We have investigated the surface-enhanced resonance Raman spectroscopy (SERRS) for 4,4′-diaminotolane absorbed on silver experimentally and theoretically. Experimental observation shows greatly enhanced bg symmetric modes ν26 and ν27, which are silent in normal Raman spectroscopy and SERS on gold. The dependence of the surface-enhanced Raman spectroscopy (SERS) spectra on five excitation lines has also been registered and classified for single bands into three profiles referring to the different contributions of resonant transitions. Theoretical calculations based on the time-dependent path integral formalism by including the Herzberg–Teller correction reproduce the experimental spectra with good agreement. The importance of the Herzberg–Teller term to improve the pattern of the spectra and the enhancement of totally and non-totally symmetric modes are recognized and elucidated. The strong charge-transfer transition with nearly close molecular excitations in this compound creates an opportunity to obtain insight into the combination of molecular and charge-transfer transitions and their effect on the chemical mechanism of SERS.

The singular charge-transfer SERS spectra of symmetric and non-symmetric benzene-like compounds and their selective enhancement under electric field are investigated.

Surface-enhanced Raman spectroscopy is a highly sensitive phenomenon and a powerful fingerprint detection tool that reflects the small changes in polarizability on the pattern and intensity of Raman signals. The SERS enhancement signals elucidate with the surface-selection rules. In this regard, molecular configuration and adsorption orientation on the surface, in addition to the direction of external electric field, can lead to different patterns of SERS spectra. To evaluate how the variation of these features influences the pattern and reproducibility of the spectra, the chemical charge-transfer SERS spectra for pyridine on silver clusters are calculated for different field directions, tilt angles, and anchoring bond distances. The impact of external electric field direction versus the adsorbed tilt angle and Ag–N bond distance on the pattern of SERS-CT spectra is established by time-dependent excited-state gradient approximation and elucidated based on the deviation of ground- and excited-state properties from the optimized condition.

Resonance Raman spectroscopy has long been established as one of the most sensitive techniques for detection, structure characterization and probing the excited-state dynamics of biochemical systems. However, the analysis of resonance Raman spectra is much facilitated when measurements are accompanied by Density Functional Theory (DFT) calculations which are expensive for large biomolecules. In this work, resonance Raman spectra are therefore computed with the Density Functional Tight-Binding (DFTB) method in the time-dependent excited-state gradient approximation. To test the accuracy of the tight-binding approximations, this method is first applied to typical resonance Raman benchmark molecules like β-carotene and compared to results obtained with pure and range-separated exchange-correlation (xc) functionals. We then demonstrate the efficiency of the approach by considering a computationally challenging heme variation. Overall, we find that the vibrational frequencies and excited-state properties (energies and gradients) which are needed to simulate the spectra are reasonably accurate and suitable for interpretation of experiments. We can therefore recommend DFTB as a fast computational method to interpret resonance Raman spectra.