VUV Photodissociation of ammonia: a dispersed fluorescence excitation spectral study

Abstract

NH₂ and NH transitions were identified in dispersed fluorescence spectra obtained by VUV excitation of NH₃ at nine different energies between 8.266 and 15.498 eV. Fluorescence excitation (FEX) spectra between 6 and 15 eV were measured at seven specific emission wavelengths for these species. This extends the usual 11.7 eV upper limit of NH₃ FEX spectra. Among the results of our work are the following: (i) a clarification of the assignment of the emission carriers; (ii) analysis of the FEX spectra of NH₂(Ã ²A₁ → ²B₁) emission and assignment of the features to members of the npe′ ← 1a^″₂, nde″ ← 1a^″₂, nsa^′₁ ← 1a^″₂ and nda^′₁ ← 1a^″₂ Rydberg series transitions of NH₃; (iii) new information on photodissociation processes leading to formation of NH₂(Ã ²A₁) and of NH in various excited states, in particular concerning the formation of NH (A ³Π) at 8.266 eV by dissociation of the ã ³A^″₂ state of NH₃, the existence of a potential barrier of 360 ± 70 meV in the dissociation to form NH (c ¹Π) at 9.85 eV and its formation from a non-planar state above 11.3 eV; (iv) information pertaining to the heat of formation of the NH radical, which we determine as ΔH_f,0°(NH) = 3.70 ± 0.01 eV, and to the thermochemical limits of the dissociation reactions of NH₃.