region for doping the droplets. 10,11 After leaving the differential region through a 2 mm skimmer, the droplet beam entered the main chamber and was ionized with VUV synchrotron undulator radiation. The bandwidth of the VUV light used was ∼35 meV and the flux was ∼10 13 photons‚s -1 .…”

“…Nevertheless, neither optical transparency of the droplet nor direct dopant ionization appears necessary for the emission of fast electrons: a similar result was seen in the single photon ionization of SF 6 inside helium droplets. 10 Here, the droplet absorbed the photon, and ionization was mediated by the droplet, yet the photoelectron spectra were relatively unperturbed from those of bare SF 6 .…”

Photoelectron Imaging of Helium Droplets Doped with Xe and Kr Atoms

“…Peterka et al 38 measured photoelectron images of droplets doped with SF 6 at a photon energy of 21.8 eV; these were the first photoelectron spectra obtained by indirect dopant ionization. The spectra were similar to those of gas phase SF 6 , although detailed comparisons were complicated by the fact that SF 6 undergoes dissociative ionization to SF 5 + + F; therefore, all the spectral features were quite broad.…”

Femtosecond extreme ultraviolet ion imaging of ultrafast dynamics in electronically excited helium nanodroplets

“…The helium droplet apparatus is described in detail elsewhere [30]. Briefly, helium droplets are generated by expanding helium gas (Airgas, 99.9999% purity) at temperatures of 10-20 K and pressures of 20-80 bar through a 5-μm orifice into a vacuum chamber.…”