Laser spectroscopy of CCH in the 36600-39700 cm^-1 region

This work reports on nine new bands of the CCH B∼ ²A′←X̃ ²∑⁺ system recorded by laser-induced fluorescence. In addition, the 37 946 and 38 107 cm^-1 bands were revisited in a supersonic molecular beam. Of these 11 bands, ten were in the 36 600-39 700 cm^-1 range; they were rotationally analyzed and assigned as transitions from low vibronic levels, X̃(0,υ₂,υ₃) (υ₂=2-4 and υ₃ = 0-1) of CCH to two vibrational levels (T,T+1221) of the B̃ state. Although previously observed as too weak, these bands were enhanced in this work by relaxing vibrationally excited CCH with SF₆, instead of He or Ar. The rotationally resolved 35 939 cm^-1 band, the band outside of the range of 36600-39700 cm^-1, is assigned as B̃(T+775)←X̃(0,9¹,0). These new bands, together with transitions previously reported, are simultaneously analyzed in a nonlinear fit. This work obtained the first spectroscopic parameters of X̃(0,2⁰,0), X̃(0,2²,0), X̃(0,2²,1), B̃(T+775), and B̃(T+1221), and further improved the spectroscopic parameters of X̃(0,3¹,0), X̃(0,3³,0), and B̃(T). Though the rotational constants of the B state have a very small vibrational dependence, their spin-rotation constants increase by a factor of 2 within 1221 cm^-1, suggesting that the B̃ state is perturbed by the nearby unobserved C̃²A″ state via the Renner-Teller effect.