Plasmonic energy can be transferred to the vibrational energy of molecules on metals. However, due to the rapid vibrational relaxation of the molecules, the achievable steady-state excited-state population is usually assumed to be negligible, and the state distribution is nearly statistical. In this talk, I will present two SERS studies showing that optical and photochemical plasmonic activation can lead to highly non-statistical molecular vibrational states. First, non-reactive single-molecules (such as 4, 4’-benzenedithiol) placed at plasmonic gaps show simultaneously blinking Stokes and anti-Stokes SERS spectra, some of which exhibit only one vibrational peak. Such stochastic activation occurs irrespective of the chemistry of the molecules. The site-specific, single-peak spectra could be explained by the angstrom-sized electromagnetic hot-spots. Second, some of the photo-reactive molecules (such as 4-nitrobenzenethiol) placed at metallic gaps, when illuminated by light, show an intense anti-Stokes vibrational peak of a specific mode during the early stage of the chemical reaction. Such activation occurs only for redox-active molecules and is temporally correlated to the kinetics of the reaction. The finding can be explained by the energy exchange between plasmonic hot-electrons and the vibronic quantum states of molecules. In both cases, effective vibrational temperatures of the SERS peaks often exceed 900 K, indicating that plasmon-induced molecular activation can be in fact efficient enough to overcome rapid vibrational energy relaxation.

Unusual molecular vibrational excitation induced by gap-plasmons

256th National Meeting (2018)

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