How dark is a grey state?

We present experimental results showing that a dark state with finite lifet ime, a grey state, becomes darker the more the atom is exposed to on-resona nt light. We discuss this phenomenon theoretically in the context of the co mplex eigenenergies and show that it can be understood qualitatively in ter ms of the quantum Zeno effect. The predicted dependence of the lifetime on the composition of the grey state and on the light intensity is experimenta lly confirmed with caesium atoms. This has implications for velocity-select ive coherent population trapping and adiabatic transfer. (C) 1999 Elsevier Science B.V. All rights reserved.