MRCI studies on ground and excited states of CBr

Abstract

Potential curves and spectroscopic constants for a large number of doublet and quartet states of CBr were obtained by multireference configuration interaction calculations, using valence triple-zeta basis sets with polarization and diffuse functions. Besides the X²Π ground state, 1⁴Σ⁻, 1²Δ and 2²Σ⁺ have been found to be stable. Spectroscopic constants calculated for 1²Δ are in excellent agreement with experimental values obtained by Dixon and Kroto in 1963. Their observed predissociation of one component of 1²Δ can be explained by the crossing of the 1²Δ potential near equilibrium by 1²Σ⁺. The 1²Σ⁺ state is calculated to have a shallow long-range minimum at 2.31 Å. The dissociation energy of X²Π is calculated to be 3.43 eV. An observed T _e of 4.97 eV for 2²Σ⁺ agrees with the theoretical value. Several Rydberg states of the 2π→Ryd and 3σ→Ryd series, starting at T _e  = 5.25 eV, were identified. Photodissociation of CBr by sunlight, important in the ozone cycle, can occur via direct dissociation of the ground state, or by excitation to 1²Δ followed by predissociation. Most dissocative repulsive states lie at higher energies, and are not expected to participate in the photodisscociation of CBr.

Keywords:

• CBr
• Ab initio spectroscopy
• Potential curves
• Spectroscopic constants
• Predissociation

Acknowledgements

The authors had the pleasure of discussing the spectroscopy of CBr with Sir Kroto on the occasion of his visit to this university. Thanks also to Dr Pablo Bruna for helpful discussions. Financial support of this work by NSERC-Canada is gratefully acknowledged.