Rovibrational spectroscopy

Spectroscopy of simultaneous changes of vibrational and rotational state. The transition wavenumbers combine vibrational energy differences with rotational constant terms. The allowed rotational changes depend on the molecule and the transition type.

A symmetric-top vibrational band whose transition dipole is perpendicular to the symmetry axis, with $\Delta K=\pm 1$. Unresolved Q subbranches can create a regular sequence controlled mainly by $A-B$.

A symmetric-top vibrational band whose transition dipole is parallel to the symmetry axis, with $\Delta K=0$ in the elementary model.

The rovibrational spectroscopy branch in which absorption raises the total rotational angular momentum quantum number by one. A label $R(J)$ conventionally specifies the lower-state value $J$.

The rovibrational spectroscopy branch with unchanged rotational angular momentum quantum number. Differences between upper and lower rotational constants can resolve its transitions into closely spaced lines. Allowed lower-state values depend on the angular momenta of both states.

The rovibrational spectroscopy branch in which absorption lowers the total rotational angular momentum quantum number by one. A label $P(J)$ conventionally specifies the lower-state value $J$.

The observed collection of lines or bands from vibration-rotation transitions. In the usual parallel diatomic $\Sigma \to \Sigma$ infrared band, P and R branches have $\Delta J=-1,+1$ and the Q branch is absent.