Delayed quantum beats as a method of subnatural-linewidth spectroscopy

The resolution of a quantum-beat experiment, which is basically limited by the atomic natural linewidth, has been improved by observing the fluorescence only from those atoms which have survived in excited states after some time delay. The resulting narrowing enables one to resolve previously overlapping lines in beam-foil quantum-beats experiments using H..cap alpha.. and /sup 4/He II (4 ..-->..3) deexcitation.