Relaxation dynamics of magic clusters

Using femtosecond time-resolved two-photon photoemission spectroscopy, relaxation rates of the optically excited states in Al₁₃⁻, Si₄⁻, and Au₆⁻ clusters are estimated to be 250 fs, 1.2 ps, and about 1 ns, respectively. These clusters show closed electronic shell structures and large gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital in the anionic (Al₁₃⁻) or neutral (Si₄ and Au₆) states. Thus the different behaviors of these clusters for the relaxation dynamics cannot be rationalized within a relaxation mechanism dominated by Auger-like electron-electron coupling. We suggest that for these small clusters, the major decay mechanism might be electron-phonon coupling.