Unusual radiation effects from atoms in gases and plasmas

New interesting effects arise, when three-level atoms interact with the pla sma, laser field, or a heat bath. If the atoms inside the plasma are excite d by the polarized laser pulse or an electron beam, the line wings of the r adiation are completely polarized, while the line core is only partially po larized. The Einstein coefficients are generalized to the case, when the at omic decay channels are subject to quantum interference. Such atoms may dev elop nonzero quantum coherence between upper levels, if they are in contact with the heat bath in thermal equilibrium. This coherency changes the emis sivity of the medium and hence can be experimentally observed. In the case when three-level atoms interact with the laser field, spontaneous emission can be suppressed over the whole frequency spectrum. (C) 1999 American Inst itute of Physics. [S1070-664X(99)93105-1].