The lifetimes of rubidium atoms in a dark magneto-optical trap are measured at different populations of the`bright' and`dark' hyperéne states of captured atoms. It is found that the lifetime of atoms in the trap decreases if they spend more time in the bright state. A simple explanation of this effect is proposed which is based on the increase in the transport cross section for collisions of thermal rubidium atoms surrounding the trap with cold rubidium atoms upon their electronic excitation.

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A theoretical model of coherent control of nuclear-spin isomers by microwave radiation has been developed. The model accounts the M degeneracy of molecular states and molecular center-of-mass motion. The model has been applied to the 13 CH 3 F molecules. Microwave radiation excites the para state (Jϭ11, Kϭ1) which is mixed by the nuclear-spin-spin interaction with the ortho state ͑9,3͒. Dependences of the isomer enrichment and conversion rates on the radiation frequency have been calculated. Both spectra consist of two resonances situated at the centers of nuclear-spin-allowed and -forbidden transitions of the molecule. Larger enrichment, up to 7%, can be produced by strong radiation resonant to the forbidden transition. The spin conversion rate can be increased by two orders of magnitude at this resonance.