Rovibrational distributions of CH(B ²Σ^-) produced in electron-C₂H₂ collisions

The excitation function and rovibrational distributions of CH(B ²Σ^-) produced in e-C₂H₂ collisions have been measured. There are two thresholds at 13.8 ± 0.5 and at 26.5 ± 1.5 eV in the excitation function. Apart from the absolute intensity, the excitation function is quite similar to that of the CH(A-X) emission. The rovibrational distributions have been estimated from a computer simulation of the emission spectra and a self-consistent graphical plot. The rotational temperature is constant at the incident electron energies below the second threshold, and the rotational distributions are easily reproduced with a single rotational temperature. Above the second threshold, two rotational temperatures are required to reproduce the rotational distributions of the emission spectra. The vibrational distribution is independent of the electron energies. Thus, it is concluded that the reaction C₂H₂** → CH(A ²Δ or B ²Σ^-) + CH(X ²Π) principally takes place below the second threshold, whereas the reaction C₂H₂** or C₂H₂⁺ → CH(A ²Δ or B ²Σ^-) + CH⁺(A ¹Π) contributes considerably to the production of CH(A) and CH(B) above the second threshold.