FEMTOSECOND TIME-RESOLVED IONIZATION SPECTROSCOPY OF NA(3)(B) AND THEQUESTION OF THE GEOMETRIC PHASE

The femtosecond time-resolved ionization spectrum of the B-state of Na 3 has been studied theoretically, taking two nuclear degrees of freedo m into account (bending mode and pseudorotation). Special emphasis is put on a comparison between different approaches to the pseudorotation al dynamics of Na3(B) proposed earlier in the literature: a Jahn-Telle r treatment invoked in the original work versus a pseudo-Jahn-Teller t reatment advanced in later studies. The time-dependent wave-packet dyn amics is found to differ drastically in the two cases although it proc eeds on (virtually) the same potential energy surface. This is interpr eted as a consequence of the geometric phase which is nontrivial only in the Jahn-Teller case. A low-energy peak in the Fourier transform of the pump-probe spectrum of Baumert et al. (Chem. Phys. Letters 209 (1 993) 29) is tentatively interpreted as evidence in favour of the pseud o-Jahn-Teller mechanism.