Atoms and molecules are the building blocks of matter and atomic and molecular research is at the root of scientific investigations in many fields. Advances in fundamental understanding of the interaction of atoms, molecules, clusters and their ions with vuv/soft x-ray photons may contribute to advances in chemical physics, plasma physics, fusion, astrophysics, and laser developments.

We have investigated, at the atomic and molecular level, inner-shell processes with unprecedented level of detail. Measurements that examine the relaxation mechanisms following photoexcitation of atoms, molecules, clusters and their negative ions will be presented.  Specifically, my research program aims to measure, quantify, characterize and identify photoelectron emission and fragmentation pathways, subsequent to Coulomb explosion, to understand the mechanisms of electronic emission and photodissociation dynamics. These arise from electronic and nuclear relaxation after site-specific absorption of single photons by molecules, clusters and their ions. In addition, we aim to study the mechanisms of cluster fragmentation and the evolution of the electronic properties of clusters as a function of cluster size. These investigations will reflects directly on the quantum mechanical wave function used in models which we seek to test or stimulate.  The understanding of how electronic and nuclear motions are coupled by electronic excitation, still present serious experimental and theoretical challenges.

We use photons from the Advanced Light Source (ALS), at Lawrence Berkeley National Laboratory as our probes, because the intense brightness, tunability and polarization of the photon beam enables new problems to be investigated with an impressive degree of precision, leading to new or better understanding of atomic and molecular physics.  We have also recently used the Free Electron Laser in Hamburg to study the excitation of atoms and clusters in a high intensity regime and are preparing an experimental program to use the SLAC x-ray FEL.  The experiments are carried out using different methods such as ion imaging and electron-ion coincidence techniques, spin- and angle-resolved photoelectron spectroscopy, and photoion-spectrometry.

Highlight investigations of inner-valence/inner-shell excitation in selected atoms, molecules, clusters and their ions with photons at different wavelengths, intensity regimes and photon pulse duration will be presented [1].