Imaging electronic motions in atoms and molecules by ultrafast electrons

Abstract

Due to the important roles played by the electrons in various kinds of reactions, direct imaging electronic motions provides insight into electron dynamics. Therefore, we investigate the possibility of using ultrafast electrons to directly image the electronic motions in atoms and molecules under diffraction and ionization processes. Ultrafast electron diffraction from time-varying coherent electronic states of the H atom is analyzed theoretically. This theoretical analysis identifies the conditions necessary to obtain time-resolved measurements. Electron diffraction from coherent electronic states exhibiting breathing and wiggling modes of electronic motion are simulated numerically in order to demonstrate the capability of attosecond electron pulses to image electron dynamics. Energy-resolved ultrafast electron diffraction is introduced to retrieve the spatial, temporal, and spectral information of target electronic motions. The impact ionization process is analyzed for the case of an ultrafast electron pulse incident upon a target prepared in a time-varying, coherent superposition of states. Results for coherent electronic motions in both the H atom and the hydrogen molecular ion are used to illustrate the capability of an ultrafast electron pulse to image time-dependent momentum densities of target electrons.