Rotation-Vibration Energy Level Transitions of a Diatomic Rotor

This Demonstration shows the energy level transitions associated with each line observed in a rotationally resolved infrared band spectrum, in which the vibrational transition is coupled with rotational transitions. For a diatomic molecule the vibrational and rotational energy levels are quantized and the selection rules are (vibration) and (rotation). Only transitions that meet the selection rule requirements are allowed, and as a result discrete spectral lines are observed, as shown in the bottom graphic. The position of a spectral line corresponds to the energy difference between the initial and final states of the transition. These energy level transitions from the ground to excited rotation-vibration states are shown in the top graphic. The spectrum consists of a branch (, , smaller wavenumbers, lower energy transitions) and an branch (, , larger wavenumbers, higher energy transitions). The central gap between the and branches represents the forbidden branch since the selection rule makes the transitions forbidden. A branch can appear, however, if the molecule has electronic angular momentum, a well-known case being the NO molecule.