Photolysis of nitrogen dioxide

Abstract

Relative quantum yields for the photolysis of NO2 to give NO have been measured at 5 or 10 nm intervals in the range of 295 to 445 nm, and also at several longer wavelengths. The behavior of the quantum yield as a function of photon energy can be separated into three distinct wavelength regions. At all wavelengths shorter than 398 nm the quantum yield is almost constant and independent of NO2 pressure in the range 0.5-4 torr. Hence, by comparison with previous work, the photodissociation probability of NO2 remains close to unity for photolysis by all wavelengths shorter than the dissociation limit at 398 nm. At wavelengths longer than 430 nm, there is a small quantum yield for NO formation which decreases slowly with increasing wavelength. Quenching studies and variations in light intensity show that these small yields are caused by reaction of an electronically excited NO2 molecule with ground state NO2. In the wavelength region between 400 and 430 nm, the quantum yield falls abruptly. Isotopic exchange experiments involving oxygen-18 confirm that free oxygen atoms are produced in this region, even though the energy of the photolyzing quantum is less than the NO 2 dissociation energy. Recent fluorescence yield measurements quantitatively complement the present photodissociation quantum yields. A previously suggested model, in which the ground state rotational energy of NO2 supplements the photon energy, gives only fair agreement with the experimental quantum yields. A slight reduction in the accepted value for the dissociation energy of NO2 would improve the agreement between theory and experiment, although differences would still remain.