Vibrational wave packet dynamics in the silver tetramer probed by NeNePo femtosecond pump-probe spectroscopy

We present results on the ultrafast dynamics of mass-selected neutral Ag-4 clusters using NeNePo (negative ion - neutral - positive ion) femtosecond p ump-probe spectroscopy. One-color pump-probe spectra of the Ag-4(-)/Ag-4/Ag -4(+) system measured at 385 nm and an internal cluster temperature of 20 K display a complex beat structure over more than 60 ps. The oscillatory str ucture is attributed to vibrational wave packet dynamics in an excited "dar k" state of neutral Ag-4. A dominant 740 fs wave packet period as well as w ave packet dephasing and rephasing are observed in the spectra. Fourier ana lysis of the spectra yields a group of frequencies centered around 45 cm(-1 ) and an anharmonicity \2 nu (0)chi (c)\ of 2.65 cm(-1) for the active vibr ational mode.