Knowledge Bank

Pure rotational transitions in excited vibrational states of the ground electronic states were predicted for molecules of $Td$ symmetry. Stark spectroscopy of methane demonstrated polarity in an excited state of P(7) in the triply-degenerate fundamental $\nu 3$. Subsequently, two $\Delta J=0$ transitions were observed in this multiplet. The present work is an explicit generalization to excited states of all infrared-active transitions in $Td$ molecules. Electric dipole moment expressions for excited vib-rot states will be given. Stark shifts of excited-state levels will be calculated. Experiments, such as those employing the coincidence of Er:YAG laser emission with methane absorption at $6077cm-1$, will be discussed.