A pH-dependent SERS study of thiophene-2-carboxylic acid adsorbed on Ag-sols Uttam K Sarkar * Department of Physics, Malda College, Malda 732101, WB, India Received 22 February 2003; in final form 30 April 2003 Abstract SERS is observed for thiophene-2-carboxylic acid (TCA) adsorbed on Ag-sols. The enhancement is maximum when TCA concentration, in Ag-sol, is 5  10 3 M. The SER spectra of TCA are different at different pH values, which suggest that the adsorption geometry is pH dependent. This is further supported by the absorption spectra at different pH values. It is inferred that the chemisorption takes place through both the S-atom and COOH group for pH 6 7 and through the p-electronic system for pH ¼ 10.5. Quick desulphurization at pH ¼ 2 and stable perpendicular and parallel surface bonding for pH ¼ 7 and 10.5, respectively, is revealed. Ó 2003 Published by Elsevier Science B.V. 1. Introduction Surface enhanced Raman Spectroscopy (SERS) is a useful technique for the study of chemical as well as physical transformation of molecules ad- sorbed on metal surfaces [1–4]. Two mechanisms, mainly, have been shown to contribute to SERS. One is long-range classical electromagnetic field effect and the other is short-range charge transfer effect [3]. The classical field effect is due to surface plasmon resonance by the incident electromagnetic field and is operative at larger distances. For iso- lated metal sphere it has been shown that the plasmon resonance occurs if the frequency of ex- citation matches with that at which the extinction by the metal particles is maximum [5]. The short- range effect, on the other hand, is because of ex- change of electrons between the molecule and the metal. This latter effect is dependent on the con- centration of the adsorbate and is maximum when monomolecular layer is formed on the surface [6]. According to the image dipole theory, Raman scattering from an adsorbed molecule arises from both a dipole induced in the molecules and from the image dipole of the molecules within the metal surface. Vibrational modes which have dipolar components perpendicular to the surface will show enhancement and the magnitude of the enhance- ment will be proportional to the projection of the dipole moment on the perpendicular to the sur- face. Thus, SERS can provide information about orientation of the molecule on the metal surface [7,8]. Chemical Physics Letters 374 (2003) 341–347 www.elsevier.com/locate/cplett * Fax: +913512220516. E-mail address: uttamsakar@yahoo.com. 0009-2614/03/$ - see front matter Ó 2003 Published by Elsevier Science B.V. doi:10.1016/S0009-2614(03)00728-0