Radiative lifetimes of CH2(b 1B1)

Abstract

Radiative lifetimes are reported for CH2(b 1B1) in a wide range of rotational states in the (0,14,0), (0,15,0), (0,16,0), and (0,17,0) vibrational levels. Laser photolysis of ketene in a supersonic-jet expansion produced CH2(a 1A1) which was excited to single rovibrational levels of the (b 1B1) state by a second laser. Analysis of the temporal evolution of the fluorescence (b 1B1)–(a 1A1) yielded collision-free radiative lifetimes for the (b 1B1) state. The measured lifetimes range from about 4 to 10 μs and decrease with increasing vibrational energy. For the (0,14,0) overtone, the lifetimes increase slightly as a function of J (Ka=0). However, the lifetimes of rotational levels with Ka=0 in the (0,16,0) vibrational state are found to be independent of the rotational state. Calculations of the vibronic lifetimes show that the considerable lifetime lengthening when Ka≥1 is due to Renner–Teller coupling to the ã 1A1 state. The random lifetime fluctuation observed in some vibronic bands is probably due to spin–orbit coupling to the X̃ 3B1 state, e.g., 413 and 414 of (0,15,0). Here the radiative lifetime of the singlet component is shorter than that of the corresponding triplet component.