This paper reviews aspects of nonlinear optical spectroscopy of interfaces. The emphasis is put on second-order nonlinear optical techniques, such as sum-frequency generation (SFG), which possess intrinsic surface or interface selectivity and can therefore be used to probe buried interfaces accessible by light. The basic concepts of the second-order nonlinear response of surfaces and interfaces are given. While SFG in the ultraviolet-visible range allows one to achieve surface-specific electronic spectroscopy, infrared-visible SFG spectroscopy allows one to have access to absolute vibrational spectra of adsorbates at an interface. The main experimental schemes commonly employed are described. Selected experimental examples are given for studies of liquid surfaces and interfaces, polymer surfaces and interfaces, solid surfaces under ultra-high vacuum conditions or in inert atmospheres, solid-gas interfaces, solid-liquid interfaces and solid-solid interfaces. Both frequency-resolved studies and time-domain measurements are addressed.

Phone: +33 1 69 15 32 75; Fax: +33 1 69 15 33 28 RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) TITLE RUNNING HEAD Non uniqueness of parameters in NLO spectroscopiesAbstract Experimental data from second-order nonlinear optical spectroscopies (SFG, DFG, SHG) provide parameters relevant to the physical chemistry of interfaces and thin films. We show that there are in general 2 N or 2 N-1 equivalent sets of parameters to fit an experimental curve comprising N resonant features, of vibrational or electronic origin for example. We provide the algorithm to calculate these sets, among which the most appropriate has to be selected. The main consequences deal with the existence of "ghost resonances", the need of a critical analysis of fit results and the procedure to search for better sets of parameters coherent with applied constraints.

Absolute vibrational spectra of the electrochemical H/Pt and H/Pt(100) interfaces in the underand overpotential ranges are measured by ir-vis sum frequency generation. The vibrational signature of the underpotential-deposited hydrogen is structure sensitive and is related to its bonding configuration and to the water molecules located in the double layer region. A new hydrogen adspecies, presumed to be the intermediate for the hydrogen evolution reaction, shows up in the overpotential range, with a similar vibrational fingerprint on both surfaces. PACS numbers: 68.45.v, 42.65. Ky, 82.45.+z Hydrogen-surface interaction is of importance in various technological fields such as heterogeneous catalysis where hydrogen is involved as reactant, intermediate,

We have measured the potential dependence of optical second-harmonic generation from Ag(111) and Au(111) electrodes at wavelengths below and above the free electron regime. In the free electron regime, the cubic nonlinear susceptibility is dominant, leading to similar effects for positive and negative charging. Above the free electron regime, the quadratic nonlinear susceptibility of the d-electrons is dominant, leading to different effects for positive and negative charging. We also followed the oxidoreduction of Au(111) and found that second-harmonic generation is very sensitive to the different steps of the reaction.

Within semiconductor quantum dots (QDs), exciton recombination processes are noteworthy for depending on the nature of surface coordination and nanocrystal/ligand bonding. The influence of the molecular surroundings on QDs optoelectronic properties is therefore intensively studied. Here, from the converse point of view, we analyse and model the influence of QDs optoelectronic properties on their ligands. As revealed by sum-frequency generation spectroscopy, the vibrational structure of ligands is critically correlated to QDs electronic structure when these are pumped into their excitonic states. Given the different hypotheses commonly put forward, such a correlation is expected to derive from either a direct overlap between the electronic wavefunctions, a charge transfer, or an energy transfer. Assuming that the polarizability of ligands is subordinate to the local electric field induced by excitons through dipolar interaction, our classical model based on nonlinear optics unambiguously supports the latter hypothesis.

We report on the first in situ study of the electronic state and the structure of underpotentially deposited submonolayers of Cu on Au(lll) by x-ray-absorption spectroscopy. We found (i) various ordered adlayer structures, dependent on coverage, time, and direction of the potential scan, and (ii) strong interactions of Cu and oxygen and charge transfer from Cu to Au. During the deposition, the initial (VJxVJ) structure at 0.3 monolayer evolved to an equilibrium with the c(5x5). The latter and the (lxl) phases were observed at 0.6 and 1 monolayer, respectively. The three structures are present at 0.6 monolayer during the stripping scan.

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