The Cooling of a Gas by Radiation

Abstract

We consider a gas composed of atoms having two electronic states. Transitions between these two states can take place by collisions, and by emission or absorption of radiation. When the radiative transition probabilities are small compared with those due to collisions, a `temperature' may be defined for the translational motion. We formulate the general problem for the case where the total concentration of the gas depends on the space coordinates in a given way, and the possibility of the `imprisonment' of the radiation emitted is taken into account. In this case the problem leads to a pair of coupled integro-differential equations, from which the `temperature' and the concentration of atoms in the excited state are obtained as functions of space and time. Neglecting the imprisonment of the resonance radiation and the spatial variation of the temperature, we have made calculations for the case when the energy difference between the two states is of the order kT, for a number of values of the ratio between radiative and collisional transition probabilities. The results are applied to the problem of the cooling of the atmospheric gas at high altitudes (100 km) at night, as a result of the magnetic dipole transitions between the components of the 3P state of the oxygen atom.