A laser-induced transient photovoltaic effect using blocked electrodes

Abstract

adsorption after heating to 300.0 °C then cooling to 60.0 °C is consistent with the latter observations. Madey et al.7 also report that methane's fraction of the hydrocarbon product at 425 °C (0.99) is considerably higher than this fraction at 175 °C (0.90). It has been suggested that while the methanation mechanism may involve the hydrogenation of a surface carbon species produced by the decomposition of CO, the formation of higher hydrocarbons may require oxygen-containing species as intermediates. This would certainly be consistent with the results of Figure 3. At T = 180.0 °C, for example, considerable adsorbed CO is available from which oxygen-containing species could be formed while at 240.0 °C there is little adsorbed CO and carbon atoms appear to be the predominant surface species.Care must be taken, however, in extrapolating our results of Figure 3 to other reactant pressure regions, since the corresponding experiments carried out in the 10'7 torr range showed a similar type of behavior but the CO band disappears at a temperature approximately 70 °C lower (Figure 4).

Photovoltaic detection method for condensed-phase photoionization

“…nitude, and temporal behavior are comparable to those reported previously by Coleman and co-workers (17). The peak photopotential in our studies (Figure 3A) is approximately 5-fold smaller and the temporal decay is approximately 2-fold longer than reported previously with n-type tin oxide electrodes irradiated by a nitrogen laser (17); however, this discrepancy may be attributed to the method of signal acquisition. Coleman and co-workers measured the photopotential arising after a single laser pulse, and noted that the magnitude decreased and the time constant increased with repeated irradiation (17).…”

“…Consequently, the reaction written above is essentially irreversible in deoxygenated solvents, due to the change in physical state of the decomposition products. By interpolation of literature data (17,18), the quantum yield for photodecomposition of permanganate in aqueous solution is estimated to be 3.5 X 10~3 at 337.1 nm and 5.5 X 10~2 at 248 nm. The quantum yield is reported to be independent of permanganate ion concentration (10-2 to 10"4 M), hydrogen ion concentration (pH 7-14), temperature (0-45 °C), and irradiance under the experimental conditions employed herein (18).…”

“…Potassium permanganate was selected as the analyte molecule for initial studies because it has been used previously to characterize the photovoltaic detection method (17) and because its photochemical properties have been studied extensively (18,19). Although the exact mechanism remains in question, the photodecomposition of permanganate ion in neutral or basic aqueous solution is reported to yield the following products (18,19):…”

“…A novel alternative to conduction measurement, proposed by Coleman and co-workers (17), is to detect the photopotential induced at the electrode-solution interface in the absence of an applied electric field. By elimination of the bias voltage, contributions from intrinsic solvent conduction are substantially reduced or eliminated.…”

A Photovoltaic Detection System for Flow Injection and Liquid Chromatography

“…One means to circumvent the problems encountered in the photoconductive technique is to use a different method of signal acquisition. Coleman and co-workers reported the detection of solute photoionization by measuring the potential induced at the electrode−solution interface in the absence of an applied field …”