Excited Species and Their Contribution to NO(β, γ) Fluorescence in the Photodissociation of N2O

Abstract

The fluorescence intensity and the yield of the NO β bands, originating from photodissociation of N₂O, was measured as a function of wavelength of incident light in the vacuum ultraviolet region. Photon energies required for the occurrence of the fluorescence are far less than the threshold energy to produce NO(A² Σ⁺, B ²Π_r) directly from N₂O, indicating that the emission is due to secondary processes. The fluorescence intensity curve follows closely the absorption curve of N₂O indicating that excited species responsible for the emission are produced from dissociation of electronically excited N₂O rather than from the direct dissociation to these species. A mechanism of fluorescence is discussed on the basis of the threshold energy above which a specific photochemical process is possible. Reactions responsible for the β emission in the absorption region 1400 to 1550 Å of N₂O are production of N(²D) and O(¹S) followed by $\mathrm{N}{(}^2\mathrm{D)+}{\mathrm{N}}_2\mathrm{O}\to {\mathrm{N}}_2+\mathrm{NO}{\mathrm{(B}}^2{\Pi }_r)$ and $\mathrm{O}{(}^1\mathrm{S)+}{\mathrm{N}}_2\mathrm{O}\to \mathrm{NO}+\mathrm{NO}{\mathrm{(B}}^2{\Pi }_r)$ . In the region below 1400 Å, N(²P) and N₂(B ³Π_g, B′ ³Σ_u⁻) also can contribute to the emission. When NO was added to N₂O, the fluorescence intensity increased considerably and the emission changed from β to γ bands. Reactions which produce the emission in the absorption region 1400 to 1550 Å are $${\mathrm{N}}_2\mathrm{O}\to {\displaystyle {lim}^{\mathrm{h\nu }}}{\mathrm{N}}_2{\mathrm{(A}}^3{\Sigma }_u^{+}\mathrm{)+}\mathrm{O}{(}^3\mathrm{P)}$$ followed by ${\mathrm{N}}_2{\mathrm{(A}}^3{\Sigma }_u^{+}\mathrm{)+}\mathrm{NO}\to {\mathrm{N}}_2+\mathrm{NO}{\mathrm{(A}}^2{\Sigma }^{+})$ . Below 1350 Å additional excited molecules N₂(B ³Π_g, B′ ³Σ_u⁻) can contribute to the emission in similar reactions.