Knowledge Bank

The expression for the integrated intensity of an infrared absorption band is normally obtained by neglecting the probability of spontaneous emission. At high temperatures, this factor cannot be neglected, and in fact gives rise to a decrease in intensity with increasing temperature. We have studied this effect experimentally with the molecular crystal, copper phthalocyanine at temperatures up to $600\circ C$. Expressions based on the assumption of a harmonic potential function have been derived for the temperature dependence of the fundamentals and certain overtones and combinations. These expressions compare favorably with the experimental data.