THE BARRIER TO FREE PSEUDOROTATION IN THE SODIUM TRIMER

$^{h}$. N. Ohashi, M. Tsuura, and J. T. Hougen, J. Mol. Spectrosc. 173, 79-99 (1995). $^{i}$ N. Ohashi, M. Tsuura, and J. T. Hougen, W. E. Ernst, and S. Rakowsky, to appear in J. Mol. Spectrosc. $^{j}$D. T. Vituccio, O. Golouzka, and W. E. Ernst, to appear in J. Mol. Spectrosc. $^{k}$R. Meiswinkel and H. K\""{o}ppel, Chem. Phys. 144, 117-128 (1990). $^{l}$M. Mayer, L.S. Cederbaum, and H. K\""{o}ppel, J. Chem. Phys. 104, 8932-8942 (1996).Author Institution: Department of Physics, The Pennsylvania State UniversityIn the $\tilde{B}$ state of $Na_{3}$, the three sodium nuclei can perform an almost free pseudorotational motion in the molecular plane. The purely vibrational movement is slightly hindered in the lowest vibronic level due to the small potential barriers which separate the three equivalent nuclear configurations of this $X_{3}$-type molecule. Optical-optical double resonance spectroscopy in conjunction with resonant two-photon ionization was used to record the lowest vibronic bands ($u=0, j=0, 1, 2$, and 3)in the $\bar{B}-\bar{X}$ system with rotational resolution. The recently developed effective rotation-pseudorotation Hamiltonian of Ohashi {et al}.$^{h,i}$ was applied in the rotational analysis of the spectra. The Hamiltonian is described in Refs. 1 and 2 in detail and contains two terms $Fj^{2}_{P} + \frac{1}{2} V_{3} (1 - \cos 3_{\chi P})$ which represent the three-identical-well particle-on-a-ring pseudorotational problem. While previously measured higher vibronic levels do not indicate the presence of any barrier a fit of the lowest bands yields a value $V_{3}=10.15 cm^{-1}$ for the pseudorotational barrier. In their theoretical treatment of the pseudo Jahn-Teller coupling of $a{^{2}} A^{\prime}_{1}$ and $a{^{2}}E^{\prime}$ state which are believed to form the three-surface potentital called the $\bar{B}$ state of $Na_{3}$, Meiswinkel and K\""{o}$ppel^{k}$ derived pseudo Jahn-Teller coupling parameters which correspond to a barrier height of $15.2 cm^{-1}$. The results will be compared and supplemented by a discussion about a recently published rovibronic coupling model.height of $15.2 cm^{-1}$. The results will be compared and supplemented by a discussion about a recently published rovibronic coupling $model.^{l}