Abstract :
We present new results concerning 17 Zeeman anticrossing spectra obtained by the excitation of 17 rotational levels (K′=0), 1, N′=0,1,2) belonging to different vibrational levels of the Ã1Au state of gas phase acetylene (HCCH). The vibrational levels studied are mainly a progression of the ν′3 mode, (ν′3 is the CCH trans-bending mode of the trans-bent excited state) with ν′3=0–4. The energy range sampled was from 42200 to 46300 cm−1 above the rotationless zero-point energy level of the X̃1Σg+ state. The magnetic field was scanned from 0 to 8 T. We used the technique of the Fourier transform to statistically analyze each Zeeman anticrossing spectrum. This technique gave the ability to determine, the distribution law of the anticrossings linewidth linked to the coupling matrix elements, the density of anticrossings, the anticrossing average half linewidth and the product of these two last quantities. The distribution law of the anticrossinss is a reproducible power law very weakly dependent on the rovibrational level. By using a model of spin-orbit singlet ∼ triplet couplings we were able to deduce the product of the density of coupled vibrational levels with the reduced average coupling ϱvib〈V〉. This quantity is usually considered as playing a fundamental role in intramolecular dynamics. We observed an exponentially increasing product ϱvib〈V〉 as a function of the excitation energy over an energy range larger than 2000 cm−1; at higher energies, saturation effects appeared, relaxing such a law. We interpreted this exponential increase as due to a trans-linear-cis isomerization barrier of a triplet surface, as we had previously reported [P. Dupré et al., Chem. Phys. 152 (1991) 293].
