Spectral Lines of Atmospheric Gases

Abstract

The density of the molecules in a gas at atmospheric pressure is at most one thousandth of the density in a liquid or solid material. The distances between the molecules are typically ten times the size of the molecules or more, therefore the mutual perturbation is weak. Any oscillation of a molecule, e.g. a vibration, can go on undisturbed on average during many periods before it becomes interrupted, e.g. due to the collision with a neighbouring molecule. The dynamic behaviour of the ensemble of molecules is usually influenced by incident electromagnetic waves. However, due to the weak mutual disturbance between molecules, the interaction of an individual molecule and a photon may essentially be regarded as independent of the behaviour of other molecules. The total radiation effect of the gas volume is, therefore, simply the sum of all individual contributions.