Spin-vibronic transition moments and rotational intensities in doublet-quartet transitions in asymmetric top molecules
Symmetry and time reversal arguments allow the rigorous determination of the nonvanishing and independent spin-vibronic transition moments for doublet-quartet transitions in asymmetric top molecules. Less strict additional relations, leading to a smaller number of independent parameters, are derived by considering an insight of the intensity borrowing mechanism in the case of weak spin-orbit interactions. In this case it is shown that there are either two or three independent transition moments in orthorhombic molecules, depending on the symmetry species of the vibronic wavefunctions involved in the transition. Case ( b) selection rules are derived, and it is shown that rotational intensities in orthorhombic molecules can be obtained in all cases by squaring certain sums of either all real or all imaginary terms. A diagrammatical method is devised to identify the form and polarization of the nonvanishing transition moments as well as the symmetry species of the products of those vibronic pairs whose spin-orbit mixing contributes to a given transition moment.