Inversion of the contribution of an isotope of small relative concentration to total absorption coefficient of a mixture of neon isotopes associated with the $3s_{2}-2p_{4}$ transition

Absorption of probe laser radiation by a mixture of even isotopes of neon in a gas discharge plasma is investigated by the method of magnetic scanning of $3s_{2}-2p_{4}$ transition. The contours of absorption lines of isotopes are resolved by means of numerical modeling. It is discovered that, upon decrease in relative concentration of one of the isotopes, its contribution to absorption is replaced by gain. The effect is found to be caused by radiative transfer of excitation energy between atoms of different kinds in the absence of a difference in level energies (the process known as optical pumping). The effect of this mechanism turned out to be substantial for the upper level transitions from which to the ground state are allowed while being absent for the lower level of the transition from which such transitions are forbidden although other decay channels are available.