Surface second harmonic generation from a mercury film electrode electrochemically deposited on an iridium substrate: Part II. Adsorbed anions

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
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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.

Introduction

In the past, mercury has played a dominant role in the development of classical double layer theories owing to its advantages with respect to other metals [1], [2]. For instance, its liquid-like structure prevents complications arising from the atomic lattice arrangement of crystalline solid surfaces and its behaviour as an ideally polarised electrode over a wide range of applied potentials has made mercury a metal of reference in electrochemistry. 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 distribution 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 media, the SH signal is generated only at the interface in the electric dipole approximation. Furthermore, in the presence of an applied electric field, the contribution arising from the coupling of the applied static field and the laser field is also surface specific since the applied field is non-vanishing only in the vicinity of the surface. In the long wavelength limit, second-order optical phenomena originate from the excitation of the metal free conduction band electrons without any interplay from interband transitions. As a result, the electronic density radiates a strong non-linear response severely modified by both the applied electric field and specific adsorption. This feature can be related to other surface physical properties such as the metal work function, also affected by the surface coverage of adsorbed species and the interface polarisation [8]. The effect of chemisorption on the SHG response has been examined for alkali metal atoms adsorbed at the metal  vacuum interface and strong SHG enhancements were reported at low coverage (θ below 0.3) [9]. These enhancements were attributed to the change with the alkali metal coverage of the optical transition probability between alkali electronic states rather than to the increase in the electronic density at the surface following chemisorption. Nevertheless, theoretical studies have clearly demonstrated the role played by the hyperpolarisability of the adsorbed adatoms and the charge transfer between the adsorbate and the metal occurring upon adsorption [10], [11]. Specific adsorption at electrochemical interfaces has also been the subject of non-linear optical studies. In particular, analysis of the close relationship between the SH signal and the surface charge density yielded values for the Gibbs energy of adsorption for bromide on silver substrates for example [12]. Pettinger and Bilger [13] recently employed interference SHG (ISHG) to study the silver  electrolyte interface. No effect from specific adsorption was observed as a function of the surface charge on the phase of the SH electromagnetic field for metal charges where anions do not form arranged adlayers [14]. One of the conclusions drawn was that the contribution from the coupling of the laser field with the interfacial static electric field dominates the SHG response. Theoretical studies based on the jellium model and on the density functional theory have been carried out in order to calculate the induced second-order polarisation under the presence of a perturbation by a static electric field [15], [16], [17]. Effects of specific adsorption were introduced as a perturbation to the metal surface electronic density and an increase in the SHG signal was predicted [16]. This emphasises the role played by adsorbed species in determining the overall second-order polarisation of the metal free electron gas. In these theoretical studies dealing with the adsorption of simple anions at metal  electrolyte interfaces, the contribution to the SHG signal from the adsorbed species themselves, through their hyperpolarisability, has never been taken into consideration. This assumption is valid when no resonance due to optical transitions of the adsorbed ions occurs at the working photon energies. Furthermore, these studies were mostly limited to adsorbates possessing low hyperpolarisabilities. Note however, that it is possible that the symmetry of the adsorbed species is so affected upon adsorption that their hyperpolarisability is enhanced. Nevertheless, this effect is expected to be negligible at the mercury surface for halide anions.

In this work, we present a study of the non-linear optical properties of a mercury film electrode immersed in iodide+fluoride mixtures of constant ionic strength. It has indeed been recognised in the past that iodide is strongly adsorbed at the mercury  electrolyte interface and enhancements of the SHG signal were observed on Pt electrodes [18], [19]. The potential dependence of the SHG signal from a mercury film  1 M KF electrolyte solution interface has been followed in the long wavelength limit, where no interband transition occurs ensuring that the free electron gas regime dominates the second harmonic response. The effect of iodide specific adsorption on the measured SHG signal was thus recorded as a function of the bulk iodide mole fraction at constant ionic strength. The function a(σ), representing the surface non-linear currents normal to the surface [20], was determined quantitatively as a function of the metal surface charge and the iodide mole fraction.

Section snippets

Experimental

The mercury film electrode was prepared as described in a previous work [21]. Briefly, an iridium electrode substrate was fixed on a flat glass support and placed inside the electrochemical cell in a vertical position with respect to the laboratory framework. The electrical contact was made with a silver conducting paste connecting the iridium film to a platinum wire. The connections were then isolated with an epoxy resin. A traditional three electrode cell was employed. The saturated calomel

Theory

In the long-wavelength approximation, metal free conduction band electrons are generally taken into account through a free electron gas immersed in a positive ionic core potential. The second harmonic light is therefore radiated by the non-linear polarisation of this free electron gas oscillating at the harmonic frequency. Mercury being a medium possessing the property of inversion symmetry, no electric dipole contribution from the bulk metal is allowed. As a result, the origin of the second

Results and discussion

In Fig. 1 the measured second harmonic generation intensity is reported as a function of the applied potential for eight x M KI+(1−x) M KF electrolyte solutions. The increase in the iodide mole fraction leads to a progressive displacement of the minimum of the SHG intensity curve (point of second harmonic minimum, (pshm)) against the applied potential towards more negative potentials. Hence, the pshm follows the same trend as the point of zero charge (pzc) with the increase in the iodide mole

Conclusion

The effect of the specific adsorption of iodide anions has been investigated for mixtures of KF and KI electrolytes of constant ionic strength. The pshm is observed to follow the pzc over several decades of iodide concentrations, albeit with a small offset the origin of which stems from the experimental geometrical arrangement only. Furthermore, at all iodide concentrations, the SH intensity takes a unique value at very negative metal surface charges in the region where no specific adsorption

Acknowledgements

The authors acknowledge fruitful discussions with Dr Alastair W. Wark and Dr David J. Fermı́n. The authors would also like to thank most sincerely Professor R. Parsons for helpful discussions and access to personal data. H.H.G. is most grateful to Professor R. Parsons for his support and friendship over the years. The support from the Fonds National Suisse de la Recherche Scientifique under grant number 2000-043381-95/1 is gratefully acknowledged. The Laboratoire d'Electrochimie is part of the

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    For Part I, see Ref. [21].

    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 Moléculaire, UMR CNRS 5579, Université Claude Bernard Lyon 1, 43 Boulevard du 11 November 1918, F-69622 Villeurbanne, France.

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