The low-lying bending vibrational levels of the CCH (X 2Σ+) radical studied by laser-induced fluorescence

The uv spectrum of the CCH radical was recorded using the laser-induced fluorescence technique on the 193 nm photolysis product of acetylene. Four 2Π–2Π bands at 38 805, 37 946, 37 010, and 36 075 cm−1 of CCH were rotationally analyzed and assigned as transitions from the (0,v21,0) (v2=1, 3, 5, 7) vibrational levels of the X̃ 2Σ+ state to a common upper vibronic state (denoted as U), which possibly belongs to the 2 2Π state. A simultaneous nonlinear least squares fit of the uv bands, in combination with the infrared transitions previously observed in the X̃ 2Σ+ state, provided improved spectroscopic parameters for the U state and the (0,31,0), (0,51,0), and (0,71,0) levels of the X̃ state.