Knowledge Bank

In the $36600-39700cm-1$ range, eight new bands of CCH $B2A\prime \leftarrow X~2\Sigma$ transition were observed by laser-induced fluorescence. By rotational analysis, they have been assigned as transitions from low vibronic levels, $X~(0,v2,v3)(v2=2-4$ and $v3=0-1$) of CCH to two vibrational levels (T, T+1221) of the $B~$ state. Most of these bands were difficult to be detected if a acetylene/argon or helium gas mixture was photolyzed to produce CCH. Due to population transfer, via collisions with $SF6$, from higher vibronic levels of CCH which were predominantly produced in the 193.3 nm photolysis of acetylene, spectral intensities of these bands were thus enhanced. Spectroscopic parameters of $\tilde{X}(0,2^{0}, 0),\tilde{X}(0,2^{2}, 0),\tilde{X}(0,2^{2},1) $ and $B~(T+1221)$ were obtained for the first time. Spectral lines of transition $B~(T)\leftarrow X~(0,30,3,0)$ recorded in a supersonic beam were also included in a simultaneous fit, spectroscopic parameters of $X~(0,30,3,0)$ were accordingly better determined in this work than our previous $reports.a,b$ The vibronic energy levels $X~(0,v2,v3)(v3=0-2)$ of CCH up to $5500cm-1$ were approximated as three anharmonic oscillators, where $v2$ is 401 cm. The anharmonicities and 911#### obtained from the fit will be given.