Patterns in two-dimensional spectra can identify broken rovibrational selection rules

Abstract

The spectroscopically allowed rovibrational transitions for a symmetric top molecule are ΔK = 0 for parallel bands and ΔK = ±1 for perpendicular bands. However, perturbations and mixings can cause quantum numbers to be compromised and the observation of forbidden transitions. Such observations are especially common when the density of states is high. This paper presents a two-dimensional spectrum of CH3D where the patterns due to changes in the rotational quantum numbers J and K are distinguishable. This 2D rovibrational spectrum contains an unusual pattern that appears to be produced by ΔK = ±1 transitions in the mid-infrared and ΔK = ±2 transitions in the near infrared. The ability to experimentally observe such differences in multidimensional patterns could be useful in complex and congested regions of the spectrum that have previously been difficult to assign.