Ab initio self-consistent field calculations have been performed for $1s$-core-orbital ionization in the halogen atoms F, Cl, and Br, and their diatomic counterparts ${\mathrm{F}}_2$, ${\mathrm{Cl}}_2$, and ${\mathrm{Br}}_2$. It is found that $E_B\left(X_2\right)<\mathrm{}{E}_B\mathrm{}\left(X\right)\mathrm{}$ in all cases. This order is determined by the relaxation-energy corrections—Koopman's theorem predicts just the opposite shift. The calculations predict that an additional 7-14% of all $1s$-ionization events lead to multiple excitation (shakeup and shakeoff) in the diatomics relative to the free atom. Within a given series, however, the probabilities are fairly insensitive functions of atomic number. The corresponding orbital transformation of Amos and Hall is shown to have particularly useful features when applied to the ionization problem.

• Received 4 April 1977

DOI:https://doi.org/10.1103/PhysRevA.16.1341