CASSCF-wave packet ab initio prediction of electronic and vibrational spectra: Application to the A(²Pi) reverse arrow X(². sigma. /sup +/) absorption of C2H at 3000 K


The combination of a b i n i t i o 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‘) ← Xabsorptions, the spectra are determined to 60 cm1 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‘) ← Xabsorption and is centered at 5500 cm1, while the other is due to A(2A’) ← Xabsorption and is centered at 9500 cm1. 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 absorptionspectrum and show it to be of much lower intensity than the pure electronic spectrum.

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Last updated on 10/07/2016