Crossing Electronic States in the Franck-Condon Zone of Carbon Dioxide: A Five-Fold Closed Seam of Conical and Glancing Intersections.

Intersections of electronic states are recognized as natural molecular hubs routing initial photoexcitation toward specific products. In this Letter, we investigate a five-fold intersection between the 2,3(1)A' and 1,2,3(1)A″ electronic states of CO2 responsible for the ultraviolet absorption of the molecule between 120 and 160 nm. It is demonstrated that the intersection is not an isolated point but a seam with the following properties: (1) The seam comprises both conical and glancing intersections. (2) The seam is a closed loop traced by the concerted displacements of two CO bonds. (3) The seam is constrained to linear geometries and confined to the CO bond plane. (4) The seam passes directly through the Franck-Condon zone. Topography and properties of the intersection seam are validated using assignment of the electronic wave functions and illustrated using circulation of the derivative couplings along closed contours. The possibility of observation of strongly curved seams is briefly discussed.