Experimental Lifetime Measurements of C₂ in the High-lying (2−4)³Δ_g States^†

The dicarbon (C₂) molecule is an important molecular species observed in many carbon-containing gaseous environments. The spectroscopic and dissociative studies of C₂ have attracted great attention for a long time for understanding its electronic characters as well as the evolution and cycling of carbon in the universe. In this study, the lifetimes of C₂ populated at the three high-lying electronic states of (2−4)³Δ_g in the vacuum ultraviolet (VUV) region have been experimentally measured using a time-of-flight mass spectrometer and the VUV-pump−UV-probe photoionization scheme. The measurements showed that all the rovibronic levels in the 2(g)³Δ_g state exhibit shorter lifetimes than the dynamical limit of the experimental method, consistent with the theoretically predicted radiative lifetimes. Dependence of the lifetime on the vibrational level was observed in the 3³Δ_g state, and a marginal rotational dependence was noticed in the vʹ=1 vibrational level. For the 4³Δ_g state, the rotationally resolved lifetimes in the vʹ=1 vibrational level were found to be slightly longer than those in the vʹ=0 and 2 vibrational levels. Such a complicated dependence of the lifetime on the rovibronic level makes further experimental and theoretical investigations highly desired for understanding the dynamics in the high-lying excited states of C₂.