Role of the Electrode Potential in the Charge-Transfer Mechanism of Surface-Enhanced Raman Scattering

Abstract

A general model concerning the role of the electrode potential on the charge-transfer enhancement mechanism of SERS is proposed, justifying the existence of a parameter β which reduces the effectiveness of the electrode potential in order to shift the excited charge-transfer level. This explains the observed deviation of β from the unity, and allows for relating the intensity/electrode-potential (SERS−CT) profiles of similar adsorbates with the respective electron affinities. The SERS of pyridine, pyrazine and methylpyrazines have been recorded on silver, being found that the electrode potential in the maximum of the SERS−CT profiles shifts toward more negative values as the number of methylsubstituents attached to the pyrazine ring increases. This result has been explained on the basis of the relative stability of the radical anions given that a correlation between those shifts and the calculated ab initio vertical transition energies between the neutral molecules and their respective radical anions can be established. This confirms the participation of a resonant charge-transfer mechanism in the SERS as well as the usefulness of this technique to gain insight into the electronic properties of the doublet states of the here studied molecules.

Inherent Complexities of Trace Detection by Surface‐Enhanced Raman Scattering

“…[47] This, though, can be compounded by the possibility of a multiplicative enhancement generated by a resonance Raman effect due to excitation into a CT transition, as was pointed out at the very beginning of SERS. The inclusion of CT in terms of the vibronic theory as a mechanism for SERS has been discussed in detail by several groups, [85,87,88] and recently corresponding experiments supported by quantum chemical computations on CT resonance Raman effects have been presented, for example, by Arenas et al [89] Alternatively, one can examine the interpretation of the SERS spectrum of a chemically adsorbed molecule that forms a surface complex, but does not have an electronic absorption in the spectral region of the excitation, that is, there is no resonance Raman effect. In the latter case of chemisorption, identification of the bonding and characterization of the surface complexes in SERS, is the fundamental task in order to report the interpretation of the observed spectra.…”

Resonant charge transfer on the nanoscale: studying doublet states of adsorbates by surface‐enhanced Raman scattering

“…6,20 We have proposed a general methodology for predicting the RR relative intensities under RR-CT conditions: according to the A-term in RR, 24 those vibrations connecting the equilibrium geometries of the two electronic states involved in the resonant process should be the most enhanced. Since these doublets of neutral adsorbates such as pyrazine, pyridine and derivatives are not well characterized, we used ab initio calculations in order to gain information on these chemical species.…”

“…Obviously, the energy of the CT transition [E CT D E ver CT S 0 ] depends on the electric potential of the interphase given that the M V C A ;CT level involve charged species. 20 Negative electrode potentials stabilize the species M C and destabilize the species A . If the energy of laser photons is kept constant, the role of the electrode potential is just to tune the energy of the CT transition E CT :…”

“…The physical meaning ofˇ(or S 1 ) has been described in Ref. 20 and quantifies the effectiveness of the electrode potential to shift the energy of the CT transition. It may depend in a complex way on the electrode potential and a wide range of values have been reported for pyridine on different conditions.…”

“…17 The profiles show a maximum at V max ³ 0.50 V as in the case of methylpyrazine, 9 it being found that the maximum shifts towards more negative values as the number of methyl substituents attached to the pyrazine ring increases. 20 The SERS-CT excitation profiles of 3,5-dimethylpyridine are shown in Fig. 10.…”