THEORETICAL-STUDY OF THE TRANSITION-PROBABILITIES OF THE DOUBLY-EXCITED STATES E1-SIGMA-G+ OF C2 AND 2(2)SIGMA-G+ OF C2+

The radiative properties of the doubly-excited states E(1)SIGMA(g)+(sigma(u)2 --> sigma(g)2) of C2 and 2(2)SIGMA(g)+(sigma(u)2 --> pi(u)2) of C2+ are investigated theoretically by using MRD-CI wave functions. Due to a heavy mixing between bonding and antibonding configurations, the E state (with a maximum contribution of sigma(u)2 --> sigma(g)2 of 45%) does not show the typical features of strongly bound states (short R(e) and high omega(e)). Because of the mixing, the experimental Freymark system E(1)SIGMA(g)+ - A(1)PI(u) of C2 exhibits a moderate intensity though. At R = 2.43 bohr (an intermediate geometry between both minima), a calculated transition moment SIGMA-R(e'e'')2 of 0.151 au2 disagrees with an experimental estimate of 2.26 +/- 1.13 au2 from Cooper and Nicholls. The same authors assigned to the Mulliken band D(1)SIGMA(u)+ - X(1)SIGMA(g)+ of C2 a SIGMA-R(e'e'')2 of 0.13 +/- 0.05 au2, whereas this work and earlier data from the literature indicate a higher value of 0.41 au2 for that quantity. A total radiative lifetime tau(0) almost-equal-to 15 ns for the E(1)SIGMA(g)+ state is mainly dicated by the radiative process E --> A, the competitive decay E --> D being about 10 times slower. The 2(2)SIGMA(g)+(sigma(u)2 --> pi(u)2) state of C2+ is strongly bound, with a shortening DELTA-R(e) = 0.35 bohr and an increase DELTA-omega(e) almost-equal-to 800 cm-1 (60%) relative to the X(4)SIGMA(g)- ground state. A lifetime tau(0)(2(2)SIGMA(g)+) almost-equal-to 28 ns results from radiative decay into both 1(2)SIGMA(u)+ and 2(2)PI(u) states. On the basis of the present spectroscopic data for C2+, an absorption band observed at 2.77 eV (assumed as belonging to C2) is tentatively reassigned to the 2(2)SIGMA(g)+ <-- 1(2)SIGMA(u)+ transition of C2+, with a calculated DELTA-E00 = 2.81 eV.