Unusual conical intersections in the Jahn-Teller effect: The electronically excited states of Li₃

Conical intersections of the 2 ²E′ and 1 ²A′₁ states of Li₃ are considered as a first step in the description of the excited state involved in the absorption at 14583 cm^-1. Quite novel results are obtained. In addition to the standard symmetry-required D_3h seam of conical intersection, C_2v symmetry-allowed seams are found and their locus determined as a function the symmetric stretch coordinate. The C_2v seams of conical intersection intersect the D_3h conical intersection seam at two distinct points producing two heptafurcations of the D_3h seam. The locus of the C_2v conical intersection points is explained in terms of a quadratic Jahn-Teller model. The overall geometric phase effect associated with the D_3h and C_2v conical intersections is well described using a previously suggested phase factor deduced from molecular properties. In addition to these conical intersections an intersection of the 1 ²A′₁ and 2 ²E′ is states is located.