The exciton transition 4T1→6A1 of Mn2+ ions in quasi one-dimensional antiferro-magnet CsMnCl3·2H2O is very weak in intensity with a shorter lifetime due to the vibration-queching. Replacing H2O by D2O), three important effects have been observed: the exciton line is shifted to the red by 18cm-1, the intensity of the exciton is increased by almost one order, the lifetime is lengthened from 0.58 ms to 9.2 ms. These facts show that the vibration-quenching has been weakened. The experimental results are analysed by means of the theory of multi-phonon radiative and non-radiative transitions. It has been shown that the isotope substitution changes the spectrum of lattice vibration, and brings forth a great influence on the non-radiative transition. The electronphonon coupling intensity does not show any obvious change, and the Huang-Rhys factor remains constant. The phonon modes involved in the multi-phonon radiative transition are different from those involved in the multi-phonon nonradiative process, their average energies are quite different from each other.
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