Elucidation of charge-transfer SERS selection rules by considering the excited state properties and the role of electrode potential

Abstract

The goal of this study is to shed light on the charge-transfer (CT) mechanism of surface-enhanced Raman scattering (SERS) by considering the properties of CT excited states. The calculations have been done by means of an excited-state gradient approximation for a pyridine molecule interacting with a silver cluster, and provided a satisfactory improvement in comparison to previous work. The effect of electrode potential on the SERS-CT spectra has been modelled theoretically by applying an external electric field for selected CT transitions and the enhancement of the ν and ν modes and a decline in the intensity of the ν mode under a negative electric field (which is directed toward the cluster) have been observed. These results match well with the experimental studies and also explain the effect of electrode potentials on the patterns of spectra, as experimental evidence of the CT mechanism. Finally, this study demonstrated that the excited state vector gradient can be used as a distinguishing factor to explain the SERS selection rules.

How Do Adsorbent Orientation and Direction of External Electric Field Affect the Charge-Transfer Surface-Enhanced Raman Spectra?

“…In our previous study, the SERS-CT spectra for Py-Ag 6 in-plane and perpendicular to the plane (of cluster) have nearly the same pattern using Franck–Condon approximation. Also, the spectrum for the perpendicular configuration has a good agreement with experiment and calculated spectrum for a larger cluster (Py-Ag 20 ) . However, for Py-Ag 6 , the effect of Herzberg–Teller term and Duschinsky rotation corrections on the pattern of the SERS-CT spectrum were obtained (with path integral formalism) .…”

“…For more than 40 years, surface-enhanced Raman spectroscopy (SERS) has attracted remarkable attention as a powerful surface sensitive technique to detect trace amounts of analytes and characterize the structures from their rich vibrational spectra. − It has a wide variety of applications in chemical detections, due to the enhancement of intensities by a factor of 10 4 −10 6 or even larger by placing the molecules in hotspot positions of gold and silver nanoparticles, enabling single-molecule detection (SM-SERS). − Experimental observations have proved the ability of SERS as a structurally sensitive probe that can characterize the molecular surface configuration and adsorption orientation, all through the change in the relative intensities of measured signals. − The high sensitivity of this phenomenon comes from the effect of small changes in the polarizability on the intensities in response to radiation. Therefore, different local parameters such as the energy of the incident lights, − configuration and composition of metallic surfaces, ,− intrinsic adsorbent properties, − and also electrode potentials − can influence the overall performance and the pattern of spectra. As an example, Haes and co-workers reviewed the influence of intra- and intermolecular interactions on SERS spectral features and considered the dependence of signals on the adsorbent affinity and distance.…”

“…SERS-CT has been extensively studied as a sensitive mechanism that can present the effect of local and environmental parameters on the pattern of spectra. ,, Moreover, in tip-enhanced Raman scattering, this resonance chemical mechanism has a main role in finding the adsorption configuration and also the relative position of the tip with respect to the adsorbent. , In our previous studies, we tried to investigate the effect of variation of the external electric field, the surface element, and the adsorbent compounds on the relative intensities of SERS-CT spectra. In agreement with the experiment, we found different patterns for the same molecule at various applied potentials and also for different benzene-like derivatives at the same potential.…”

“…The amplitude and direction of the excited-state gradient vectors of this specific state can be used as a distinguishing factor to explain the intensity and pattern of spectra. In our previous studies, the validity of this approach was demonstrated for SERS-CT spectra of pyridine and benzene-like derivative compounds under the effect of uniform finite electric field. Herein, we try to consider the effect of changing the orientation and distance of molecular adsorption and also the direction of the external electric field on the pattern of SERS-CT spectra for pyridine on silver clusters.…”

Theoretical Approaches for Modeling the Effect of the Electrode Potential in the SERS Vibrational Wavenumbers of Pyridine Adsorbed on a Charged Silver Surface

“…This approach has been very frequently used in the earlier similar studies. For instance, Mohammadpour et al (2017) used an asymmetric range of 120·10 −4 a.u. by using a planar-triangular Agtext6text0 cluster to reproduce the trends observed on the SERS intensities of pyridine recorded by us on silver in the range −0.5 to −1.2 V. Zhao and Chen (2013) studied pyridine on a gold electrode modeled as a Au 20 tetrahedral cluster using a range of ±10·10 −4 a.u.…”