ANOMALOUS BEHAVIOUR OF THE ANTICROSSING DENSITY AS A FUNCTION OF EXCITATION ENERGY IN THE C2H2 MOLECULE

$^{1}$ P. Dupre, R. Jost, M. Lombardi, P. G. Green, E. Abramson and R. W. Field, in preparation. $^{2}$ P.P. G. Green, PhD Thesis, MIT (1989). CAMBRIDGE MA (U.S.A.).Author Institution: Service National des Champs Intenses.; Service National des Champs Intenses.; Mail Stop 170-25, California Institute of Technology; Mail Stop 170-25, Service National des Champs Intenses.; Department of Chemistry, MITWe have recorded Zeenan Anticrossing (ZAC) spectra of gas phase acetylene $(HC=CH)$ in the $\bar{A} ^{1}A_{u} v_{3}= O-3 (v_{3}$ is the trans-bending normal mode of the trans-bent excited electronic state). $J = K = I = O$. levels. The energy range thus sampled was from 42,200 to 45,300 cm1 above the $\bar{X} ^{1}\Sigma_{g}$ state. The eugnetic field scanned from 0 to 8 Tesin and the ZAC spectre were recorded as variations (decreases) in the intensity of the fluorescence excited by a pulsed, frequency doubled dye laser. The ZAC spectra wore unassignably complex. We report a surprisingly rapid increase in the density of anticrossing (AC) over a $3100 cm^{-1}$ (only 7\% of the total excitation energy) energy interval, In the $v = 3$ level the ZAC spectra are unassignably complex and the anticrossing density is $10^2$ Limes larger than the maximum computed density of triplet vibrational states and quite comparable to the computed density of $\tilde{X} ^{1}\Sigma^{+}_{g}$ vibrational states. This constrasts with the AC level density observed in the $v_{3} = 0$ vibrational level where the agreement with the calculated level density is good enough. We suggest three plausible mechanisms to explain this behaviour : the existence of a dissociation limit, the potential curve crossing or the vicinity of a triplet trans- cia-bent isomerization barrier. We prefer this last explanation because this barrier should increase the $S_{1} \longleftrightarrow T$ and the $S_{0} \longleftrightarrow T$ couplings, Indeed, we suggest that the large level densities are due to $S_{1}\longleftrightarrow S_{0}$ tran-sitions induced by a triplet $level^{3}$. Moreover, the existence of the isomerization barrier is suggested by recent work using the Stark quantus beat $technique^{2}$