The response of atomic and molecular species to photoionization is being studied for decades now. These atoms and molecules respond characteristically upon absorption of photons. The interesting question is: how is the energy deposited into the atoms and molecules by this purest form of energy ultimately dissipated? The advent and development of X-ray synchrotons, free-electron lasers and the strong field optical lasers keep ushering new dimensions into this question in terms of the ability to follow rapid sequence of events, and, otherwise, hidden relaxation mechanisms.
Through the means of resonant excitation of valence and inner shell electrons of simple atom such as neon and cluster-like complex molecular system such as fullerene and endohedral fullerene, we investigated their relaxation mechanisms by absorption of single and multiple photons. At excitation above their characteristic resonances, we observed hitherto unknown time delayed relaxation mechanisms in the structural change of fullerene, interatomic Coulombic decay and incomplete charge transfer processes in endohedral fullerene, and two photon absorption in neon through observation of fluorescence. Furthermore, we also investigated the fragmentation and hydrogen migration dynamics in hydrocarbons to explore the energy dissipation question for biologically relevant systems. The work is worthwhile in that it contributes to the understanding of the ultrafast internal dynamics of various processes and systems relevant to physics, chemistry, and biology
