Evidence for quantum interference in collision-induced intramolecular energy transfer within CO singlet–triplet mixed states

Abstract

The quantum interference effect associated with one type of radiationless transition, the collisional energy transfer between singlet–triplet mixed molecular states, is studied. The experiments are conducted on CO(A 1Π,e 3Σ−)–He(Ar) via the ultrasensitive optical–optical double resonance multiphoton ionization (OODR-MPI) technique which measures state-to-state cross sections to an accuracy of ±10%, irrespective of the lifetime of the excited state. Distinct evidence of a quantum interference effect on the transfer rate has been obtained for mixed state CO intramolecular energy transfer processes. A simple, explicit expression for the cross section for mixed state energy transfer, based on the first order Born approximation of time dependent perturbation theory, is derived. The use of a transition phase angle θST is incorporated in the expression to describe the phase angle difference between singlet and triplet channels. This greatly refines the existing theory of quantum interference for collisional processes in that it successfully calculates the energy transfer cross sections and interprets the observed quantum interference effect under various quantum transitions. Experimental values of θST obtained for the CO–He system are 66° for J=9 and 73° for J=13. These results indicate that the infinite-order-sudden approximation which calls for θST=0 cannot satisfactorily account for the quantum interference effect. In addition, measured θST values for CO–Ar system are consistent with our theoretical expectations.