Varshni,Y.P., Nasser,R.M.: 1986, Astrophys.Space Sci., 125, 341.
Available in Adobe Acrobat PDF format.Model calculations for laser action in He I are carried out, when helium plasma is rapidly cooled by expansion. Results are presented for four transitions, two of which, 3S to 2P singlet (7281 Å) and 3D to 2P singlet (6678 Å), show strong population inversion. Available observational evidence for possible laser action in these two lines in Wolf-Rayet and emission-line stars is summarized and discussed.'