The pressure broadening of spectral lines

Interatomic forces. It is often possible to represent the long-range interaction between the excited atom and a perturber by a potential of the form 

The quasi-static and the impact approximations. The representation of the interatomic forces by equation (8.18) refers to the simple case of a binary system of an excited atom and a single perturber having a fixed separation and orientation. In general, however, the excited atom may interact with several perturbers and it is necessary to average over the different orientations and paths of these perturbers. This averaging can only be performed satisfactorily in two limiting cases known as the quasi-static and the impact or 

From the properties of the Fourier transform, equation (8.8), we see that the shape of the line profile at a frequency separation A(o=a)Q-(o from line centre is determined by the wavetrain emitted during the time interval At, where At 1/Aco. If we imagine the atom to emit a wavetrain until perturbed by a strong collision, the time of interest. At, 

When the duration of a collision is much greater than the time between collisions, the line broadening is produced while the perturber moves only a short distance. It is therefore possible to ignore the motion of the perturbers completely and we have the quasi-static approximation  

The excited atom is assumed to emit a wave shifted in frequency by equation (8.17) and the lineshape for the sample as a whole is obtained by averaging over the probability distribution of the stationary perturbers. This approximation is used to calculate the Stark broadening produced by ions in a plasma and for neutral atom broadening at pressures s 100 Torr. From equations (8.19) and (8.20) we see that the quasi-static approximation is likely to be better at high densities, where T is short, and at low temperatures. 

---

This article reviews the basic theory of the pressure broadening of spectral lines. 

---