ANOMALOUS TIME DECAY OF TRANSIENT NUTATIONS IN S=1 2 SPIN SYSTEMS/

The damping of the transient nutations has been investigated experimen tally at microwave frequency in two-level spin systems (E' centers in silica and [AlO4]degrees in quartz). Transient nutation signals decay exponentially but with a rate much faster than expected from a phenome nological Bloch model. We find a linear dependence of the damping rate on the Rabi frequency. We have not yet found a satisfactory explanati on of this anomalous behavior.