Lifetime lengthening and the Renner–Teller effect in the HCF (Ã1A″←X̃1A′) system

We report new measurements of fluorescence excitation spectra and single rovibronic level fluorescence lifetimes for (0,υ2′,0)←(0,0,0) bands with υ2′=0–6 in the HCF Ã1A″←X̃1A′ system. Due primarily to the Renner–Teller interaction, upper state levels with Ka⩾1 display an obvious lengthening of lifetime with increasing energy which is strongly correlated with the presence of vibronic and rovibronic perturbations, and increases dramatically as the barrier to linearity is approached. This system provides a textbook example of the small molecule limit of radiationless transitions.