Photoinduced Thermionic Emission from [M25(SR)18]- (M = Au, Ag) Revealed by Anion Photoelectron Spectroscopy

Photoelectron (PE) spectra of thiolate-protected coinage metal clusters [Au-25(SC2H4Ph)(18)](-) and [Ag-25(SPhMe2)(18)](-) isolated in vacuum were recorded at 355 and 266 nm. Their adiabatic electron affinities were determined to be 2.36 +/- 0.01 and 2.02 +/- 0.02 eV, respectively, from the onsets of the PE spectra recorded at 355 nm. Upon irradiation with a 266 nm light, the emission of slow electrons was observed as the major process, while suppressing the direct electron detachment and dissociation into anionic fragments. Curve fitting analysis of the PE spectra at 266 nm suggests that the slow electron emission is assigned to thermionic emission (TE) from the vibrationally hot M-13 core in [M-25(SR)(18)](-) (M = Au, Ag). The novel photoinduced TE observed here is ascribed to the selective transition to an electronically excited state [M-25(SR)(18)](-)* embedded in the electron detachment continuum followed by quick internal conversion to a vibrationally excited state in the electronically ground state.