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

The effect of the specific adsorption of iodide on the optical second harmo nic (SH) signal generated at the surface of a mercury film electrode has be en 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 anion s 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 a dsorption occurs. The potential corresponding to the minimum of the SH inte nsity parabolic curve follows the shift of the point of zero charge with th e bulk solution iodide concentration, although with a small offset, the mag nitude of which depends on the geometrical configuration of the experiment. For positive metal charges, the effect of specific adsorption is non-linea r with the bulk solution iodide concentration. A strong enhancement of the SH intensity is indeed observed at iodide concentrations below 10(-4) M. Th is enhancement is attributed to the specific interaction between the mercur y electronic density and the iodide anions and more specifically to the dis crete nature of the adsorbed charge. (C) 2001 Elsevier Science B.V. All rig hts reserved.