RELATIVE CONTRIBUTIONS OF DISCRETE AND CONTINUUM ABSORPTION TO THE PHOTODISSOCIATION OF VIBRATIONALLY EXCITED O2 (V=12-20)

It has been proposed that photodissociation of highly vibrationally excited O2 in the B3-SIGMA-u(-)-X3-SIGMA-g(-) Schumann-Range bands is a source of ozone in the upper stratosphere. Simultaneously, this O2(vib) population could be excited into the continua of the O2(B) and 1(3)PI-u states, providing additional ozone. Laboratory measurements indicate that both discrete and continuous processes are operative. Calculations of the Schumann-Runge continuum cross sections have now been carried out for v" = 12-20, spanning the 200- to 320-nm wavelength range. The maximum cross sections are in the 1-2 x 10(-18) cm2 range, with the O2(B) state contributing 2-3 orders of magnitude greater absorption than the 1(3)PI-u state. For v" = 12, it is shown that the Schumann-Runge band and continuum cross sections merge smoothly at 250 nm. In laboratory experiments on ozone photodissociation at 248 nm, the principal absorption in product O2(vib) involves the Schumann-Runge 2-7 band and Schumann-Runge continuum absorption from vibrational levels at v" = 12 or higher. Although the cross section for line absorption is much higher than that for the continuum, it is shown that a result of a variety of processes and mechanisms, photodissociation yields in the bands and the continuum are comparable in magnitude, thereby providing consistency with the laboratory observations. For linear absorption systems, such as the terrestrial atmosphere, high v" absorption in the Schumann-Runge bands should be substantially more important than continuum absorption.