Abstract
Rotationally resolved excitation spectra of the acetylene à 1Au←X̃ 1∑g+ transition were recorded by measuring separately the total emission in two spectral regions: UV (200–400 nm) and NIR (0.9–1.5 μm). UV emission is due to Ã→X̃ transitions. The source of the NIR signal is suggested by the observation that the NIR:UV intensity ratio is approximately constant throughout the V02K01 subband. (V denotes the trans-bending mode.) This is consistent with C2H à 2Π→X̃ 2∑+ NIR transitions occurring as a result of resonance-enhanced two-photon excitation of predissociated Rydberg states. The V03K01 subband shows significant fluctuations in the NIR:UV intensity ratio. The increase in trans-bending excitation from 2ν3′ to 3ν3′ enables accidental resonances with background states (both triplet and S1) to alter the NIR emission intensity. Triplet perturbers can facilitate Tn (n>1)→T1 NIR transitions or modify the resonance enhancement of C2H Ã̃-state production. Extra lines in the V03K01 subband, due to an S1 anharmonic perturber, show dramatically diminished NIR but enhanced UV emission intensity. This indicates that the perturbing state is anomalously resistant to photodissociation.
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