Knowledge Bank

The relative emission intensity of vibrational bands due to the $A2\Sigma \to X2\Pi$ transition in $HCl+$, $DCl+$ and $HBr+$ have been measured. The intensities of bands in a progression from a fixed upper state were compared with the Franck-Condon factors. The observed intensities became increasingly smaller than the Franck-Condon factors predicted as $v\prime \prime$ increased in each progression. The results were interpreted as a decrease in the transition moment with increasing internuclear separation. When treated in this fashion $DCl+$ gives the same quautitative decrease as $HCl+$. The intensity variation can be fit by using a power series expansion in the internuclear separation for the electric dipole moment.