The effects of triplet perturbers on photophysical processes in the (A)over-tilde(1)A(u) state of acetylene

Rotationally resolved excitation spectra of the acetylene (A) over tilde (1)A(u) <-- (X) over tilde (1) Sigma(g)(+) transition were recorded by measuring separately the total emission in two spectral regions: UV (200-400 nm) and NIR (0.9-1.5 mu m). UV emission is due to (A) over tilde --> (X) over tilde transitions. The source of the NIR signal is suggested by the observation that the NIR:UV intensity ratio is approximately constant throughout the (V0K01)-K-2 subband. (V denotes the trans-bending mode.) This is consistent with C2H (A) over tilde (2) Pi --> (X) over tilde (2) Sigma(+) NIR transitions occurring as a result of resonance-enhanced two-photon excitation of predissociated Rydberg states. The (V0K01)-K-3 subband shows significant fluctuations in the NIR:UV intensity ratio. The increase in trans-bending excitation 2 nu'(3) to 3 nu'(3) enables accidental resonances with background states (both tripler and S-1) to alter the NIR emission intensity. Triplet perturbers can facilitate T-n (n>1) --> T-1 NIR transitions or modify the resonance enhancement of C2H <(A)over bar-state> production. Extra lines in the (V0K01)-K-3 subband, due to an S-1 anharmonic perturber, show dramatically diminished NIR but enhanced UV emission intensity. This indicates that the perturbing state is anomalously resistant to photodissociation. (C) 1997 American Institute of Physics.