Analysis of the Ultraviolet Absorption Cross Sections of Six Isotopically Substituted Nitrous Oxide Species Using 3D Wave Packet Propagation

Abstract

The ultraviolet absorption cross sections of six isotopically substituted nitrous oxide species ( 1 4 N 1 4 N 1 6 O, 1 4 N 1 4 N 1 7 O, 1 4 N 1 4 N 1 8 O, 1 4 N 1 5 N 1 6 O, 1 5 N 1 4 N 1 6 O, and 1 5 N 1 5 N 1 6 O) were computed using the wave packet propagation technique to explore the influence of excited-state dynamics, transition dipole surface, and initial vibrational state. Three-dimensional potential energy surfaces for the electronic states of N 2 O related to the experimentally observed photoabsorption between 170 and 220 nm were calculated using the ab initio molecular orbital configuration interaction method. The transition dipole moment surfaces between these states were also calculated. Numerous wave packet simulations were carried out and used to calculate the temperature-dependent photodissociation cross sections of the six isotopically substituted species. The photolytic isotopic fractionation constants determined using the calculated cross sections are in good agreement with recent experiments. The results show that, in addition to the effect of the changed shape of the ground-state vibrational wave function with isotopic substitution, photodissociation dynamics play a central role in determining isotopic fractionation constants.