Self-broadening in singlet spectral lines of helium

Abstract

The emission profiles of five lines in the singlet spectrum of helium, λλ492.2, 501.5, 504.7, 667.8 and 728.1 nm, have been ’studied under high resolution to test some aspects of line-broadening theory, particularly as it is applied to the resonance interaction. The source was a weak d.c. discharge at 77 or 273 K; the pressure range was 0-10 Torr. Self-absorption was shown to be negligible in separate experiments. Profiles were recorded digitally and analysed by computer into Lorentzian and Gaussian components. The results show that the observed profiles of all the lines are influenced to varying degrees by excitation mechanisms; a discussion of these effects is given. They prevent a quantitative interpretation of λλ492.2 and 667.8 nm, and limit the accuracy of the results for the other lines. Of these, λ728.1nm shows the closest approach to pure resonance broadening; the measured broadening constants are 7.7 ± 0.2 (77K) and 9.3 ± 0.9 (273 K), both in units of 10 -20 cm -1 cm 3 per atom. This line also shows at 77 K a shift to the red of 0.11 ± 0.03 in the same units; this and the temperature dependence are attributed to a non-resonant contribution to the interatomic potential. Preliminary results of recent calculations based on a model potential are in fair agreement with our data. The Lorentzian width of λ728.1nm extrapolated to zero density is found to exceed the radiation width by (1.2 ± 0.5) 10~ 3 cm_ 1 , in agreement with the results of earlier workers, but the possibilities of systematic error (apparatus function, excitation mechanisms) reduce the significance of this finding. Possible experiments to circumvent the difficulties encountered in the present work are discussed.