On the role of conical intersections in photodissociation. IV. Conical intersections and the geometric phase in the 2 3A″ and 3 3A″ states of CH2

The 2 3A″–3 3A″(1 3A2–2 3B1) conical intersection seam in CH2 is characterized directly, that is without prior determination of the potential energy surfaces. Nuclear configurations in the vicinity of the seam are characterized in terms of a set of natural polar coordinates defined directly from the seam mapping procedure. Using these coordinates the geometric, or Berry phase, effect is demonstrated and the topology and interstate couplings in the vicinity of the seam are analyzed. The techniques used to study this surface of intersection are readily applied to larger polyatomic systems.