Self broadening and radiation width in the singlet spectrum of helium

Abstract

By means of a direct recording Fabry–Perot interferometer, the emission profiles of seventeen lines in helium were studied over a wide range of density and temperature. A number of different analysis techniques were used in the accurate separation of Doppler and Lorentz contributions. Linear extrapolation of the Lorentz width to zero density establishes the existence of an additional 'coupling width' which persists at very low density. In two of the lines studied, λ 7281 Ǻ (21P–31S) and λ 6678 Ǻ (21P–31D), the slope of the curve of the Lorentz width against gas density could be attributed entirely to resonance interaction in the lower 21Plevel. The measurement of this resonance broadening constant is in excellent agreement with the results of a number of recent calculations. In the lines 21P–n1S,n1D(n= 4, 5, 6, 7, 8), considerable pressure broadening was accompanied by large shifts of the line centre towards higher frequencies. In contrast, lines of the type 21S–n1P(n= 3, 4, 5, 6, 7, 8) were strongly broadened but showed small shifts to lower frequencies. These effects are attributed to the upper levels involved. Evidently the potential curves of1Sand1Dlevels show very marked differences from those of1Plevels. This result is discussed in terms of different types of broadening interaction. It is concluded that in these highly excited states the interaction is dominated by forces of electrostatic repulsion.