The pumping radiation couses the transition of atoms from the ground state to the high energy excited state. From this short-living state the atoms go by non-radiative transition to the long-living metastable state. Once in the metastable state many atoms can be accomulated. The laser beam, stimulated emission, arises when all atoms simultaneously make a transition to the ground state.
© Copyright 1997, Sergey Kiselev and Tanya Yanovsky-Kiselev