Ab initio potential energy surfaces, total absorption cross sections, and product quantum state distributions for the low-lying electronic states of N₂O

Abstract

Adiabatic potential energy surfaces for the six lowest singlet electronic states of N₂O (X ¹A', 2 ¹A', 3 ¹A', 1 ¹A", 2 ¹A" and 3 ¹A") 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 photodissociation cross 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 FranckCondon region the potential energy curves 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 N₂O initially 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 [¹S (1 ¹A"), ¹D(2 ¹A'), and ¹D(2 ¹A")] have zero transition dipole moments from the ground state [¹S⁺(1 ¹A')] in its equilibrium linear configuration. The absorption becomes possible only through the bending motion of the molecule. The ¹D(2 ¹A')<--X ¹S⁺(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 ¹D(2 ¹A") 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.

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