Photoconductivity in Rigid Organic Solutions. II. Initial-Rate Studies of Charge-Carrier Production and the Two-Photon Requirement

The initial rises of photocurrents in certain rigid organic solutions are examined both as a function of light intensity and wavelength. The quadratic nature of the intensity dependence is taken to signify a two-photon mechanism for charge-carrier production. The initial rate of charge-carrier production is linear in solute concentration and is not sensitive to O2 levels. Double-beam experiments allow the isolation of each of the two one-photon steps and the wavelength dependence of each step is studied. The first photon step shows specific vibronic activity for transfer into the intermediate state. The relative quantum yield rises sharply over the first absorption band of the solute molecule and then abruptly drops as the second absorption band is entered. The second step is relatively wavelength independent once its near-ultraviolet threshold is reached. A dark lifetime of 4.5 sec is observed for the intermediate state. The triplet state of the solute molecule cannot be implicated. Additional initial-rise studies reveal that of the two induced photocurrent signals one, the so-called ``365'' pseudostationary signal, arises from a state of the system which is one photon produced and one photon ionized; the other, referred to as the spike signal, arises from a state which is two photon produced and one photon ionized. The general problem of creating full charge separation in low-dielectric-constant media is briefly reviewed to show how the two-photon requirement is entirely to be expected. The results are treated within the framework of a straightforward kinetic scheme. Possible mechanisms, including a proton-transfer intermediate, are briefly but inconclusively examined.