Atomic-site-dependent light emission from Au(110)-(2 × 1) surface induced by scanning tunneling microscope

We have investigated the scanning-tunneling microscope (STM) light-emission mechanism of the Au(110)-(2 × 1) surface. We found that the light stimulated by the STM is emitted through three different channels. The first channel is the emission through excitation of localized surface plasmons (LSP). The other two channels are through the recombination of d-band holes and s-p electrons in Au. When the sample bias voltage is positive (i.e., electrons are injected into the sample), d-band holes are created by impact ionization. The intensity due to this process is greater when the tip is located between the Au atomic rows than over the row. This process is the origin of the atomic-site-dependent spectra that we reported in a previous paper [Y. Uehara, T. Fujita, and S. Ushioda, Phys. Rev. Lett. 83, 2445 (1999)]. When the bias voltage is negative, d-band holes are created by tunneling of d electrons from Au to the tip. The light-emission intensity due to the recombination of these d holes with sp electrons is about twice as strong as that emitted through the excitation of LSP.