Breakdown of the Spectator Concept in Low-Electron-Energy Resonant Decay Processes

We suggest that low-energy electrons, released by resonant decay processes, experience substantial scattering on the electron density of excited electrons, which remain a spectator during the decay. As a result, the angular emission distribution is altered significantly. This effect is expected to be a common feature of low-energy secondary electron emission. In this Letter, we exemplify our idea by examining the spectator resonant interatomic Coulombic decay of Ne dimers. Our theoretical predictions are confirmed by a corresponding coincidence experiment.

Figure 1

Dimer-frame angular emission distributions of ICD electrons emitted after $2s$ ionization. The dimer is oriented horizontally. Panel (a): Present calculations performed for an ICD electron energy of 0.65 eV, which corresponds to the maximum of the electron spectrum and to a decay at the equilibrium internuclear distance of 3.1 Å [42]. The total distribution is shown by the solid curve while contributions from the two initial states $2{\sigma }_{g/u}^{-1}$ are depicted by the broken curves. Panel (b): Experimental results, integrating over all occurring ICD electron energies. The full line is a fit with Legendre polynomials to guide the eye. Note that the distribution is independent of the orientation of the polarization vector of the ionizing photons with respect to the dimer axis [41].

Figure 2

Theoretical and experimental electron angular emission distributions of SRICD electrons in the dimer frame. Panel (a): the $5p\sigma$ and $5p\pi$ spectator electrons are included in the potential for the calculation of the continuum SRICD electron waves (see legends). Panels (b) and (c): the dimer is oriented in parallel or perpendicularly to the polarization direction of the linearly polarized light used for the excitation (shown by double arrows). Panel (d): calculations for the $5p\sigma$ state and two different kinetic energies of the SRICD electron (see legends). Panels (e) and (f): measurements for the lower and higher kinetic energies of the SRICD electron and horizontal orientation of the electric field vector. Data in all panels include contributions from almost degenerate states of $g$ and $u$ symmetry.