Pinto-Robledo, V., V. Aboites, ., and J. Soto B. “He-CuII Hollow Cathode Laser in Buffer Gas Pressures in the Range of 6 to 100 MBar”. Revista Mexicana De Física, vol. 45, no. 6, Jan. 1999, pp. 604-7, https://rmf.smf.mx/ojs/index.php/rmf/article/view/2915.