Knowledge Bank

A diatomic molecule in the vibronic state $|\Delta s\Sigma ;v>$ has a dominant magnetic moment given by $(\Delta +2\Sigma )$ (eh/4nmc) due to which weak rotational transitions within this state are possible. Magnetic dipole transitions of this kind are $known1$ in the $X3\Sigma g-$ state of $02$. Extending the results to other electronic states, expressions for the intensity factors are derived for rotational transitions in $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Delta$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Delta$ states. A noteworthy feature is that both parallel and perpendicular moments can contribute to the rotational intensities. An allied aspect, perhaps more interesting, is that the magnetic moment in the state $|\Delta s\Sigma ;r>$ has no dependence on the internuclear distance. Thus in a given electronic state rotation- vibration transitions of magnetic dipole origin seem forbidden notwithstanding the occurrence of pure rotational transitions. Nevertheless one could invoke indirect mechanisms that can impart non-zero intensities to ro-vibrational transitions. In the simplest case of a $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Delta (\Delta +0)$ state centrifugal distortion is one such mechanism. In states of higher multiplicity other mechanisms are conceivable. Possible application of these results will be indicated.