Tracing orbital images on ultrafast time scales

Abstract

Electron dynamics in time and space Following molecular excitation and electron transfer processes in time and space within a single experiment is a long-standing goal of spectroscopy in the field of chemistry. Wallauer et al. combined tomographic photoemission imaging with a femtosecond pump-probe scheme to trace the excited state molecular orbitals of surface-adsorbed molecules with both spatial and temporal resolution. The present demonstration opens a new window for investigating the ultrafast electron transfer dynamics in such processes as chemical reactions on surfaces and intermolecular charge transfers. Science, this issue p. 1056 Femtosecond-resolved photoemission tomography is used to trace orbital images of excited states of surface-adsorbed molecules. Frontier orbitals determine fundamental molecular properties such as chemical reactivities. Although electron distributions of occupied orbitals can be imaged in momentum space by photoemission tomography, it has so far been impossible to follow the momentum-space dynamics of a molecular orbital in time, for example, through an excitation or a chemical reaction. Here, we combined time-resolved photoemission using high laser harmonics and a momentum microscope to establish a tomographic, femtosecond pump-probe experiment of unoccupied molecular orbitals. We measured the full momentum-space distribution of transiently excited electrons, connecting their excited-state dynamics to real-space excitation pathways. Because in molecules this distribution is closely linked to orbital shapes, our experiment may, in the future, offer the possibility of observing ultrafast electron motion in time and space.