Laser-induced fluorescence spectroscopy of CCH (X̃²Σ ⁺) in vibrationally excited levels up to 4500 cm^-1

Vibrationally excited levels of the CCH radical in the X̃ ²Σ⁺ state up to 4500 cm^-1 above the ground vibrational level have been studied both at high pressure and in a supersonic jet by laser-induced fluorescence (LIF). The CCH radical was produced by photolyzing acetylene with 193 nm laser light. Nineteen UV bands are rotationally analyzed and assigned as transitions of parallel type from X(0,v₂ ^l,v₃), (v₂=0-10, v₃ = 0-2, l = 0-3) to four K sublevels (K=0-3) of a common vibrational level, T, of an upper electronic state most likely B̃²A′. Among them, bands at 37 010 and 36 075 cm^-1 were previously reported [J. Chem. Phys. 98, 6690 (1993)]; an improved spin-orbit constant of X̃(0,7¹, 0) was determined in this work. Vibrational energies of the l=0 and 1 levels were determined in a simultaneous analysis of UV and IR bands. Assignment of a vibrational level T as the common upper state of the observed LIF bands permits us to determine spectroscopic parameters of the l=2 and 3 levels of the X̃ state, which are reported for the first time. The geometry of level T at 39 157.41 cm^-1 is accordingly deduced and agrees with the ab initio prediction of the B state. Our vibrational assignments of the X state, particularly those of bending progressions, are supported by observations of the l=2 and 3 levels.