A. Tadjeddine

Sum-frequency generation spectroscopy of interfaces

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.

Electrochemical study of diamond thin films in neutral and basic solutions of nitrate

Abstract The electrochemical behavior of boron-doped diamond films has been investigated in a number of neutral and alkaline solutions with and without nitrate ions. Two kinds of diamond electrode were studied: self-supported film (100 μm) (sample A) and diamond film (10 μm) supported on a Si substrate (sample B). It was found that water oxidation and reduction appear at much larger polarizations for diamond electrodes, as compared with platinum and platinized platinum electrodes. In particular, the higher (cathodic) overpotential for hydrogen reduction permits efficient nitrate reduction to ammonia. The underlying Si substrate is shown to take part in the electrochemistry of the diamond electrodes. In the case of the Si-supported electrode (sample B) the reaction with the Si substrate was imminent. For the free-standing diamond electrode (sample A) various impurities in the grain boundaries and at the back of the electrode, including the back metallic contact, intervened with the electrochemistry of the diamond electrode, but to a much lesser extent than with sample B. Meticulous cleaning and careful working practices permitted this interference to be excluded altogether in sample A.

Spectroscopic investigations of adsorbates at the metal—electrolyte interface using sum frequency generation

Abstract The adsorption of carbon monoxide and cyanide on a platinum electrode is studied by the nonlinear optical technique of sum frequency generation. Two bands are observed for the stretching vibration of adsorbed cyanide. They are in antiphase and correspond to N-bound and C-bound species.

Electrochemical deposition of hydrogen on platinum single crystals studied by infrared‐visible sum‐frequency generation

Under‐ and overpotential deposition of hydrogen on polycrystalline platinum, Pt(100), Pt(110), and Pt(111) surfaces in sulfuric acidic medium is monitored in situ by infrared‐visible sum‐frequency generation (SFG). In the underpotential range, the vibrational signature between 1800 and 2020 cm−1 reveals hydrogen bonding between the stable hydrogen adsorbed in terminal sites and the water molecules in the electrolyte. For Pt(100) and Pt(111), the observed double SFG resonance is most consistent with a bonding configuration where water dimers are each bounded to three hydrogen atoms adsorbed on the relaxed substrate. In the overpotential range, an additional hydrogen adspecies appears with a similar vibrational fingerprint on all the investigated platinum surfaces and is tentatively assigned to the intermediate of the hydrogen evolution reaction. Its SFG resonance frequency near 1770 cm−1 suggests a dihydride adsorption configuration. The detection at under‐ or overpotential of high frequency SFG resonances...

Vibrational spectroscopy of the electrochemical interface by visible infrared sum frequency generation

Vibrational properties of species adsorbed on platinum electrodes in the electrochemical environment are investigated by visible-infrared sum-frequency generation (SFG). Absolute vibrational spectra of adsorbed species are obtained by combining the inherent interfacial selectivity of SFG between centrosymmetric media and the use of a free-electron laser as infrared source. SFG spectra of hydrogen on polycrystal and single crystal platinum electrodes in acid medium are measured both in the under- and in the overpotential region. The vibrational fingerprints of the underpotential deposited hydrogen are structure sensitive and are related to its bonding to interfacial water molecules. On Pt(111) electrode, we have detected adsorbed hydrogen at potential as high as 0.35 V, within the so-called unusual adsorption state. A new hydrogen adspecies shows up in the overpotential range. Its vibrational signature is not dependent on the electrode surface orientation and we have assigned it to the intermediate of hydrogen evolution.

INFRARED LASER INDUCED ISOMERIZATION OF METHANOL POLYMERS TRAPPED IN NITROGEN MATRIX. I. TRIMERS

Selective vibrational excitations of OH stretching modes of methanol aggregates trapped in solid nitrogen were carried out in the range 3500–3370 cm−1. This proved an efficient means of interconverting trimers and possibly branched polymers absorbing in this domain. Irradiation in the absorption band of the cyclic trimer (CT) around 3450 cm−1 leads to a cycle opening and to the appearance of several groups of bands assigned to different forms of open chain trimer (OTi, i=1–3). Irradiations at the absorption frequencies of OT1 and OT2 interconverted these open species and partially regenerate CT. Isotopic dilution experiments enable to improve vibrational analysis of the open trimers, previously advanced from data obtained with irradiations performed with polychromatic light [Chem. Phys. Lett. 217, 123 (1994)]. Two other bands at 3424 and 3370 cm−1, growing upon annealing as the high polymer bands around 3250 cm−1, also turn out to be sensitive to selective irradiation. OH/OD and 16O/18O isotopic substitut...

Vibrational spectroscopy of a Au(hkl)-electrolyte interface by in situ visible-infrared difference frequency generation

Abstract We demonstrate a significant decrease in the non-resonant response of gold substrates in IR-VIS nonlinear optical spectroscopy by switching from sum- to difference-frequency generation. This has allowed DFG spectroscopy to show the significant effect of the surface orientation on the adsorption strength of CN − , which decreases from (110) to (100) and (111). Both the linewidth and the potential tuning rate of the DFG resonances are independent of the electrode structure, pointing to CN − being tightly and linearly bound to each nearest surface gold atom.

Vibrational spectroscopic study of the adsorption of pyridine at the Au(111)-electrolyte interface by in situ difference frequency generation

The adsorption geometry of pyridine at the Au(111)/electrolyte interface has been studied for the first time by in situ visible-infrared difference frequency generation (DFG) in the spectral range 900–1150 cm−1 using the CLIO free electron laser. The pyridine molecule was found to adsorb using the nitrogen lone pair, in an end-on configuration, the molecular plane being tilted. The potential dependence of the DFG spectra showed evidences of a reorientation of the molecule around the potential of zero charge (pzc). At a positively charged interface, the molecule tilts towards the surface normal and undergoes a rotation around its C2 axis.

Adsorbate vibrational spectroscopy by IR-visible sum-frequency generation using CLIO-FEL: CO from CH3OH electrochemical decomposition on Pt

The use of infrared-FEL allows adsorbate vibrational spectroscopy by IR-visible sum-frequency generation (SFG) beyond the frequency range accessible by benchtop IR lasers. Therefore, we synchronized a frequency doubled mode-locked YAG laser (532 nm) with the CLIO-FEL, which is tuneable from 2 to 17.5 μm. The feasibility is demonstrated by recording SFG-vibrational spectra near 5-μm of the CO adsorbate at the electrochemical interface Pt/HClO4 solution containing CH3OH. These spectra indicate the coexistence of linearly and doubly-bounded species. This work is a preliminary step towards adsorbate SFG-vibrational spectroscopy at longer wavelengths.

Sum and Difference Frequency Generation at the Electrochemical Interface

The surface electronic properties and the vibrational spectroscopy of the electrochemical interface are investigated by in situ Infrared-Visible Sum (SFG) and Difference (DFG) Frequency Generation. We first present the theoretical background of SFG/DFG and their generation at the interface and calculate the interface second-order susceptibility including the resonant contribution of the adsorbate and the non resonant contribution of the metal substrate electronic intraband and interband transitions. The effect of the latter is addressed by comparing the experimental SFG and DFG spectra recorded on silver, platinum and gold substrates. The experimental set-up and procedure are then described, and some selected experimental examples are shown and discussed.