EVIDENCE OF A TRIPLET BARRIER ISOMERIZATION IN THE ACETYLENE MOLECULE ($C_{2}H_{2}$)?

[1]P. Dupre, R. Jost, M. Lombardi, P.G. Green, E. Abramson and R.W. Field. Chemical Physics, 152, 293-318(1991). [2] P. Dupre, These detat, GRENOBLE (1992). [3] P. Dupre and P.G. Green, submitted to Chemical Physics Letters.Author Institution: Service National des Champs Inienses, C.N.R.S.BP 166, 38042 GRENOBLE Cedex 09, FRANCE PDUPRE@ grenet.frWe investigated the evolution of the quantity pt - (V) (product of the vibrational coupled level density by the average coupling) over a range of $4000 cm^{-1}$ by using different kinds of experimental methods such as Quantum Beats and Zeeman Anticrossing Spectra (ZAS). Different rovibrational levels (with a few quantum number of rotation) of the first excited electronic state ($\tilde{A}^{1} A_{u}$) have been studied using a high magnetic fied (up to 8 traIn). The quantum beat analyzis shows, strong perturbations of the interacting triplet state (dark level). These perturbations are attributed to coupling among triplet states (couplings $T_{1} - T_{2}$). A statistical analysis, based principally on Fourier transform calculations, reveals a dependarice of the pseudo-probability law of the anticrossing of the energy levels as a function of the linewidth $(w) : p(w) = w^{6}$ with $- 1.65 \leq \varsigma \leq = 1$. Moreover, we analyze a large anticrossing on the $3\nu_{3}$ vibrational level, the characteristics of this anticrossing are near the one of a pure singlet-triplet anticrossing: $V_{at} = 3.3(4)GHz, g = 1.7(6)$ (g: Lande g factor). By assuming a spin-orbit coupling hamiltonian (and pur singlet-triplet anticrossing), the FT analysis permits to determine the average coupling, the vibrational coupled level density and the product of these 2 quantities. Over the limited energy range that we could explore, that latter clearly shows an explosion expressed by the relation: $P_{vit} (V)\propto \exp (E(cm^{-1})/585)$ (where E is the excitation energy). The exponential increase of the quantity $P_{vit}$ (V) could be due to a triplet ($^{3}B_{u}$ trans-bent $\longrightarrow ^{3}B_{2}$ as-bent) acetylene linear barrier. Near the top of the barrier (about 45500cm-1) the probability density of vibrational wave function becomes higher in the linear geometry configuration, i.c. the Franck-Condon overlap between the excited $\bar{A}$ state (trans) and the triplet state (cis) becomes higher. The vibrational mode $v_{3} (\widehat{CCH}$ bending) used for increasing the energy excitation seems play a major past near the top of the isomerization barrier [1,2,3]