The frequency of the radio signal from a transmitting station was exactly matched with the oscillating frequency of a circuit in a radio. The mechanical examples described earlier included the resonance of a variable power source with hacksaw blades that resonated at different natural frequencies. This same coupling can be achieved on the molecular scale. Now, however, we will pay much closer attention to the particular frequency at which resonance occurs. Consider the OH bond in a molecule of water using the mechanical analogy of a simple harmonic oscillator.
Molecular vibrations consist of displacements about the equilibrium positions of the atoms.
Simple harmonic model of two atoms and a bond between them. The energy of these molecular vibrations can also be thought of as having a ground, or rest, state and a series of excited states. These energy levels are illustrated in Figure 18. Potential energy as a function of displacement from equilibrium. The vibrational energy levels are illustrated.