Knowledge Bank

Anticrossing 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 $S1(A~1\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Au)$. 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 $Ti(i=1-3)$ --- and highly vibrationally excited levels of the ground electronic state $S0(X~\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma g\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>})$ - 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 $T2$ potential energy surface. This barrier would be located near or below the level, thus at about $44,000cm-1$ above the zero point level of Sa