Enrique López-Moreno., and V. Beltrán-López. “EPR and Resonant Cavity Measurements of Free Electrons and Metastable $N$ Atoms Produced in a He-N$_2$ Microwave Discharge”. Revista Mexicana De Física, vol. 36, no. 2, Jan. 1989, pp. 236-4, https://rmf.smf.mx/ojs/index.php/rmf/article/view/2111.