276-nm absorption band system of m-dichlorobenzene: Rotational band contour analysis

The 276-nm absorption band system (${}^1\text{B}{}_2\text{←}{}^1\text{A}{}_1$) of m-dichlorobenzene was photographed under high resolution. The electronic origin band (0, 0) and a band at (0 + 380) cm^?1 were subjected to rotational “band contour” analysis. As a result, it is found that the origin band has a type A band contour and that at (0 + 380) cm^?1 exhibits a type B band contour. The band contour analysis also yields an accurate determination of the excited state parameters, viz., A′ = 0.0911 ± 0.0003, B′ = 0.02852 ± 0.00005, and C′ = 0.02175 ± 0.00001 cm^?1. A model geometry for the molecule m-DCB in its first excited singlet state has been proposed.