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

We have recorded Zeeman anticrossing (ZAC) spectra of gas phase acetylene (HC = CH) in the A approximately 1A(u)v'3 = 0-3 (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 42200 to 45300 cm-1 above the rotationless zero-point level of the X approximately 1-SIGMA-g+ state. The magnetic field was scanned from 0 to 8 T and the ZAC spectra were recorded as decreases of intensity of the fluorescence excited by a pulsed, frequency doubled dye laser. The ZAC spectra were unassignably complex. We report here a surprisingly rapid increase in the density of detectable anticrossings (AC) over a 3100 cm-1 energy interval (only 7% of the total excitation energy). In the v'3 = 3 level the detected anticrossing density is 10(2) times larger than the maximum computed density of triplet T1-T3 vibrational levels and quite comparable to the computed density of X approximately 1-SIGMA-g+ vibrational levels. Plausible mechanisms for the rapid increase in the ZAC density are discussed, especially one involving access to the highly excited vibrational levels of the S0 state via a cis-bent isomer of the triplet acetylene near the top of a cis-trans isomerization barrier on one triplet surface.