Line Breadths and Absorption Probabilities in Sodium Vapor

Molecular broadening of the absorption lines of the principal series of sodium.—From recently published experimental data the theoretical half-breadths of the lines 2 to 8 are calculated, together with the half-breadths in frequency units ($\frac{P}{2\pi }$), which are proportional to the probabilities. The broadening of the first line is at least 100 times as great as that to be expected from Stark effect and collision broadening and the breadths of succeeding lines are found to decrease rapidly with increasing term number at the saturated vapor pressures used (450 to 600°C), a result contrary to that commonly found at low vapor pressures. It is suggested that this high pressure broadening is due to the presence of a large proportion of diatomic sodium molecules in varying states of stability which disturb the absorbing atoms far more than an inert gas would, the perturbation being greater the lower the quantum state. As evidence for this the absolute values of the probabilities $A_{\mathrm{ij}}$ and $B_{\mathrm{ij}}$ come out only about one fiftieth as large as would be expected if all the gas was in the atomic condition; moreover, the bands associated with line 3303 have about 50 times the total intensity of the line. Incidentally, the half-breadth is found to vary approximately as the square root of the number of absorbing atoms.