Direct visualization of photoinduced tunneling charge transfer (TCT) in an Au5/para-aminothiophenol (PATP)/Ag6 junction in which Au and Ag clusters form the first and second layer, respectively, is provided by the charge difference density (see picture; green and red stand for holes and electrons, respectively).
We theoretically investigate the mechanism of chemical enhancement of surface-enhanced resonance Raman scattering (SERRS) of para-aminothiophenol (PATP)/metal complexes and metal/PATP/metal junctions. The method of charge difference density is used to visualize intracluster excitation and charge transfer (CT) between PATP and metal during the process of resonant electronic transitions. It is found that the selective enhancement of the b2 mode in SERRS spectra result not only from Albrecht’s A term (the Frank–Condon term), but also from the Herzberg–Teller term (Albrecht’s B mechanism) via resonant CT. For the metal/PATP/metal junctions, the calculated results reveal that the Raman spectrum is of SERRS nature and the nontotally symmetric b2 mode is strongly enhanced at the incident wavelength of 1064 nm when Au and Ag nanoparticles are the first and second layer, respectively, and the dominant enhancement mechanism is the Herzberg–Teller term in chemical enhancement via tunneling charge transfer (intervalence electron transfer from the Ag cluster to the Au cluster). When the first and second layers were inverted (i.e. the Ag and Au nanoparticles are the first and second layers, respectively), the Raman spectrum at an incident wavelength of 1064 nm is due to normal Raman scattering, and the nontotally symmetric b2 mode is not strongly enhanced. Our theoretical results not only support the experimental findings, but also provide a clear physical interpretation.