Alan Campion

Surface-enhanced Raman scattering

We present an introduction to surface-enhanced Raman scattering (SERS) which reviews the basic experimental facts and the essential features of the mechanisms which have been proposed to account for the observations. We then review very recent fundamental developments which include: SERS from single particles and single molecules; SERS from fractal clusters and surfaces; and new insights into the chemical enhancement mechanism of SERS.

On the chemical mechanism of surface enhanced Raman scattering: Experiment and theory

We have investigated the chemical mechanism of surface enhanced Raman scattering (SERS) on an atomically smooth metal surface using electron energy loss spectroscopy (EELS) and molecular spectroscopy simulations. The EEL spectra of pyromellitic dianhydride (PMDA) adsorbed on Cu(100) and Cu(111) are reported. Simulations of the surface-enhanced Raman spectra and electron energy loss spectra (EELS) of pyromellitic dianhydride adsorbed on Cu(100) and Cu(111) are reported. The surface enhanced Raman spectra [J. Chem. Soc. Faraday Trans. 92, 4775 (1996)] and the EEL spectra are shown to be sensitive to crystal face. The relevant excited state observed in the EEL spectrum is not intrinsic to molecular PMDA, but results from chemisorption. The Raman spectra are sensitive to the incident laser polarization on both the (100) and (111) surfaces but in different ways. These observations are shown to be a result of the excited state potential energy surface having different shape, and the respective transition dipole...

Size quantization effects in cadmium sulfide layers formed by a Langmuir-Blodgett technique

Abstract Layers of Q-state particles ( 2 S to yield cadmium sulfide particles in a lattice of arachidic acid. The onset of absorption of this layer is blue-shifted by 0.38 eV with respect to bulk CdS. The adherence to Beers law and insensitivity of the absorbance edge to the number of LB layers deposited (up to seven) suggest no strong electronic interactions between successive layers. Ellipsometric measurements show that exposure of the LB films of cadmium arachidic acid to H 2 S gas results in an increase in thickness of about 3 A per layer.

Surface photochemistry. II. Wavelength dependences of photoinduced dissociation, desorption, and rearrangement of O2 on Pt(111)

A Pt(111) surface saturated with molecular oxygen was irradiated by UV light from a mercury arc lamp with various cutoff filters. High‐resolution electron energy‐loss spectroscopy (HREELS) and temperature programmed desorption mass spectroscopy (TPD) were used to characterize the products retained on the surface. Upon UV irradiation at 95 K, chemisorbed O2 (peroxo) undergoes dissociation, desorption and rearrangement. Different wavelength dependences were observed for the three processes: dissociation was not observed at wavelengths longer than 295 nm, in agreement with gas phase photodissociation of the O–O bond in hydrogen peroxide; desorption and rearrangement became negligible at wavelengths longer than 420 nm, in agreement with the photolysis of an organometallic peroxoplatinum complex. For wavelengths between 230 and 315 nm, the average cross sections of dissociation and desorption were estimated to be 5.7×10−20 and 1.2×10−19 cm2 , respectively. The possible origins of the three processes are discussed.

Hydrogen photoproduction by Nafion/cadmium sulfide/platinum films in water/sulfide ion solutions

The construction, characterization, and evaluation of a novel system for photocatalytic hydrogen generation that comprises semiconductor (CdS) crystallites embedded in a polymer (Nafion) matrix that also contains a hydrogen-evolution catalyst (Pt) are reported. Hydrogen-production efficiencies from water containing a sacrificial electron donor (sulfide ion) were greater than those commonly obtained with unsupported colloidal or powdered semiconductors under similar conditions. Several factors dramatically influence performance: the crystalline form and surface composition of the semiconductor and the extent of catalyst dispersion throughout the film.

Selection rules for surface Raman spectroscopy: Experimental results

We report the results of an experimental survey determining the contributing factors for Raman spectral activity of molecules adsorbed on metal surfaces. The systems investigated are benzene, pyrazine, and s‐triazine physically adsorbed onto the (111) and (110) faces of single crystal silver. We consider the relative importance of three factors: symmetry reduction, image dipoles (metal optics), and quadrupole polarizability contributions. The experimental data may be explained satisfactorily by involving both the adsite geometry and image effects in the theoretical selection rules. Quadrupole interactions appear neither necessary nor sufficient to predict the proper surface spectra.

Normal raman scattering from pyridine adsorbed on the low-index faces of silver

Abstract We find no enhancement of the Raman scattering cross section for pyridine adsorbed at 120 K on the (100), (110) and (111) faces of silver in ultrahigh vacuum. The frequencies we observe are essentially unshifted from those of liquid pyridine, intensity ratios are similar to the liquid, the signal intensity is linear in coverage from submonolayer to multilayer, and the depolarization ratio is low. Since these observations are in marked contrast to those associated with surface-enhanced Raman scattering (SERS), we conclude that our spectra result from normal Raman scattering. Our results support the hypothesis that special adsorption sites are responsible for a substantial fraction of the total enhancement of the Raman cross section for the pyridine - silver system.

XPS and IR study of X-ray induced degradation of PVA polymer film

Abstract The X-ray induced compositional modification of a solvent cast polyvinyl alcohol [PVA, -(CH 2 -HCOH) n -] polymer film was studied using X-ray photoelectron (XPS) and transmission infra-red (IR) spectroscopies. While the -CH 2 group was stable, the alcoholic -COH moiety was unstable under the X-ray beam (Mg K α radiation: 1253.6 eV) and reached continuosly to produce three new C(1s) peaks assignable to carbonyl C=O (oxidation), carboxylate -COO (oxidative chain scission) and ether C-O-C linkages (crosslinking, dehydroxylation). Transmission FT-IR spectra of X-ray treated films showed the carbonyl stretch at 1700 cm -1 and a loss of intensity in the O-H stretching mode near 3340 cm -1 . The results indicate that degradation occurs throughout the polymer bulk and is caused either directly, by X-ray photons, or by the secondary electrons generated within the polymer. The ease with which degration can be detected is sensitive to analyzer resolution, X-ray flux and precautionary measures.

Variations in homogeneous fluorescence linewidth and electron–phonon coupling within an inhomogeneous spectral profile

Abstract : The fluorescence line narrowing technique (FLN) is used to determine the homogeneous width of the 5D(0) to 7D(0) resonance fluorescence line of Eu(3+) in an amorphous silicate matrix as a function of laser excitation wavelength and temperature (200-350 K). A smooth increase in delta nu sub hom is observed as the excitation wavelength is decreased across the inhomogeneous profile (excitation of sites of progressively higher average crystal field. The combined temperature and excitation wavelength dependence identify the broadening mechanism as a two-phonon Raman scattering process and suggest that the observed increase of delta nu sub hom with increasing excitation energy is due to stronger electron-phonon coupling at sites of stronger crystal field. (Author)

Photoactivity of ternary lead-group IVB oxides for hydrogen and oxygen evolution

The relative photocatalytic activity of a series of lead-IVB group oxides (PbCrO4, PbMoO4, and PbWO4) was studied for hydrogen evolution from aqueous methanol solution and for oxygen evolution from aqueous silver nitrate solution. Among the compounds, only PbMoO4 and platinized PbMoO4 powders acted as photocatalysts for oxygen and hydrogen evolution reactions from aqueous solutions, with an activity for oxygen evolution on PbMoO4 comparable to that observed on TiO2.