Knowledge Bank

It is sometimes assumed that the electronic transition moment does not vary appreciably with changes in the internuclear distance, or that the variation is merely linear (R-centroid method). These criteria are not usually satisfied for many molecular transitions. The $X1\Sigma g+-B1\Sigma g+$ (Lyman Bands) transition of hydrogen provides an interesting example of a case in which an appreciable variation of the transition moment occurs. The transition moment was evaluated over the range 1 $\le$ R $\le$ 6 a u. using an SCF $X1\Sigma g+$ ground state wave function with configuration interaction, and a frozen core $B1\Sigma g+$ excited state wavefunction with configuration interaction. The variation of the electronic transition moment with internuclear distance is explained in terms of contributions from various configurations of the ground and excited state wavefunctions. Other factors which influence the dependence of the electronic transition moment on internuclear distance, and the validity of the R-centroid method are also discussed.