Ab initio study of the photodissociation of ammonia

Abstract

An ab initio SCF and CI treatment of the electronic spectrum of ammonia in both the pyramidal and planar conformation is reported which employs an [8, 6, 1 4 , 1] AO basis of near Hartree-Fock quality; the ground state CI energy obtained for the equilibrium conformation is −56.4241 a.u. In addition, further calculations have been carried out at the SCF level to study various photodissociation reactions of NH 3. The calculated CI transition energies are seen to agree with corresponding experimental values to within 0.0–0.3 eV, usually in the 0.1-eV range. Photodissociation to the NH 2 ( 2 B ̃ 1) + H( 2S) products is confirmed thereby to proceed via the A ̃ 1a″ 2 → 3s state of ammonia, but contrary to earlier speculation it is found that the transformation between reactant and products is already satisfactorily described at the SCF or orbital level, i.e., a Rydberg 3 s of NH 3 is seen to be gradually converted into a pure hydrogenic 1 s species as dissociation proceeds. In addition the photolysis of NH 3 to NH 2 ( 2A 1) + H( 2S) is argued to occur via the C ̃ 1a″ 2 → 3pz 1A′ 1 state and as such is seen to be a symmetry allowed process, in contrast to the previous assignment involving the B ̃ 1a″ 2 → 3px , y 1E″ species. Finally an attempt is made to analyze the mechanisms of various NH + H 2 photodissociation processes with the help of SCF calculations and symmetry arguments for various higher-lying excited states of ammonia.