VACUUM-UV (172 NM) ACTINOMETRY - THE QUANTUM YIELD OF THE PHOTOLYSIS OF WATER

With the development of new light sources (Xe-excimer light sources), the vacuum-UV (VUV) photochemistry on a preparative scale is becoming technically feasible. Among the first potential technical applications , VUV photolysis of aqueous systems must be considered as a potential alternative to established ''advanced oxidation procedures'' (AOP). Fo r the design and dimensioning of corresponding reactors, incident phot on rates must be determined. The standard VUV actinometry in condensed phase is the cis-trans isomerization of cyclooctene in n-pentane. The incident photon rate of these new light sources depends on their geom etry, the configuration of their electrodes, and the dielectric consta nt of the solvent in the case where the substrate solution is part of the dielectric barrier; thus actinometric experiments should be made u nder operational conditions. However, the radiant power density of the excimer sources will be different if n-pentane (standard actinometry) is used as part of the dielectric barrier from when using water (oxid ative degradation experiments), because the voltage drop across the fl uids is different. Consequently, for projects involving aqueous reacti ons systems, operational conditions cannot be met by the standard acti nometer. Water exhibits a high absorption cross-section for VUV irradi ation (lambda < 190 nm) and homolyzes mainly into hydroxyl radicals an d hydrogen atoms. Hydroxyl radicals, but not hydrogen atoms, are very efficiently scavenged by methanol molecules, and under defined conditi ons, the rate of production of hydroxyl radicals may be determined fro m the rate of degradation of methanol dissolved in the aqueous reactio n system. The parameters affecting primarily the rate of methanol degr adation, i.e., the incident photon rate, the concentration of dissolve d molecular oxygen, the initial methanol concentration, and the flow p arameters in the photochemical reactor, were determined and optimized for a general actinometric procedure. A normalized diagram of the inci dent photon rate versus the initial methanol concentration allows one to determine the boundary conditions under which the rate of methanol degradation may be used to evaluate the production rate of hydroxyl ra dicals for Xe-excimer light sources of different radiant power and ind ependent of their geometry. Having determined both the rate of product ion of hydroxyl radicals and the rate of methanol degradation, and hav ing calibrated the corresponding incident photon rates by the cis-tran s isomerization of cyclooctene (standard actinometry) in an experiment al setup in which the emitted photon rate does not depend on the react ion medium, the quantum yield of the homolysis of water by VUV irradia tion from Xe-excimer Tamps may finally be calculated.