Charge-Transfer Processes in Surface-Enhanced Raman Scattering. Franck-Condon Active Vibrations of Pyrazine Juan Francisco Arenas, Mark Steven Woolley, Juan Carlos Otero,* and Juan Ignacio Marcos Department of Physical Chemistry, Faculty of Sciences, UniVersity of Ma ´ laga, E-29071 Ma ´ laga, Spain ReceiVed: August 4, 1995; In Final Form: NoVember 9, 1995 X SERS spectra of pyrazine on silver electrode have been recorded and analyzed, assuming a charge transfer effect and using selection rules analogous to those of resonance Raman. With the aim of predicting the effect of this mechanism on the selective enhancement of fundamentals, a method has been proposed based on an analysis of the results of geometry optimizations carried out by ab initio calculations. The strongest SERS bands coincide with those assigned to the normal modes connecting the equilibrium geometries of the neutral molecule and the radical anion. These results support the presence of a charge transfer process from the metal to the adsorbate in the SERS spectra of pyrazine where a significant enhancement of vibrations 8a, 9a, 1, and 6a can be observed. The prediction capability of the analysis proposed here has been checked with published resonance Raman spectra of pyrazine and pyrazine-d 4 . In all of the cases the strongest bands in the spectra are directly related to the largest ΔQ values obtained through the transformation ΔQ ) L -1 ΔR. Introduction Twenty years after the discovery of surface enhanced Raman scattering (SERS), 1,2 there now exists a general agreement that the enormous observed enhancement of the Raman signal is due mainly to two mechanisms, the electromagnetic (EM) and the charge transfer (CT) or chemical effect. 3-5 The selection rules derived from the EM mechanism are simple to apply as they resemble those of surface infrared spectroscopy and have been widely used. However, well defined selection rules that are capable of at least detecting the presence of the CT mechanism are not available given that this mechanism depends on the electronic structure of the particular metal-adsorbate system. Some of the published SERS results for the pyrazine molecule are a good example of this. 6-9 In some works, a marked enhancement of the bands belonging to totally symmetric vibrations is seen that have been analyzed on the basis of the surface orientation of the adsorbate. However, similar results observed in the SERS of pyridine at sufficiently negative eletrode potentials have been satisfactorily explained assuming the existence of a charge transfer state accessible to the system under the experimental conditions. 4,10,11 In general there are no simple rules that allow the recognition of which mechanism or mechanisms have given rise to a particular spectrum and therefore which class of selection rules should be applied. This puts in serious doubt the type of information that can be extracted from a SERS experiment: the surface orientation of the adsorbate (EM) or the properties of the electronic states involved in the charge transfer process (CT). In this respect, electron transmission and electron energy loss (EELS) experiments under resonance conditions (shape reso- nances 11-15 ) have been quite useful in explaining the SERS- CT results. If the charge transfer in SERS-CT amounts to a complete electron, the transient state would correspond to the radical anion in both cases. 4 This parallelism has allowed Otto et al. to explain the SERS results of benzene on cold deposited silver or sodium, 4,16 as the most intense bands coincide essentially with those observed in electron impact experiments. 17 The selective enhancement mechanism of these bands is similar in both cases. These active bands are closely related to the differences between the equilibrium geometries of the two states involved; 18 the ground electronic state of the neutral molecule and that of the radical anion or the CT state in EELS or SERS, respectively. When the charge is trapped by the molecule, the nuclear structure relaxes in the direction of the potential energy surface minimum of the excited state. When the molecule comes back to its ground state, the normal modes connecting the equilibrium structures of the upper and the lower states remain excited much more probably than any other. Unfortu- nately, published works on shape resonances is rather scarce and the selection rules that have been proposed based on symmetry considerations are restricted to benzene. 15,19 The origin of the selective enhancement of fundamentals that has been described is analogous to that of resonance Raman (RR) spectroscopy via Franck-Condon factors (A term). 20 In practice, the derived selection rules of this mechanism can be summarized by the well-known empirical rule of Tsuboi: 21 if a Raman line becomes stronger when the excited frequency is brought into resonance with an electronic band, the equilibrium conformation of the molecule will be distorted along the normal coordinate of the giVen Raman line in the transition from the ground to the excited electronic state. In a SERS-CT experi- ment, the situation is somewhat more complicated since the radical anion or the charge transfer states of a molecule such as pyrazine are not as well-known as are the excited singlets relevant in RR spectroscopy. In this work we propose the use of geometry optimization results carried out by using ab initio calculations in order to detect the presence or absence of the CT mechanism in SERS. For this, we have assumed the hypothesis that in a SERS-CT experiment, an electron is transferred from the metal to the adsorbate, for which the CT state is equivalent to that of the radical anion from the molecular point of view. Since this chemical species is a doublet in its electronic ground state it will be named as the D 0 state. The analysis will consist of comparing the equilibrium geometries of the S 0 and the D 0 states in order to find out which portion of the molecule is the actual chromophore. In small aromatic molecules such as pyrazine the chromophore extends over its whole structure and as such the transferred electron density will be delocalized throughout X Abstract published in AdVance ACS Abstracts, January 15, 1996. 3199 J. Phys. Chem. 1996, 100, 3199-3206 0022-3654/96/20100-3199$12.00/0 © 1996 American Chemical Society