KrF excimer laser-induced ozone formation in supercritical carbon dioxide

Laser-induced reactions by a pulsed KrF excimer laser were studied using UV absorption spectroscopy in sub- and supercritical O-2/CO2 mixtures up to t he pressure of 15 MPa (corresponding density, 17 mol dm(-3)). Although the 248 nm excimer laser photon energy is smaller than the energy required for dissociating O-2 ozone formation was observed in O-2/CO2 mixtures. Under th e laser irradiation, O-3 concentration increased monotonically with the inc rease of the irradiation time and then stayed constant, which is satisfacto rily expressed by the equation d[O-3]/dt = a - b[O-3]. a corresponds to O-3 formation rate and b to O-3 decomposition rate constant. The value of a in creased with the increase of CO2 density up to 3 mol dm(-3) and was then ke pt almost constant with further increase. O-2 absorbs a photon to yield an oxygen molecule in the Herzberg III state O-2(A' (3)Delta(u)), being augmen ted along with the increase of CO2 density. In pure Or, the predominant pat hway of O-3 formation is the reaction between excited O-2 in Herzberg state s and ground state O-2 to yield O-3 and atomic oxygen. In high-density O-2/ CO2 mixtures, O-3 is considered to be produced through reaction between the Herzberg states O-2 and CO2. Taking account of the quenching effect for th e above reaction together with the augmentation of O-2 absorption of laser light by the high-density CO2, the behavior of a with respect to CO2 densit y was satisfactorily explained. The behavior of b suggested a certain inhib ition of O-3 recovery in high-density CO2 after the photodecomposition of t he product O-3, which was ascribed to the formation of CO3 from the O(D-1) reaction with Co-2. A certain cage effect for the O-3 photodecomposition wa s also suggested. No specific pressure effect was observed near the critica l point.