The low-lying bending vibrational levels of the CCH (X̃ 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,υ₂ ¹,0) (υ₂=1, 3, 5, 7) vibrational levels of the X̃ ²Π⁺ 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.