TRANSITION-PROBABILITIES FOR THE B 1-DELTA-G-A1-PI-U AND B' 1-SIGMA-G+-A1-PI-U INFRARED BANDS OF C2 - AN ABINITIO STUDY

Several radiative data (e.g., transition moment R(e'e"), oscillator strength f-upsilon' upsilon", Einstein emission coefficient A-upsilon' upsilon" and lifetime tau-upsilon') are derived for the B 1-DELTA-g-A 1-PI-u and B' 1-SIGMA-g+-A 1-PI-u infrared bands of C2 by using ab initio MRD-CI wavefunctions. The predicted f00 values are 0.75 x 10(-3) (B <-- A) and 1.54 x 10(-3) (B' <-- A), both results being somewhat smaller than the best literature estimate f00 almost-equal-to 2.21 x 10(-3) for the A <-- X (Phillips) band. The computed tau-0(B 1-DELTA-g) of 141.3-mu-s represents the longest radiative lifetime so far reported for any dipole-allowed C2 transition. The lifetime tau-0(B' 1-SIGMA-g+) is only 8-mu-s, a value which is approximately 50% shorter than the time range from 13 to 19-mu-s assigned to tau-0(A 1-PI-u); however, for upsilon greater-than-or-equal-to 2 the lifetimes of both B' and A states differ by less than 3-mu-s. Differences between the radiative data for the A, B, B' and b states are rationalized on the basis of excitation energies and Franck-Condon factors. According to the computed B and B' lifetimes, the cascading process B' --> A --> X might have been responsible for the longer A 1-PI-u lifetimes reported experimentally.