STARK AND ZEEMAN SPECTROSCOPY OF ACETYLENE: LOCATING A TRIPLET ISOMERIZATION BARRIER

Author Institution: Mail Stop 170-25 Caltech; Service National des Champs Intenses, C.N.R.S.; Weiss School of Natural Sciences, Rice University; Department of Chemistry, Massachusetts Institute of Technology; Department of Chemistry, University of Southern California; Department of Chemistry and George R. Harrison Spectroscopy Laboratory, Massachusetts Institute of TechnologyAnticrossing and quantum beat spectroscopies (employing magnetic fields from zero to 8 Tesla = 80 kGauss, or electric fields of up to 113 kV/cm) have been used to study intramolecular interactions in the acetylene molecule. Both Zeeman and Stark activity occurs among the low-lying rovibrational levels of the first excited singlet electronic state $S_{1} (\tilde{A}^{1}{^{1}}A_{u})$. These properties, as well as the quantity and strength of interactions between the selected levels and those nearby levels belonging to the lower-lying triplet electronic states $T_{i} (i=1-3)$ --- and highly vibrationally excited levels of the ground electronic state $S_{0} (\tilde{X}{^{1}}\Sigma_{g}{^{+}})$ - change rapidly with small changes of total energy in the molecule. The variety of observations are best explained by the surmounting of a triplet isomerization barrier between the trans- and cis-bent minima on the $T_{2}$ potential energy surface. This barrier would be located near or below the $v_{3}{^{\prime}}=2$ level, thus at about $44,000 cm^{-1}$ above the zero point level of S