UNAMBIGUOUS PROOF FOR BERRYS PHASE IN THE SODIUM TRIMER - ANALYSIS OFTHE TRANSITION A(2)E''[-(XE)-E-2'

Precise nb initio calculations of the rovibrational structures of the A(2)E '' and (XE)-E-2' electronic states of Na-3 prompt a new vibratio nal assignment of the A <-- X transition and provide the basis for the rotational analysis of the vibrational band A(v(s) = 1, v(b) = 0, v(a ) = 0) <-- X(0, 0, 0) by means of high-resolution optical-optical doub le resonance. The calculations, which use the single-surface adiabatic approach, reproduce our experimental data only if, as required by the ory, a geometric phase of pi under pseudorotation around the equilater al configuration is imposed. We consider this the first verification o f Berry's phase in high-resolution molecular spectroscopy. [S0031-9007 (98)07707-2].