Classification of Rotational Energy Levels for Symmetric-Top Molecules

Abstract

It is shown that the rotational energy levels of a symmetric‐top molecule can be classified according to symmetry species of the full symmetry group of the molecule. Useful selection rules for electric‐dipole transitions between rotational energy levels thus classified are presented. It is shown that the rotational energy levels can be labeled in addition by a convenient quantum number G, which is essentially a generalization of the +l, —l labels. Selection rules for this quantum number are presented, which, together with those for the symmetry species, allow one to determine easily the allowed branches for a transition between any two vibronic states in a symmetric‐top molecule.