Charge, transient generation

Transient Photoconductivity. A solution of neutral molecules in a polar solvent shows only ohmic conductivity, but if ions are formed by the action of the photolytic flash these charge carriers generate an additional current which is proportional to the ion concentration. The observation of such transient photocurrents is the most direct experimental evidence for the formation of free, solvated ions in electron transfer reactions. The quantum yield of ion formation can be obtained through proper calibration procedures and the kinetics of ion recombination can be determined. Figure 7.37 gives an example of such transient photocurrent rise and decay. 

Transient states generated from PH Hj. Absorbance transients generated from the P%Hm state provide a calibration for the extent of the absorbance changes which can be expected for a charge separation with a quantum yield of 1. Fig. 1 shows transient spectra in the 760 to 900 nm region recorded with 1.5 ps time resolution from the PHlHj Q state at 90 K. The multichannel detection was not sensitive beyond 900 nm. 

VShen a suspension of chloroplasts is illuminated with a non-saturating flash a transient electric potential difference can be measured between two electrodes inserted into the suspension if the electrodes are separated along the direction of the li t path (Fowler and Kok, 1974 Witt and Zickler 1973) Hiis effect has been interpreted to reflect the transient generation of a transvesicular electric potential difference. Due to the partial li t absorption a hi ier nmher of charge separations occur at that half of the vesicle directed to the li t entrance side than at the li t exit side. (See scheme in Fig. 1.)... 

The flash photolysis of s-trinitrobenzene (TNB) aerated solns in alcohols generated a transient species with absorption maxima at 430 and 51 Onm (Ref 27). The yield of the transient was a function of oxygen concn, and its rate of formation was viscosity dependent. In deaerated solns, instead of the transient, a brown permanent product, identified as a charge transfer... 

It is important to note that there may be at least two reasons for obtaining deviations from a purely exponential behavior for a PMC transient. These are a too high excess carrier generation, which may cause interfacial rate constants that are dependent on carrier concentration, and an interfacial band bending AU, which changes during and after the flash. For fast charge transfer, a more complicated differential equation has to be solved. 

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