Photodissociation of NH3 in the Vacuum Ultraviolet

Two photodissociation processes of NH₃ producing electronically excited radicals have been studied in the vacuum ultraviolet: $${\mathrm{NH}}_3\to {\mathrm{NH}}_2{Ã}^2{A}_1+\mathrm{H},$$ $${\mathrm{NH}}_3\to \mathrm{NH}{c}^1\mathrm{\Pi +}{\mathrm{H}}_2.$$ The intensity of the fluorescence from these radicals, lying in the spectral regions 4000 to 6000 Å and at 3240 Å, respectively, has been measured as a function of wavelength of incident light with a resolution of 7 Å. The fluorescence curves thus obtained were compared with the absorption spectrum of NH₃. It is shown that the process (1) may be correlated with the ${\mathrm{B̃}}^1{E}^{\mathrm{\prime \prime }}$ (1665‐Å bands), D¹A₂″ (1434‐Å bands), ${Ẽ}^1{A}_2^{\mathrm{\prime \prime }}$ (1330‐Å bands), 1286‐Å bands and the continuum (1250 to 1600 Å). On the other hand, the process (2) may be associated with the continua (1250 to 1600 Å and below 1250 Å). The threshold wavelength of Process (2) is 1325±7 Å. The electronic energy a of the first singlet state NH a ¹Δ with respect to the ground triplet state X³Σ⁻ is a≤1.6±0.1 eV, which is obtained from the threshold wavelength together with the heat of reaction (3.9±0.1 eV) of NH₃→NH X³Σ⁻+H₂, and the electronic of the NH c¹Π (3.813 eV) with respect to the a¹Δ. Various other primary processes are discussed in conjunction with the absorption spectrum. For incident light of wavelength below 1200 Å the ionization process $${\mathrm{NH}}_3\to {\mathrm{NH}}_3^{+}\mathrm{+e,}$$ and the process (2) both of which may arise from the continuum become predominant. Dissociation processes appear to be subject to the spin conservation rule.