Rotational Structure of Singlet–triplet Transitions in Near Symmetric Tops

Expressions are presented for the intensities of the rotational lines in singlet–triplet transitions in molecules which belong to asymmetric-top point groups, but which are nevertheless near symmetric tops. The general selection rules for the rotational branches in such singlet–triplet transitions are: ΔN = 0, ± 1, ± 2 and ΔK = 0, ± 1, ± 2. They differ from the selection rules for singlet–singlet transitions in near symmetric tops by the occurrence of branches with ΔN = ± 2 and ΔK = ± 2. For certain asymmetric-top molecules, it is possible to divide transitions into two mutually exclusive types: those characterized by the selection rules ΔK = 0, ± 2 and those characterized by ΔK = ± 1.