KrF Excimer Laser-induced Ozone Formation in Supercritical Carbon Dioxide

Abstract

Laser-induced reactions by a pulsed KrF excimer laser were studied using UV absorption spectroscopy in sub-and supercritical O 2 /CO 2 mixtures up to the 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 /CO 2 mixtures. Under the laser irradiation, O 3 concentration increased monotonically with the increase of the irradiation time and then stayed constant, which is satisfactorily expressed by the equation d. a corresponds to O 3 formation rate and b to O 3 decomposition rate constant. The value of a increased with the increase of CO 2 density up to 3 mol dm -3 and was then kept almost constant with further increase. O 2 absorbs a photon to yield an oxygen molecule in the Herzberg III state O 2 (A′ 3 ∆ u ), being augmented along with the increase of CO 2 density. In pure O 2 , the predominant pathway of O 3 formation is the reaction between excited O 2 in Herzberg states and ground state O 2 to yield O 3 and atomic oxygen. In high-density O 2 /CO 2 mixtures, O 3 is considered to be produced through reaction between the Herzberg states O 2 and CO 2 . Taking account of the quenching effect for the above reaction together with the augmentation of O 2 absorption of laser light by the high-density CO 2 , the behavior of a with respect to CO 2 density was satisfactorily explained. The behavior of b suggested a certain inhibition of O 3 recovery in high-density CO 2 after the photodecomposition of the product O 3 , which was ascribed to the formation of CO 3 from the O( 1 D) reaction with CO 2 . A certain cage effect for the O 3 photodecomposition was also suggested. No specific pressure effect was observed near the critical point.

Wavelength Dependence of the Primary Ozone Formation in High-Pressure O2 and O2/CO2 Mixtures under Irradiation from 232 to 255 nm

“…This feature is the same as obtained before with the KrF laser irradiation at 248 nm. 17 The saturation is mainly caused by the subsequent photolysis of O 3 . It is expected that the primary O 3 quantum yield without any influence by subsequent catalytic O 3 reactions can be estimated from the initial time profile of the O 3 concentration.…”

“…The time evolution of O 3 was measured as a function of irradiation time using a pulsed dye laser at the desired wavelength for high-pressure O 2 and O 2 /CO 2 mixtures. The high-pressure photolysis cell and procedures for the measurement of O 3 formation were almost the same as before 17 except for the irradiation laser. Briefly, a high-pressure cross-shaped optical cell made from stainless steel (SS 316) of 5.0 cm 3 inner volume that contained four sapphire windows of 8 mm thickness was employed.…”

“…In the O 3 concentration measurements, we evaluated inhomogeneities in the spatial distribution of O 3 concentration in the cell by using two different diameters of the spot of D 2 lamp light as before, 17 i.e., 4 and 8 mm. The spot size of the laser beam was fixed at 5 mm in the longitudinal direction and 3 mm in the lateral direction.…”

“…A one-photon process was also established at 248 nm in our previous paper. 17 (4) Dependence of the Quantum Yield on the Excitation Wavelength. The threshold for O 2 dissociation is located at 242.4 nm, and we are interested whether the wavelength dependence of the quantum yield changes through the threshold as well as with the absolute value itself.…”

Creation of spherical carbon nanoparticles and clusters from carbon dioxide via UV dissociation at the critical point

“…In contrast to our own results, laser irradiation of a mixture of oxygen and supercritical carbon dioxide at 248 nm generated ozone but no carbon monoxide. 32 As the amount of ozone was proportional to the concentration of oxygen, ozone was thought to form under a reaction of excited carbon dioxide and oxygen molecules. In our experiments the fluid is virtually oxygen-free and this reaction cannot proceed.…”