Solid State and Solution Nitrate Photochemistry: Photochemical Evolution of the Solid State Lattice

We examined the deep UV 229 nm photochemistry of NaNO3 in solution and in the solid state. In aqueous solution excitation within the deep UV NO3- strong pi -> pi* transition causes the photochemical reaction NO3- -> NO2- + O center dot. We used UV resonance Raman spectroscopy to examine the photon dose dependence of the NO2- band intensities and measure a photochemical quantum yield of 0.04 at pH 6.5. We also examined the response of solid NaNO3 samples to 229 nm excitation and also observe formation of NO2-. The quantum yield is much smaller at similar to 10(-8). The solid state NaNO3 photochemistry phenomena appear complex by showing a significant dependence on the UV excitation flux and dose. At low flux/dose conditions NO2- resonance Raman bands appear, accompanied by perturbed NO3- bands, indicating stress in the NaNO3 lattice. Higher flux/dose conditions show less lattice perturbation but SEM shows surface eruptions that alleviate the stress induced by the photochemistry. Higher flux/dose measurements cause cratering and destruction of the NaNO3 surface as the surface layers are converted to NO2-. Modest laser excitation UV beams excavate surface layers in the solid NaNO3 samples. At the lowest incident fluxes a pressure buildup competes with effusion to reach a steady state giving rise to perturbed NO3- bands. Increased fluxes result in pressures that cause the sample to erupt, relieving the pressure.