Resonant Emission under Excitation of IsotopicLevel in NaNO₂

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Temperature dependence of spectral and temporal behaviors of resonant emission in NaNO₂ has been investigated under the excitation of the isotopic level due to ¹⁵NO₂^- at low temperatures. The emission spectrum consists of series of isotopic, exciton and Raman lines. The sum of the intensities of these lines remains constant up to about 18, K, while those of the isotopic and exciton lines change remarkably above -- 10, K. It is concluded that excitation energy is transferred from the isotopic level to the exciton state mainly by two-phonon scattering process. Time response of the isotopic and exciton lines is interpreted by taking account of the transfer. Spectral change of the exciton line is discussed qualitatively in connection with the thermalization of the excitons within the exciton band.