Abstract
The combination of ab initio calculation of electronic wave functions with a wave packet calculation of the nuclear motion is used, within the Born–Oppenheimer approximation to compute the vibrational and electronic absorption of a polyatomic molecule. A particular virtue of this approach is that high as well as low temperature spectra are both calculable. This method is applied to C2H, for which the complete active space self-consistent field (CASSCF) method is used to determine full Born–Oppenheimer potential surfaces. Using the assumption that the A(2Π) ← X(2Σ+) absorption can be written as the sum of the A(2A′) ← X and A(2A″) ← X absorptions, the spectra are determined to 60 cm−1 resolution at a temperature of 3000 K. As a result of the large thermal bending amplitude at 3000 K, the calculated spectra are broad and have little resolved structure. Two bands are resolvable, one is due to the A(2A″) ← X absorption and is centered at 5500 cm−1, while the other is due to A(2A′) ← X absorption and is centered at 9500 cm−1. The dramatic blue shift of the A(2A′) ← X band results from the combination of the large X state thermal bending amplitude and high bending frequency of the A(2A′) state. We also determine the X state pure vibrational absorption spectrum and show it to be of much lower intensity than the pure electronic spectrum.
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