NO3 PHOTOLYSIS PRODUCT CHANNELS - QUANTUM YIELDS FROM OBSERVED ENERGYTHRESHOLDS

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 qu antum yields for the NO3 molecule, using measured energy thresholds se parating NO3 and its product channels. The assumptions of this model a re the following: (i) NO3 internal energy (photon plus vibrations plus rotations) gives the necessary and sufficient condition to select eac h of the three product channels, as justified by the observed large di fferences in reaction times for the three products. (ii) The unresolve d complexities of ground-state NO3 spectra and quantum states are appr oximated by standard separable re-vibrational expressions for statisti cal mechanical probability functions. These results may be of interest to both physical chemists and atmospheric chemists. The NO3() vibron ic precursors of the three product channels are identified. We evaluat e and plot vibrational state-specific absolute quantum yields as a fun ction of wavelength Phi(vib)(lambda) for each product channel. We sum over vibrational states to give the macroscopic quantum yield as a fun ction of wavelength Phi(lambda), obtained here from 401 to 690 nm and at 190, 230, and 298 K. By adding considerations of light absorption c ross sections sigma(lambda) at 230 and 298 K and a stratospheric radia tion distribution I(lambda) from 401 to 690 nm, we evaluate the wavele ngth dependent photochemical rate coefficients j(lambda) for each of t he three product channels, and we find the integrated photolysis const ants, j(NO), j(NO2), and j(fluorescence). At 298 K, our Phi(lambda) fo r NO2 + O products agree with the major features observed by Orlando e t 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 agre e with those observed by Magnotta et al. (1980) within their experimen tal scatter. Experimental error in our method for measuring quantum yi elds arises only from errors in measuring the wavelengths at which var ious product yields approach zero; there is no dependence and, thus, n o error arising from light absorption cross sections, light intensitie s, or species concentrations, which contribute errors to the method of laser photolysis and resonance fluorescence. The results reported her e are unique in including quantum yields at 190 and 230 K, which may b e useful for modeling atmospheric photochemistry.