Experimental evidence for anharmonic and rotational effects in the evaporation of sodium clusters

Time-of-flight mass spectrometry is used to measure the translational kinet ic energy release during the unimolecular decay of Na-n(+) (n = 5-21) clust ers. The detection of neutral fragments of evaporation allows measurement o f translational kinetic energies as small as 10 meV. The kinetic energy rel ease predicted in the frame of a harmonic model is compared with the measur ed value. The discrepancy, interpreted in the frame of a novel extension of phase-space theory, emphasizes the effect of the sub-cluster rotation in t he kinetic energy release and the role of vibrational anharmonicity. (C) 20 01 Elsevier Science B.V. All rights reserved.