Introduction

In this chapter we are going to discuss some of the more elementary ideas in connection with the absorption spectra of gases. The energy E of a molecule may be expressed as the sum of the rotational energy Erot, vibrational energy Evib and electronic energy Eel. Of these three types of energy, Erot is generally the smallest, typically a few hundredths of an electron Volt. Vibrational energies are of the order of a few tenths of one electron Volt, while the largest energies are of the electronic type which generally amount to a few electron Volts.

The absorption (emission) spectrum arising from the rotational and vibrational motion of a molecule which is not electronically excited will be located in the infrared region. In infrared absorption experiments light from a suitable source penetrates an absorption chamber containing the gas to be studied and then enters a spectrograph. If the instrument is of low resolving power, a series of wide bands is observed which correspond to the vibrational transitions. If an instrument of high resolving power is used, these bands are seen to consist of numerous spectral lines resulting from the energy levels of rotation.

In the next section we are going to discuss the vibrational motion of two relatively simple molecules (CO2 and H2O) which are particularly important in the study of radiative transfer in the atmosphere.