Contrasting singlet-triplet dynamical behavior of two vibrational levels of the acetylene S1, 2131B2 polyad

Surface electron ejection by laser-excited metastables (SEELEM) and LIF spectra of acetylene were simultaneously recorded in the regions of the (A) over tilde (1)A(u)-(X) over tilde (1)Sigma(+)(g) nominal 2(1)3(1)4(2) K-a = 1 <- 0(0) and 2(1)3(1)6(2) K-a = 1 <- 0(0) bands near 46 140 cm(-1). The upper states of these two bands are separated by only similar to 100 cm(-1), and the two S, vibrational levels are known to be strongly mixed by anharmonic and Coriolis interactions. Strikingly different patterns were observed in the SEELEM spectra in the regions of the 21314 2 and 213161 vibrational levels. Because the equilibrium structure of the T-3 electronic state is known to be nonplanar, excitation Of nu(4) (torsion) and nu(6) (antisymmetric in-plane bend) are expected respectively to promote and suppress vibrational overlap between low-lying S-1 and T-3 vibrational levels. The nearly 50:50 mixed 2(1)3(1)4(2) -2(1)3(1)6(2) character of the S, vibrational levels rules out this simple Franck-Condon explanation for the different appearance of the SEELEM spectra. A simple model is applied to the SEELEM/LIF spectra to explain the differences between spectral patterns in terms of a T3 doorway-mediated singlet-triplet coupling model.