Symmetric rotors, such as ammonia and benzene, are molecules containing two inertial axes with equal moments of inertia and a principal inertial axis with a unique moment of inertia. The vibrational selection rule is for an absorption spectrum. Since the change in the dipole moment is parallel to the principal axis for a parallel band spectrum of a symmetric rotor, the rotational selection rules are , if and , if , with the further restriction that . With two rotational quantum numbers, and , each value of is associated with a series of energy levels, as shown in the top graphic.

Because for a parallel band, a rotation-vibration transition can only occur between lower and excited state energy levels with the same value of . Transitions that satisfy the vibrational and rotational selection rules produce a spectrum of lines, as shown in the bottom graphic. The energy difference between the lower and excited state of a given transition determines the position of the line. The spectrum consists of a series of sub-bands, one for each value. Each sub-band is composed of a P branch (, , ), Q branch (, , , appears when ), and R branch (, , ). Since there are a decreasing number of lines at the beginning of each branch as the value of increases.

In this Demonstration, you can view the energy level transitions for the full spectrum or you can choose to focus on the individual branches. The transition label and energy can be found to the left within the controls area. The axis lower and upper limit controls will allow you to zoom in on any region of the spectrum.