Photodissociation of NH3 in the Vacuum Ultraviolet

Abstract

Two photodissociation processes of NH3 producing electronically excited radicals have been studied in the vacuum ultraviolet: NH3→NH2 Ã2A1+H,NH3→NH c 1Π+H2.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 B̃1E′′ (1665-Å bands), D1A2″ (1434-Å bands), Ẽ1A2′′ (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 NH3→NH3++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.