Nonadiabatic tunneling via conical intersections and the role of the geometric phase

As a ubiquitous quantum effect, tunneling has attracted attention ever since the dawn of quantum mechanics. However, recent evidence suggests that nonadiabatic atomic tunneling near a conical intersection (CI) behaves differently from its adiabatic counterpart, producing lifetime differences of up to two orders of magnitude. Using two-dimensional models, we demonstrate here that the failure of the adiabatic model in describing tunneling near a CI can be attributed largely to the neglect of the geometric phase, which is associated with the adiabatic electronic wave function transported around a CI. The geometric phase--induced destructive interference among wave functions following different paths around the CI, manifested as a node in the adiabatic wave function, retards tunneling.