Spontaneous emission properties of a quasi-continuum

We study the spontaneous emission properties of two distinct quasi-continua sets of levels of atomic and molecular systems so close in energy that the y behave in some respect as a continuum, yet preserving some aspects of the discrete level structure, depending on the timescales of interest. The fir st quasi-continuum considered consists of N degenerate excited levels that decay to a single ground level, while the second consists of a pair of ener gy separated quasi-continua of N degenerate excited levels that decay to th e same ground level. We find that the first quasi-continuum can trap popula tion in the system and can completely inhibit spontaneous emission, dependi ng on the initial conditions. For the second quasi-continuum, no population trapping is possible but partial inhibition of spontaneous emission via 'd ark' Lines in the spectrum can occur. Our results are compared with those o btained from the V-type system. (C) 2000 Elsevier Science B.V. All rights r eserved.