Quantum chemical study and experimental observation of a new band system of C(2), e 3Pi(g)-c 3Sigma(u)+.

A new band system of C(2), e (3)Pi(g)-c (3)Sigma(u)(+) was studied by ab initio quantum chemical and experimental methods. The calculations were carried out at the multireference configuration interaction level of theory with Davidson's correction using aug-cc-pV6Z basis set and include core and core-valence correlation as well as relativistic corrections computed with aug-cc-pCVQZ and cc-pVQZ bases, respectively. The vibrational energies and rotational constants of the upper e (3)Pi(g) state were calculated from the computed ab initio potential energy curve. The ab initio results indicate that the electronic transition moment of the e (3)Pi(g)-c (3)Sigma(u)(+) system is approximately one-half that of the Fox-Herzberg e (3)Pi(g)-a (3)Pi(u) system. Franck-Condon factors were calculated for both systems and used to guide experiments aimed at discovering the e (3)Pi(g)-c (3)Sigma(u)(+) system. The e (3)Pi(g)(v(') = 4)-c (3)Sigma(u)(+)(v(") = 3) band of jet-cooled C(2) was successfully observed by laser-induced fluorescence spectroscopy by monitoring the ensuing e (3)Pi(g)-a (3)Pi(u) emission.

The d (3)Pi(g)-c (3)Sigma(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 (3)Pi(g)<--c (3)Sigma(u) (+) and d (3)Pi(g)<--a (3)Pi(u) excitations. Rotationally resolved excitation spectra of the (4<--1), (5<--1), (5<--2), and (7<--3) bands in the d (3)Pi(g)<--c (3)Sigma(u) (+) 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 (3)Sigma(u) (+) state. The principal molecular constants for the c (3)Sigma(u) (+) state are B(e)=1.9319(19) cm(-1), alpha(e)=0.018 55(69) cm(-1), omega(e)=2061.9 cm(-1), omega(e)x(e)=14.84 cm(-1), and T(0)(c-a)=8662.925(3) cm(-1). We report also the first experimental observations of dispersed fluorescence from the d (3)Pi(g) state to the c (3)Sigma(u) (+) state, namely, d (3)Pi(g)(v=3)-->c (3)Sigma(u) (+)(v=0,1).