### Citation:

### Abstract:

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 C_{2}H, 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*(^{2}Π) ← *X*(^{2}Σ^{+}) absorption can be written as the sum of the *A*(^{2}*A*’) ← *X* and *A*(^{2}*A*‘) ← *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*(^{2}*A*‘) ← *X*absorption and is centered at 5500 cm^{−}^{1}, while the other is due to *A*(^{2}*A*’) ← *X*absorption and is centered at 9500 cm^{−}^{1}. The dramatic blue shift of the *A*(^{2}*A*’) ← *X* band results from the combination of the large *X* state thermal bending amplitude and high bending frequency of the *A*(^{2}*A*’) state. We also determine the *X* state pure vibrational absorptionspectrum and show it to be of much lower intensity than the pure electronic spectrum.

*Last updated on 10/07/2016*