Abstract
In the Franck–Condon approximation, the major contribution to the vibronic second-order Jahn–Teller (JT) reduction factors (RFs) originates from vertical virtual transitions between the Born–Oppenheimer electronic ground and excited states associated with the corresponding sheets of the adiabatic potential energy surfaces. However, an additional contribution to the polarizability arises from the vibrations of the nuclear frame of the system. To include such nuclear contributions, we must lift the restriction of the rigid-frame approximation by providing the electron wave functions an additional flexibility through allowing them to follow the nuclear vibrations. The transitions between the sheets are then no longer vertical. These non-Condon corrections to the RFs are calculated for the T⊗t JT system by perturbation theory. The new values obtained for the RFs are found to be much closer to those obtained earlier by numerical methods than the original analytical calculations. This means that calculations of second-order RFs are now available which are valid from very strong coupling to intermediate coupling.