CASSCF-wave packet ab initio prediction of electronic and vibrational spectra: Application to the A(/sup 2/Pi) reverse arrow X(/sup 2/. sigma. /sup +/) absorption of C/sub 2/H at 3000 K
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 C/sub 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(/sup 2/Pi) reverse arrow X(/sup 2/..sigma../sup +/) absorption can be written as the sum of the A(/sup 2/A') reverse arrow X and A(/sup 2/A'') reverse arrow X absorptions, the spectra are determined to 60 cm/sup -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(/sup 2/A'') reverse arrow X absorption and is centered at 5500 cm/sup -1/, while the other is due to A(/sup 2/A') reverse arrow X absorption and is centered at 9500 cm/sup -1/. The dramatic blue shift of the A(/sup 2/A') reverse arrow X band results from the combination of the large X state thermal bending amplitude and high bending frequency of the A(/sup 2/A') 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.
- Research Organization:
- Department of Chemistry, University of California, San Diego, La Jolla, California 92093
- OSTI ID:
- 5748460
- Journal Information:
- J. Chem. Phys.; (United States), Vol. 82:11
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
POLYATOMIC MOLECULES
ABSORPTION SPECTRA
ELECTRONIC STRUCTURE
BORN-OPPENHEIMER APPROXIMATION
RADICALS
SELF-CONSISTENT FIELD
VERY HIGH TEMPERATURE
VIBRATIONAL STATES
WAVE PACKETS
ENERGY LEVELS
EXCITED STATES
MOLECULES
SPECTRA
640305* - Atomic
Molecular & Chemical Physics- Atomic & Molecular Theory- (-1987)