<P> 3 N − 6 (\ displaystyle 3N - 6) </P> <P> The frequencies of molecular vibrations range from less than 10 to approximately 10 Hz . These frequencies correspond to radiation in the infrared (IR) region of the electromagnetic spectrum . At any given instant, each molecule in a sample has a certain amount of vibrational energy . However, the amount of vibrational energy that a molecule has continually changes due to collisions and other interactions with other molecules in the sample . </P> <P> At room temperature, most of the molecules will be in the lowest energy state, which is known as the ground state . A few molecules will be in higher energy states, which are known as excited states . The fraction of molecules occupying a given vibrational mode at a given temperature can be calculated using the Boltzmann distribution . Performing such a calculation shows that, for relatively low temperatures (such as those used for most routine spectroscopy), most of the molecules occupy the ground vibrational state . Such a molecule can be excited to a higher vibrational mode through the direct absorption of a photon of the appropriate energy . This is the mechanism by which IR spectroscopy operates: infrared radiation is passed through the sample, and the intensity of the transmitted light is compared with that of the incident light . A reduction in intensity at a given wavelength of light indicates the absorption of energy by a vibrational transition . The energy, E (\ displaystyle E), of a photon is </P> <P> E = h ν (\ displaystyle E = h \ nu), </P>

Difference between classical and quantum theory of raman effect