Precise ab initio calculations of the rovibrational structures of the $A^2{E}^{\prime \prime }$ and $X^2{E}^{\prime }$ electronic states of ${\mathrm{Na}}_3$ prompt a new vibrational assignment of the $A\leftarrow X$ transition and provide the basis for the rotational analysis of the vibrational band $A(v_s{=1,v}_b{=0,v}_a=0\mathrm{})\leftarrow X(0,0,0)$ by means of high-resolution optical-optical double resonance. The calculations, which use the single-surface adiabatic approach, reproduce our experimental data only if, as required by theory, a geometric phase of $\pi$ under pseudorotation around the equilateral configuration is imposed. We consider this the first verification of Berry's phase in high-resolution molecular spectroscopy.

• Received 29 June 1998

DOI:https://doi.org/10.1103/PhysRevLett.81.4584