Enhancing Elastic Scattering via Transient Resonances

Abstract

In recent years, X-ray Free Electron Lasers (XFELs) sources have seen a tremendous increase in their performance. X-ray pulses with duration breaching the femtosecond frontier – entering the attosecond domain – and intensities reaching the 10^20 W cm−2 regime have become available and, most importantly, a reliable tool for scientific exploration. Growing on the backbone of these capabilities, the field of nonlinear and ultrafast X-ray science has expanded, with a plethora of novel phenomena being observed and investigated. Upon irradiation with ultrashort and intense pulses, a defining feature of non-linear X-ray science emerges: the ability to transiently excite atoms into non-equilibrium core-excited configurations opens otherwise forbidden transitions. Under particular circumstances, these transient resonances can provide additional scattering channels and substantially increase the atomic scattering factor.

In this thesis, we demonstrate that the Lα (2p3/2 − 3d) transition at 929.7 eV transiently enhances the elastic scattering factor of copper atoms. The process is initiated by 2p photo-ionization, followed by Auger decay that triggers a cascade of inter-atomic electron-impact ionizations, ultimately populating metastable states with multiple 3d holes. Using 15 fs pulses reflected from a Cu-containing [SiC/Cu/B4C] multilayer, we observe an intensity-dependent peak around the Lα energy whose amplitude and width evolve non-linearly with excitation intensity. Theoretically, a self-consistent model linking the transient electronic population to the scattering response reproduces the observed behavior. A minimal two-level description quantifies a 12-fold enhancement of the imaginary scattering factor f2.

Our findings confirm the role of transient resonances in elastic scattering and establish a route towards innovative methods for improved contrast and resolution in ultrafast X-ray imaging.