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

Abstract

Photoelectron (PE) spectra of thiolate-protected coinage metal clusters [Au25(SC2H4Ph)18]− and [Ag25(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 M13 core in [M25(SR)18]− (M = Au, Ag). The novel photoinduced TE observed here is ascribed to the selective transition to an electronically excited state [M25(SR)18]−* embedded in the electron detachment continuum followed by quick internal conversion to a vibrationally excited state in the electronically ground state.

Ligand-protected gold/silver superatoms: current status and emerging trends

“…131 Unexpectedly, PES on [M 25 (PET) 18 ] − (M = Au, Ag) revealed that thermionic emission (TE) of a slow electron dominates over direct photodetachment upon photoirradiation at 266 nm. 128 Johnson established high-resolution UV-visible absorption spectroscopy at cryogenic temperature in the gas phase. [132][133][134] Condensation of inert gases (He, N 2 ) as a tag can freeze the internal motion of the superatoms without noticeable influence on the stable structures.…”

Elucidating the Doping Effect on the Electronic Structure of Thiolate‐Protected Silver Superatoms by Photoelectron Spectroscopy

“…Although doping effects on the electronic structures have been studied using conventional spectroscopic and electrochemical methods, the fundamental question of how the energy levels of the superatomic orbitals are shifted upon doping has not been addressed. Photoelectron spectroscopy (PES) in an isolated environment is a direct probe of the energy levels with respect to the vacuum level and density of states of the occupied superatomic orbitals, as recently demonstrated for [Au 25 (SC 12 H 25 ) 18 ] − and [Au 25 (SC 2 H 4 Ph) 18 ] − . Furthermore, the electron affinities of chemically‐modified superatoms determined by the gas‐phase PES can be directly compared with those theoretically predicted for the “unsolvated” state.…”

“…The optical spectra of the synthesized samples (Figure a) agreed well with those in the literature . The clusters were introduced via an electrospray‐ionization (ESI) source into a time‐of‐flight (TOF) mass spectrometer combined with a photoelectron spectrometer . TOF mass spectra in the negative‐ion mode (Figure b) are dominated by the mass peaks of the intact species, supporting the high purities of the samples.…”

“…Although the vertical detachment energy (VDE) of [Ag 25 (SR) 18 ] − could not be unambiguously determined due to the vague spectral shape, the adiabatic electron affinity (AEA) was determined from the spectral onset as 2.02±0.02 eV. The AEA of [Ag 25 (SR) 18 ] 0 is close to the values of the gold analogues [Au 25 (SC 12 H 25 ) 18 ] 0 (≈2.2 eV) and [Au 25 (SC 2 H 4 Ph) 18 ] 0 (2.36±0.01 eV) . The similarity in the AEA suggests that replacement of the central Ag atom with Au does not affect the electronic structure.…”

“…As anticipated, the PE spectrum of [AuAg 24 (SR) 18 ] − at 355 nm (Figure b) shows a profile that is very similar to that of [Ag 25 (SR) 18 ] − (Figure a): the AEA was determined as 2.08±0.02 eV. Upon irradiation with 266 nm, both [Ag 25 (SR) 18 ] − and [AuAg 24 (SR) 18 ] − exclusively underwent thermionic emission of a slow electron from vibrationally excited states generated via internal conversion from the photoexcited state (Figure a,b), as in the case of [Au 25 (SC 2 H 4 Ph) 18 ] − …”