The 4051-Å band of C ₃ (Ã ¹ Π _u-X̃ ¹∑ _g ⁺, 000-000): Perturbed low- J lines and lifetime measurements

Rotational analyses have been carried out at high resolution for the 000-000 and 000-100 bands of the à Πu1 -X̃ ∑g+1 transition of supersonic jet-cooled C3. Two different spectra have been recorded for each band, using time gatings of 20-150 and 800-2300 ns. At the shorter time delay the spectra show only the lines observed by many previous workers. At the longer time delay many extra lines appear, some of which have been observed previously by [McCall Chem. Phys. Lett. 374, 583 (2003)] in cavity ring-down spectra of jet-cooled C3. Detailed analysis of these extra lines shows that at least two long-lived states perturb the à Πu1, 000 state. One of these appears to be a ∑u-3 vibronic state, which may possibly be a high vibrational level of the b̃ Πg3 state, and the other appears to be a P=1 state with a low rotational constant B. Our spectra also confirm the reassignment by McCall of the R (0) line of the 000-000 band, which is consistent with the spectra recorded towards a number of stars that indicate the presence of C3 in the interstellar medium. Fluorescence lifetimes have been measured for a number of upper-state rotational levels. The rotational levels of the à Πu1 state have lifetimes in the range of 230-190 ns, decreasing slightly with J; the levels of the perturbing states have much longer lifetimes, with some of them showing biexponential decays. An improved value has been obtained for the ν1 vibrational frequency of the ground state, ν1=1224.4933±0.0029 cm-1.