Journal of Electroanalytical Chemistry 500 (2001) 365 – 373 www.elsevier.nl/locate/jelechem Surface second harmonic generation from a mercury film electrode electrochemically deposited on an iridium substrate  Part II. Adsorbed anions P. Galletto, S. Loridant 1 , R. Antoine 2 , P.F. Brevet 2 , H.H. Girault * Laboratoire d’Electrochimie, E ´ cole Polytechnique Fe ´de ´rale de Lausanne, CH-1015 Lausanne, Switzerland Received 12 June 2000; received in revised form 19 July 2000; accepted 17 August 2000 Dedicated to Professor Roger Parsons on the occasion of his retirement from the position of the Editor in Chief of the Journal of Electroanalytical Chemistry and in recognition of his contribution to electrochemistry Abstract The effect of the specific adsorption of iodide on the optical second harmonic (SH) signal generated at the surface of a mercury film electrode has been investigated. The overall SH response depends on both the metal surface charge and the iodide coverage. The latter contribution is responsible for deviations from the case of purely non-adsorbing anions like fluoride anions where the SH signal is a parabolic function of the metal surface charge. In particular, the effect of specific adsorption on the SH signal intensity is seen as an overall increase in the SH signal intensity where specific adsorption occurs. The potential corresponding to the minimum of the SH intensity parabolic curve follows the shift of the point of zero charge with the bulk solution iodide concentration, although with a small offset, the magnitude of which depends on the geometrical configuration of the experiment. For positive metal charges, the effect of specific adsorption is non-linear with the bulk solution iodide concentration. A strong enhancement of the SH intensity is indeed observed at iodide concentrations below 10 -4 M. This enhancement is attributed to the specific interaction between the mercury electronic density and the iodide anions and more specifically to the discrete nature of the adsorbed charge. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Point of zero charge; Double layer; Second harmonic generation; Specific adsorption 1. Introduction In the past, mercury has played a dominant role in the development of classical double layer theories ow- ing to its advantages with respect to other metals [1,2]. For instance, its liquid-like structure prevents complica- tions arising from the atomic lattice arrangement of crystalline solid surfaces and its behaviour as an ideally polarised electrode over a wide range of applied poten- tials has made mercury a metal of reference in electro- chemistry. Since the pioneering works on the classical double layer models, much effort has been devoted to the understanding of adsorption phenomena occurring at the metal surfaces. Most of the recent studies all agree in emphasising the role played by the metal surface in the determination of the interface properties [3,4]. The acceptance of the existence of a charge distri- bution on the metal surface as well as in the electrolyte solution has led to the conclusion that the metal surface also contributes to the total double layer capacity. Hence, optical techniques for the study of interfaces have been used with the aim of achieving a better characterisation of the electronic properties of metal surfaces [5]. As a specific surface technique, second harmonic generation (SHG) has been applied to the investigation of double layer problems [6,7]. According to the inversion symmetry property in centrosymmetric  For Part I, see Ref. [21]. * Corresponding author. Tel.: +41-21-6933151; fax: +41-21- 6933667. E-mail address: hubert.girault@epfl.ch (H.H. Girault). 1 Present address: Institut de Recherches sur la Catalyse-CNRS, 2 Avenue Albert Einstein, F-69626 Villeurbanne Cedex, France. 2 Present address: Laboratoire de Spectrometrie Ionique et Mole ´cu- laire, UMR CNRS 5579, Universite ´ Claude Bernard Lyon 1, 43 Boulevard du 11 November 1918, F-69622 Villeurbanne, France. 0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0022-0728(00)00319-3