Anion photoelectron spectroscopy on xenon cluster anions, the simplest solvated electrons

Xenon clusters had been predicted to form polarizability-bound anions and/or correlation-bound anions, Xen-, but despite their importance in understanding weak binding by excess electrons and the theoretical studies probing them, only their mass spectrometric observation by Haberland et al. [Phys. Rev. Lett. 63, 1219 (1989)] had revealed their viability. In this study, xenon atomic and cluster anions, Xen- (n = 1-54), were formed, and their anion photoelectron spectra were measured for n = 20-54. Each of these spectra exhibited two photoelectron peaks, i.e., one at an electron binding energy (EBE) around 0.2 eV and another at an EBE around 0.8 eV. The Xen- cluster anions associated with the lower EBE peaks were attributed to those in which their excess electrons are bound by aggregates of highly polarizable xenon atoms. The smallest stable size for this type of binding interaction had been predicted to be n ∼ 5. This is consistent with the extrapolation of a plot of the observed vertical detachment energy (VDE) values for the lower EBE peaks vs n-1/3 toward VDE = 0. These Xen- atomic cluster anions are presumably the simplest solvated electrons. The Xen- cluster anions associated with the higher EBE peaks were attributed to those in which an electronically metastable xenon atomic anion, Xe*-, is physisorbed, i.e., "solvated," by n - 1 Xe atoms, i.e., Xe*-(Xe)n-1.