Optical control of collisional population flow between molecular electronic states of different spin multiplicity

The adiabatic description of molecular electronic states is based on potential energy surfaces, defined by the motion of the electrons, on which the slower nuclear motions (vibrations and rotations) occur. However, this model breaks down when relativistic effects such as the coupling between the electron spin and its orbital angular momentum (spin-orbit coupling) are taken into account. The result is that conical intersections develop between the adiabatic potential surfaces resulting in molecular states with mixed spin (multiplicity) character. A growing number of theoretical results [1–3] indicates the importance of such intersections in shaping the pathways for nonadiabatic dynamical processes. For example, conical intersections play a crucial role in the processes of intersystem crossings (nonradiative quenching) [4,5] of excited electronic states. When diatomic molecules are considered, the electronic potential energy surfaces reduce to two-dimensional curves and the conical intersections become avoided crossings of the intersecting diabatic potentials. The dipole selection rule on spin, S = 0, prohibits direct excitation between states of different spin (singlet ←/→ triplet). However, nearly degenerate singlet and triplet levels, with the same rotational quantum number J , can couple together by the spin-orbit interaction, creating levels of mixed singlet-triplet character. In diatomic alkali-metal molecules, for example, such states are used in the transfer of ultracold molecules formed in the triplet a 3 + state to deeply bound levels of the singlet ground state X1 + [6–8], as well as to access spectroscopically the dark triplet excited states starting from the singlet ground state X1 + [9–14]. Thus the use of these mixed spin-multiplicity character levels called “window” levels as intermediates in double resonance excitations allows the selection rule on spin for electric dipole transitions to be circumvented. A related, but separate phenomenon is the so-called “gateway” effect [14–17], in which transfer of population between singlet and triplet manifolds occurs by way of collisional pathways through levels of mixed character. Direct transfer of population from a pure singlet level of an alkali diatomic molecule to a pure triplet level lying very close in energy, via collisions with noble gas atoms, is an extremely weak process. Instead, it has been observed that the transfer is very much more likely