NO3 photolysis product channels: Quantum yields from observed energy thresholds

The absorption of visible light by NO3 leads to three products: NO + O-2, NO2 + O, and fluorescence. We report a new method for obtaining quantum yields for the NO3 molecule, using measured energy thresholds separating NO3 and its product channels. The assumptions of this model are the following: (i) NO3 internal energy (photon plus vibrations plus rotations) gives the necessary and sufficient condition to select each of the three product channels, as justified by the observed large differences in reaction times for the three products. (ii) The unresolved complexities of ground-state NO3 spectra and quantum states are approximated by standard separable re-vibrational expressions for statistical mechanical probability functions. These results may be of interest to both physical chemists and atmospheric chemists. The NO3(*) vibronic precursors of the three product channels are identified. We evaluate and plot vibrational state-specific absolute quantum yields as a function of wavelength Phi(vib)(lambda) for each product channel. We sum over vibrational states to give the macroscopic quantum yield as a function of wavelength Phi(lambda), obtained here from 401 to 690 nm and at 190, 230, and 298 K. By adding considerations of light absorption cross sections sigma(lambda) at 230 and 298 K and a stratospheric radiation distribution I(lambda) from 401 to 690 nm, we evaluate the wavelength dependent photochemical rate coefficients j(lambda) for each of the three product channels, and we find the integrated photolysis constants, j(NO), j(NO2), and j(fluorescence). At 298 K, our Phi(lambda) for NO2 + O products agree with the major features observed by Orlando et al. (1993), but show significant systematic offset in the 605-620 nm wavelength range. Our Phi(lambda) for NO + O-2 products at 298 K agree with those observed by Magnotta et al. (1980) within their experimental scatter. Experimental error in our method for measuring quantum yields arises only from errors in measuring the wavelengths at which various product yields approach zero; there is no dependence and, thus, no error arising from light absorption cross sections, light intensities, or species concentrations, which contribute errors to the method of laser photolysis and resonance fluorescence. The results reported here are unique in including quantum yields at 190 and 230 K, which may be useful for modeling atmospheric photochemistry.