Photoelectron imaging spectroscopy of small tungsten clusters: Direct observation of thermionic emission

Abstract

Single-photon photodetachment of mass-selected W n Ϫ clusters has been studied by photoelectron imaging spectroscopy. Velocity map imaging allows us to measure simultaneously the kinetic-energy spectrum and the angular distribution of photoelectrons. This provides a clear distinction between the two major decay mechanisms: isotropic thermionic emission and anisotropic direct photoemission. A careful study of threshold electrons shows that the thermal distribution cannot be described by a bulklike formula or a simple exponentially decreasing Boltzmann function. On the contrary, our results are in excellent agreement with more refined theoretical models taking into account the spherical symmetry of the cluster. The kinetic-energy distribution of thermal electrons corresponding to thermionic emission is found to vary as p(⑀)ϰ⑀ 1/2 exp(Ϫ⑀/k B T). Our results indicate that a transition toward a bulklike statistical behavior of the internal-energy redistribution occurs in very small systems owing to the high density of states in metal clusters. Moreover, the angular distribution of direct photoelectrons is obtained and the evolution as a function of size is discussed. The asymmetry parameter ␤ of the most intense band observed in direct photoemission for each cluster decreases monotonically with size: the direct photoemission of smaller systems is strongly anisotropic, becoming isotropic as the size of the system increases. This probably indicates the loss of coherence induced by electron-electron collisions occurring in large systems.

Ultrafast above-threshold dynamics of the radical anion of a prototypical quinone electron-acceptor

“…32,33 Consequently, these electrons at eKE ~ 0 must arise through indirect processes. The appearance of the sharp zero eKE feature is typical of that expected for thermionic emission [34][35][36] and suggests that the 2 A u can undergo internal conversion to generate vibrationally hot ground state pBQ• -. This can subsequently loose an electron in a statistical fashion.…”

“…Despite this, some fraction of the 2 A u population can access the ground state as both the 2.58 eV and 3.10 eV PE spectra exhibit clear signatures of thermionic emission, which is necessarily reliant on internal conversion from the 2 A u state. 34 From the conical intersection between the 2 B 3u and 2 A u states, the system propagates towards the minimum energy along the 2 A u state. The nuclear motion associated with this involves the same modes (C=O and C=C stretches) as those on the 2 B 3u state.…”

Photoionization Dynamics in Pure Helium Droplets

“…27,76,77 The energy released in this process will heat the cluster and can result in ejection of the weakly bound Rydberg electron, analogous to thermionic emission seen in much hotter clusters. [78][79][80] The idea of slow electrons arising from autoionizing surface states has considerable appeal, but must be considered in light of other effects. First, there is the issue of how surface states would be formed via photoexcitation.…”

The repopulation of electronic states upon vibrational excitation of niobium carbide clusters

“…"Bulk-like" effects such as non-adiabatic energy transfer between the electronic and vibrational systems in clusters have been studied for decades [7][8][9][10][11][12][13] . Usually a direct excitation of the cluster electronic system is performed with subsequent observation of the relaxation time through the energy exchange with the nuclear system.…”