Significance of Herzberg-Teller effects on allowed transitions of defects in solids

The Franck-Condon principle and Condon approximation are key to understanding optical transitions. This study uses first-principles calculations to simulate luminescence and absorption line shapes of hexavalent Mn ions in alkali-metal sulfates, revealing a significant Herzberg-Teller effect in this electric-dipole allowed transition. These findings show the limitations of the Condon approximation and the need for more sophisticated approaches to accurately simulate optical phenomena.