Absolute photolysis cross-sections for NaHCO3 , NaOH, NaO, NaO2 and NaO3 : implications for sodium chemistry in the upper mesosphere

Abstract

The compounds NaHCO₃ , NaOH, NaO and NaO₂ are formed from meteor-ablated Na in the upper mesosphere. This paper reports the absolute photolysis cross-sections of these species at selected wavelengths between 193 and 423 nm, measured by laser photofragment spectroscopy. The sodium species were produced in both a slow flow reactor and a fast flow tube, by the addition of appropriate reagents to a flow of atomic Na from a heat-pipe oven. The pulsed photolysis source was either a Raman-shifted excimer laser or a Nd:YAG laser, and the resulting Na atoms were probed by laser induced fluorescence at 589.0 nm (Na(3 ²P_3/2–²S_1/2)). Measurements are reported at 200 and 300 K. A limited set of cross-sections for NaO₃ was also obtained at 300 K. In the case of NaHCO₃ , NaOH and NaO, the onset of photolysis is close to the thermodynamic threshold calculated from quantum theory. Calculations using the CI-singles method predict excited states whose relative energies are in sensible accord with the bands in the experimental spectra, but which are too high in energy with respect to their respective ground states. Photodissociation coefficients are then derived which are appropriate for modelling sodium chemistry in the upper mesosphere. Photolysis of the major sodium reservoir species, NaHCO₃ , is shown to be an important route for recycling this compound, and is also predicted to produce the HCO₃ radical, which should be stable under mesospheric conditions.