Observation of the d^Πg₃ →c ³∑_u⁺ band system of C₃

A new band system of C2, d Πg3 c u+3 is observed by laser induced fluorescence spectroscopy, constituting the first direct detection of the c u+3 state of C2. Observations were made by laser excitation of c u+3 (v″ =0) C2, produced in an acetylene discharge, to the d Πg3 (v′ =3) level, followed by detection of Swan band fluorescence. Rotational analysis of this band yielded rotational constants for the c u+3 (v″ =0) state: B0 =1.9218 (2) cm-1, λ0 =-0.335 (4) cm-1 and γ0 =0.011 (2) cm-1. The vibrational band origin was determined to be 3-0 =15861.28 cm-1.