Study of the a 3Π and X 1Σ + states of the MgO molecule: Spin-forbidden transitions and breakdown of the Born-Oppenheimer approximation

• Published in: Chemical physics 1989-08, Vol.135 (2), p.229-245
• Main Authors: Thümmel, Helmar, Klotz, Rainer, Peyerimhoff, Sigrid D.
• Format: Article
• Language: English
• ISSN: 0301-0104
• DOI: 10.1016/0301-0104(89)87023-5

The perturbation of the a 3Π 0+ and X 1Σ + vibrational levels in the MgO molecule due to spin-orbit interaction has been investigated. The magnitude depends on the size of the energy difference between the unperturbed levels and on the spin-orbit matrix elements which are calculated employing the Breit-Pauli one- and two-electron spin-orbit operator. The theoretical procedure for the calculation of spin-forbidden transitions in cases where the zero-order states involved cannot be treated within the Born-Oppenheimer approximation is given. The intensity distribution of the intercombination bands a 3Π 0+→X 1Σ + is evaluated. The weak perturber states B 1Σ +, 2,3 3Π and 1,2 3Σ − are explicitly considered in addition to the interaction of a 2Π 0+ with the “strong” perturber state X 1Σ +. The mechanisms are discussed in terms of intensity borrowing. The decrease/increase in intensity of the “infrared” transitions a 3Π 0+ (ν′)→a 3Π 0+(ν′) and X 1Σ + (ν″)→ X 1Σ + (ν″), due to spin-orbit interaction is discussed and the perturbed and unperturbed dipole moments, μ(X 1Σ +, ν″) and μ(a 3Π 0+, ν′), are given.