Theoretical investigation of the ultrafast NeNePo spectroscopy of Au-4 andAg-4 clusters

Ultrafast ground state nuclear dynamics of Au-4 and Ag-4 is theoretically e xplored in the framework of negative ion - to neutral - to positive ion (Ne NePo) pump-probe spectroscopy based on the ab initio Wigner distribution ap proach. This involves the preparation of a nonequilibrium neutral ensemble by pump induced photodetachment of a thermal anionic ground state distribut ion, gradient corrected DFT classical trajectory simulations "on the fly" o n the neutral ground state, and detection of the relaxation process of the ensemble in the cationic ground state by a time-delayed probe pulse. In Au- 4, the initially prepared linear structure is close to a local minimum of t he neutral state giving rise to characteristic vibrations in the signals fo r probe wavelength near the initial Franck-Condon transition. A timescale o f similar to1 ps for the structural relaxation towards the stable rhombic D -2h neutral isomer was determined by the increase of the signal for probe w avelength in vicinity of the vertical ionization energy of the rhombic stru cture. In contrast, the relaxation dynamics in Ag-4 is characterized by nor mal mode vibrations since both the initially prepared anionic ground state and the neutral ground state have rhombic minimum geometries. Thus, time-re solved. oscillations of pump-probe signals are fingerprints of structural b ehaviour which can be used experimentally for the identification of particu lar isomers in the framework of NeNePo spectroscopy.