M.N. Daud, G.G. Balint-Kurti, and A. Brown*
J. Chem. Phys 122, 054305.
Publication year: 2005

Adiabatic potential energy surfaces for the six lowest singlet electronic states of N2O (X 1A′,2 1A′, 3 1A′, 11A,2 1A″ and 31A have been computed using an ab initio multireference configuration interaction (MRCI) method and a large orbital basis set (aug-cc-pVQZ). The potential energy surfaces display several symmetry related and some nonsymmetry related conical intersections. Total photodissociationcross sections and product rotational state distributions have been calculated for the first ultraviolet absorption band of the system using the adiabatic ab initio potential energy and transition dipole moment surfaces corresponding to the lowest three excited electronic states. In the Franck–Condon region the potential energycurves corresponding to these three states lie very close in energy and they all contribute to the absorption cross section in the first ultraviolet band. The total angular momentum is treated correctly in both the initial and final states. The total photodissociation spectra and product rotational distributions are determined for N2Oinitially in its ground vibrational state (0,0,0) and in the vibrationally excited (0,1,0) (bending) state. The resulting total absorption spectra are in good quantitative agreement with the experimental results over the region of the first ultraviolet absorption band, from 150 to 220 nm. All of the lowest three electronically excited states [1Σ(1 1A″), 1Δ(21A), and 1Δ(2 1A″)] have zero transition dipole moments from the ground state [1Σ+(11A)] in its equilibrium linear configuration. The absorption becomes possible only through the bending motion of the molecule. The 1Δ(21A)←X1Σ+(1A) absorption dominates the absorption cross section with absorption to the other two electronic states contributing to the shape and diffuse structure of the band. It is suggested that absorption to the bound 1Δ(21A)state makes an important contribution to the experimentally observed diffuse structure in the first ultraviolet absorption band. The predicted product rotational quantum state distribution at 203 nm agrees well with experimental observations.