Spectroscopic schemes for optical pumping of a $^{15}NH_3$ laser

Nine possible spectroscopic schemes for resonant optical pumping of molecules of the isotopically substituted ammonia $^{15}NH_3$ by $^{12}C^{16}O_2$ laser radiation were studied with the aim of achieving lasing in the middle infrared range. It was found that three of these schemes were promising for three-level lasing and one for frequency-tunable lasing. Stimulated emission from $^{15}NH_3$ was obtained as a result of three-level lasing at a frequency of 758.67 cm$^{-1}$ [transition $\alpha P$ {9.2)] when pumping was provided by the $9R$ (44) $CO_2$ laser line. A mixture of ammonia $^{15}NH_3$ with nitrogen $^{14}N_2$ did not exhibit a significant isotopic exchange of the $^{15}NH_3+{}^{14}N_2\rightleftarrows{}^{14}NH_3+{}^{14}N^{15}N$ type, which could influence the operation of a $^{15}NH_3-{}^{14}N_2$ laser.