Calculated potential surfaces for the description of the emission spectrum of the C2H radical

Ab initio MRD-CI calculations are reported for the CC stretch and CCH bending potential surfaces for the lowest-lying electronic states of the ethynyl radical. It is found that the energies of the fully relaxed $\text{π → π}{}^1$ states of this system are as much as 2.3 eV below the corresponding values at the ²Σ⁺ ground state (linear) equilibrium structure. The fact that the energy minima for the C₂H excited states occur for widely different internuclear arrangements than that of the ground state is shown to be very consistent with the experimental observation that the electronic spectrum of C₂H has a much different appearance in emission than it does in absorption studies. On the basis of both calculated energy differences and oscillator strength results it is found that the most likely assignment for the broad emission feature in the 4000–6000 Å region of the C₂H spectrum is caused by vertical excitation from the lowest vibrational levels of the 3²A′ (and also 2²A″) upper states to the 2²A′, ²A′ and ²A″ species which correlate with X²Σ⁺ and A²Π for linear nuclear arrangements; the possible mechanisms for production of 3²A′ and 2²A″ upon photolysis of acetylene are also discussed. The possibility that spinforbidden transitions are involved at longer wavelengths in this spectral region is also considered. Finally it is pointed out that a potential crossing of the linear $\text{X}\text{?}{}^2\text{Σ}{}^{\mathrm{+}}$ and A²Π species calculated to occur in the neighborhood of R_CC = 2.6 bohr, which becomes sharply avoided upon molecular bending, would be expected to lead to the type of irregular vibronic structure in the 3600 cm^?1 region of the C₂H IR spectrum recently observed by Jacox.