This experimental research program is aimed at pursuing fundamental investigations of physical phenomena important to understanding x-ray interactions with atoms and molecules. The primary scientific focus is probing the limits of the dipole approximation, the simplest reasonable treatment of photon interactions, for soft-x-ray photoionization and inelastic x-ray scattering (x-ray emission) from mostly molecular targets. Photoemission studies done as part of this program have demonstrated, in some cases rather unexpectedly, quantitatively large non-dipole effects at surprisingly low photon energies, leading to a minor paradigm shift within the photoemission community regarding the general validity of the dipole approximation. The proposed research will be carried out using three unique home-built instruments, one for gas-phase photoemission using time-of-flight electron-energy analysis, one for high-resolution polarized x-ray emission of gases, and a third for measuring macroscopic currents in gas-phase samples irradiated with beams of x-rays. The broader impact of the program involves student training as well as applications to material science.
