The photoionization of large pure and doped helium droplets

Abstract

The photoionization of neutral liquid helium droplets (mean particle number 〈N〉=102–107) was studied using synchrotron radiation at photon energies ranging from 15 to 30 eV. Mass spectra as well as total and mass selective ion yields were measured as a function of the photon energy for different droplet sizes. The experiments indicate that ionization occurs not only by a direct process at photon energies above the atomic ionization potential but also at energies below the threshold by an autoionization process. The latter ionization mechanism proceeds via the electronically excited states of the neutral droplet, which show a strong neutral droplet size dependence. For large neutral droplets HeN(〈N〉≳104) retarding field measurements established that a predominant part of the total ion yield results from larger cluster ions He+k(k≳103). These measurements also show that a decay by fluorescence emission is much more probable than one by ionization following the photoexcitation process. In droplets with embedded SF6 molecules these are ionized indirectly by Penning ionization via excitons which leads to a large ion signal on the mass of the embedded species. No evidence for direct photoionization of the impurities was found.

Photoionization Dynamics in Pure Helium Droplets

“…The importance of Rydberg-like excitations in He droplets below IE(He) has been established in the fluorescence excitation spectra by Möller and co-workers, 53,55 and the role of molecular autoionizing states was first suggested by Fröchtenicht et al 9 to explain the onset of photoionization below IE(He). The idea of electron thermalization was motivated by the observation that much of the eKE distribution below IE(He) could be fit using the functional form for thermionic emission from a very cold cluster (T < 6 K).…”

“…Fröchtenicht et al 9 performed retarding field measurements of the cluster ion fragments following photoionization and showed that, under similar source conditions, the fragmentation process resulted in cluster ions with N + > 5 × 10 3 atoms. As we cannot effectively detect ions of this size, there is a disparity between the electron imaging and ion time-of-flight measurements.…”

Photoelectron Imaging of Helium Droplets Doped with Xe and Kr Atoms

“…It appears to be the same feature that was seen in our photoelectron images of pure He droplets. At 23.5 eV, 25 an energy below the ionization potential of atomic He (24.6 eV) but above the ionization threshold of He droplets (∼23 eV), 24 this was the only electron signal seen. It was also present in our photoelectron images of pure droplets above 24.6 eV, which were dominated by fast electrons from direct ionization of He atoms within the droplet.…”

“…24,26 The eKEs of the peaks comprising feature A show that the full excitation energy of 21.6 eV is available for reaction 1; that is, the 0.4 eV difference compared to the atomic transition is not dissipated in the droplet prior to dopant ionization. Instead, it is likely that a series of resonant excitation transfer collisions occurs within the droplet where the electronic excitation hops from one atom to another until a He atom adjacent to the dopant is excited, after which reaction 1 can proceed.…”

“…In the first photoionization experiment on He droplets, carried out by Fröchtenicht et al, 24 pure droplets and droplets doped with SF 6 were ionized with tunable VUV synchrotron radiation. The ionization threshold for pure droplets was found to be around 23 eV, significantly lower than IP(He) ) 24.6 eV.…”