Vibrational wave-packet dynamics of the silver pentamer probed by femtosecond NeNePo spectroscopy.

Vibrational wave-packet dynamics on the ground electronic state of the neutral silver pentamer (Ag) are studied by femtosecond (fs) pump-probe spectroscopy using the 'negative ion - to neutral - to positive ion' (NeNePo) excitation scheme. A vibrational wave packet is prepared on the A state of Ag photodetachment of mass-selected, cryogenically cooled Ag anions using a fs pump pulse. The temporal evolution of the vibrational wave packet is then probed by an ultrafast probe pulse resonant multiphoton ionization to Ag. Frequency analysis of the fs NeNePo transients for pump-probe delay times from 0.2 to 8 ps reveals three primary beating frequencies at 157 cm, 101 cm and 56 cm as well as four weaker features. A comparison of these experimentally obtained beating frequencies to harmonic normal mode frequencies calculated from electronic structure calculations confirms that Ag in the gas phase adopts a planar trapezoidal geometry, similar to that previously observed in solid argon. The dependence of the ionization yield on the laser polarization indicates a s-d wave electron photodetachment from a 'p-type' occupied molecular orbital of Ag. Franck-Condon analysis shows that both processes, photodetachment and subsequent photoionization determine the beating frequencies probed in the time-dependent cation yield. The present study extends the applicability of fs NeNePo spectroscopy to characterize the vibrational spectra in the far-IR frequency range in the absence of perturbations from a medium or a messenger atom to mass-selected neutral metal clusters with more than three atoms in the ground electronic states.