Observation and mechanism of photon emission at metal-solution interfaces

Photon emission from a metal surface (Au and Pt) induced by electron-transfer reactions was observed in an electrolyte solution containing solvated electrons. The quantum efficiency Φ increases as the electrode potential ${\mathit{U}}_{\mathit{w}}$ becomes positive. The high-energy threshold of the spectrum, ${\mathit{E}}_{\mathrm{th}}$, increases linearly with increasing ${\mathit{U}}_{\mathit{w}}$ at the rate of 1 eV/V. The relaxation energy of solvated electrons in the electrolyte solution is estimated from the values of ${\mathit{E}}_{\mathrm{th}}$ and ${\mathit{U}}_{\mathit{w}}$. The peak energy ${\mathit{E}}_{\mathit{p}}$ of the spectrum also increases as ${\mathit{U}}_{\mathit{w}}$ becomes positive although the magnitude of the shift is much smaller than that of ${\mathit{E}}_{\mathrm{th}}$. ${\mathit{E}}_{\mathit{p}}$ and the low-energy threshold ${\mathit{E}}_{\mathit{l}}$ of the spectrum are metal dependent. The emitted-photon spectra are simulated by considering the transition between bulk band states of the metal while ignoring k-vector conservation. The simulated results are compared with experiment and the validity of this model is discussed.