Experimental study of forbidden optical transitions in a dense, laser-produced plasma

The intensities and line profiles of several forbidden optical transitions, together with their companion allowed lines were measured in a helium plasma having densities N ranging between {~} 1 ×{} 10¹⁶ and {~} 5 ×{} 10¹⁶ cm^-3. The plasma was produced by irradiating helium gas with light from a repetitively pulsed, high-power CO₂ laser. The time and space-resolved spectra of three forbidden lines of He 1 were examined: the 2¹P-3¹P transition at 6632 {}, the 2³P-4³F transition at 4470 {}, and the 2³P-4³P transition at 4517 {}. The Abel-transformed line shapes were compared with recent theories and, while some discrepancies exist, their magnitude is considerably less than has been reported hitherto. A series of tests was made from which it is concluded that the existing disagreements are not the result of measuring deficiencies or errors in data processing, and are probably due to some remaining inadequacies of the theoretical models.