Electronic-vibrational couplings are pivotal in many photo-induced processes at interfaces and surfaces. Two-dimensional electronic-vibrational sum frequency spectroscopy (2D-EVSFG) is a unique tool to decipher electronic-vibrational couplings for excited states of molecules at interfaces and surfaces, enabling the correlation of photo-induced electronic-vibrational manifolds with their subsequent time evolution of high-frequency vibrational modes. In this talk, I will present the development of interface-specific two-dimensional electronic-vibrational sum frequency spectroscopy (2D-EVSFG) for electronic-vibrational couplings for excited states at interfaces and surfaces. We demonstrate this 2D-EVSFG technique by investigating photoexcited interface-active AP3 molecules at the air/water interface as an example. Our 2D-EVSFG experiments show strong vibronic couplings of interfacial AP3 molecules upon photoexcitation and subsequent relaxation of a locally excited (LE) state. Time-dependent 2D-EVSFG experiments indicate that the relaxation of the LE state, S2, is strongly coupled with two high-frequency modes of 1529.1 cm-1 and 1568.1 cm-1. Quantum chemistry calculations further verify that the strong vibronic couplings of the two vibrations promote the transition from the S2 state to the lower excited state S1. This technique is especially powerful to monitor ultrafast processes such as charge transfer, energy transfer, proton transfer, and proton-coupled charge transfer at interfaces and surfaces on a real-time basis.

Development of interface-specific two-dimensional electronic-vibrational sum frequency generation

ACS Spring 2022

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