The d³Π_g-c ³∑_u ⁺ band system of C₂

A two-dimensional fluorescence (excitation/emission) spectrum of C2 produced in an acetylene discharge was used to identify and separate emission bands from the d Πg3 c u+3 and d Πg3 a Πu3 excitations. Rotationally resolved excitation spectra of the (41), (51), (52), and (73) bands in the d Πg3 c u+3 system of C2 were observed by laser-induced fluorescence spectroscopy. The molecular constants of each vibrational level, determined from rotational analysis, were used to calculate the spectroscopic constants of the c u+3 state. The principal molecular constants for the c u+3 state are Be =1.9319 (19) cm-1, αe =0.018 55 (69) cm-1, ωe =2061.9 cm-1, ωe xe =14.84 cm-1, and T0 (c-a) =8662.925 (3) cm-1. We report also the first experimental observations of dispersed fluorescence from the d Πg3 state to the c u+3 state, namely, d Πg3 (v=3) →c u+3 (v=0,1).