A complete and general study of the dependence of life and concentration of metastable mercury atoms, optically excited, on experimental conditions, on foreign gas-pressures, and on intensity of illumination is given. This study explains why the life can not be larger than ${10}^{\ensuremath{-}2}$ second under laboratory conditions and why foreign gases act so differently in regard to the accumulation of excited atoms. The theoretical results are in good agreement with all experimental data available. An improved and more general formula for the quenching of mercury resonance radiation is obtained. The efficiencies of collisions of the second kind of metastable mercury atoms with
N2, A, and He molecules are calculated numerically. It is shown that the concentration of metastable atoms increases only with the square root of the intensity of the exciting light and not with the exciting light itself as generally assumed.
The scenario of the experiment on the amplification of gamma rays upon stimulated transitions from metastable states of isomeric nuclei in a Bose—Einstein condensate (BEC) is presented. The experiment involves the successive transport of atoms along the so-called quantum conveyer. It is shown that if the excess broadening of the gamma-ray emission line is eliminated, the total gain G > 1 can be achieved despite the presence of negative factors such as the 'laser lethargy', spontaneous decay of metastable states, etc.