John J. McMahon

Surface enhanced raman scattering and photodimerization of pyridyl-substituted ethylenes at a silver electrode surface

Pyridyl-substituted ethylenes have been observed to photodimerize while adsorbed at a polycrystalline silver electrode surface. Surface Raman spectra reveal that vinylpyridine and bis-pyridyl ethylene dimerize to products in which ethylenic bonds have been reduced. The potential dependence of the photochemical threshold wavelength implicates the role of adsorbateto-metal charge transfer excitation in the dimerization mechanism.Adsorbate-to-metal charge transfer leaves a cation radical which initiates dimerization with nearest neighbor adsorbate molecules. Photochemistry occurs only under conditions of heavy surface coverage. A prerequisite to the topochemical dimerization of ethylenic compounds in the solid state requires that the separation between neighboring ethylene groups lie between 3.7 and 4.1 A. The silver lattice parameter of 4.08 A and the absence of photochemistry under light surface coverages appear to correlate well to the solid state topochemical prerequisite. Observed isotopic shifts in the surface Raman spectra of the photoproducts prepared in H2O/D2O electrolytes indicates that solvent proton transfer occurs during dimerization. Adsorbate surface orientation was determined and the structures of photoproducts are identified.

High-temperature “in-situ” surface Raman spectroelectrochemistry in aqueous solutions

Etude, au moyen de ces methodes couplees, de la corrosion et de la passivation du plomb dans une solution de sulfate tres diluee, de 25 a 290°C et a 100 atm.

Photochemical charge transfer excitation of trans-4-stilbazole at a silver electrode

Abstract When adsorbed at the surface of a polycrystalline silver electrode, trans-4-stilbazole photoreacts, in the presence of dissolved oxygen, to form trans-4′-hydroxy-4-stilbazole. The structure of the photoproduct is confirmed by (1) a comparison of the surface Raman results with the normal Raman of trans-4′-hydroxy-4-stilbazole, and by (2) by ex-situ analysis of the irradiated electrode surface using high-resolution mass spectrometry. The surface photochemistry appears to be a one-photon process whose threshold wavelength lies in the middle of the visible range though neat trans-4-stilbazole absorbs only in the ultraviolet. The surface Raman intensity of the photoproduct increases synchronously with the Faradaic current of oxygen reduction. Attendant incorporation of oxygen exclusively at the 4′ position of end-on trans-4-stilbazole evidences a photo-initiated electron transfer from the electrode to oxygen reducing at the other end of the molecule. Scattering from stilbazole adsorbed at two different surface sites is evident in the surface Raman spectrum: charged (Ag+) sites and more neutral surface sites. We assign excitation profile maxima for stilbazole at Ag+ sites and the photochemical initiation to a silver-to-stilbazole charge transfer at those sites. Ab-initio molecular orbital calculations on a model composed of a ground state Li0-trans-4-stilbazole and the anion radical excited state Li+-trans-4-stilbazole− reveal a low-energy transition that supports assignment of the visible excitation to a silver-to-stilbazole electron transfer.

Raman Investigations of Biselenates. The Raman Spectra of RbHSeO4 in the Range 20–300K

Abstract In 1977 we undertook investigations 1 of the Raman spectra of RbHSeO4 and NH4HSeO4, prior to the discovery of their ferroelectric nature by a group of workers in Poland.2,3 Our studies did not reveal some of the behavior reported by this group, and other studies4,5 have indicated phase transitions not observed by us or the Polish group.6–9 Consequently we have withheld publishing the results of our earlier studies pending the completion of investigations designed to verify our initial findings and to determine the relationships between our Raman spectra and the reported behavior of these compounds. This paper represents the first in a planned series of communications relating our experiences with these biselenates and our interpretations of those points of disparity between the various studies that have been reported.

Resonance Raman Studies of Copper Pheophytinate

Abstract In recent years there has been considerable interest in the spectroscopic properties of chlorophyll molecules. Among the techniques employed is resonance Raman spectroscopy which can provide vibrational spectra with high selectivity, from chlorophyll a present in intact chloroplasts or cells.1 However, detecting resonance Raman scattering from chlorophyll a is hindered when excitation in the red region is used, owing to the strong fluorescence of the molecule near 670 nm. Copper pheophytinate, on the other hand, is largely non-fluorescent and thus lends itself to resonance Raman study in the red region. (The mechanism for fluorescence quenching in copper pheophytinate is believed to involve charge transfer from the porpnyrin ring to the Cu2+ ion upon excitation, although direct experimental evidence is still lacking.) we felt that it would be of interest to carry out a resonance Raman study to copper pheophytinate in the red region and compare the results with what is already known about the reso...