Excited mercury ammonia system. Part 2.—Attachment of NH3 clusters to Hg(63P0) and estimates of the bond energies

Abstract

The spectral profiles of the ultra-violet luminescence from the reaction of Hg(6³P₀) with NH₃ have been investigated at 295 and 404 K, over a wide range of pressures. At low pressures (∼0.3 Torr), the emission arises from the bimolecular reaction, Hg(6³P₀)+ NH₃→ Hg(6¹S₀)+ NH₃+hν, and the short wavelength threshold extends back to 2650 Å, equivalent to the full excitation energy of Hg(6³P₀). Addition of foreign gas induced a pressure narrowing of the band, shifting the threshold to 3000 Å; the emitter of the narrowed band is a vibrationally relaxed complex produced by termolecular attachment: Hg(6³P₀)+ NH₃ M → HgNH^*₃+ M.

However, with the total pressure maintained at 600 Torr, increase of the partial NH₃ pressure continues to shift the ultra-violet band towards the red and also causes a marked broadening to occur. It is demonstrated that these spectral shifts occur because Hg(6³P₀) has the capacity to form cluster complexes, Hg(NH₃)^*_n, with n up to at least 5.

By postulating that at high pressures the complexes are in thermodynamic equilibrium with NH₃ and that each NH₃ addition causes a red shift of the threshold, from an analysis of the pressure variation of the intensities close to the thresholds, dissociation constants and the individual spectra of the intermediates have been derived. Energies for dissociation of NH₃ from HgNH^*₃, Hg(NH₃)^*₂ and Hg(NH₃)₃ are reported to be 48(±8), 29(±5) and 10(±3) kJ mol^–1, respectively.