Radiation width and resonance broadening in helium

Abstract

By means of a direct recording Fabry-Perot interferometer, the contours of several singlet lines of helium were studied over a wide range of pressure and temperature. The source was a microwave discharge cooled with liquid nitrogen or liquid helium. Analysis of the tracings allowed the Doppler and Lorentz components to be separated quantitatively. A detailed study was carried out for the line λ 7281 Å (2 1 P - 3 1 S ). By extrapolation of the Lorentz width to zero pressure the radiation width of the level 2 was determined and led to an ƒ-value 0·377 + 0·035 of the resonance line λ 584 Å (1 1 S — 2 1 P ) The resonance broadening is found to be independent of the speed of the atoms, over a range of a factor five in the speed, in agreement with theories of optical collision by Weisskopf and others. The absolute value of the resonance broadening constant is compared with various theories. The strong line λ 6678 Å (2 1 P -3 1 D ) showed self-absorption due to imprisonment of resonance radiation.