Imaging electron dynamics with time- and angle-resolved photoelectron spectroscopy

Abstract

We theoretically study how time-and angle-resolved photoemission spectroscopy can be applied for imaging coherent electron dynamics in molecules. We consider a process in which a pump pulse triggers coherent electronic dynamics in a molecule by creating a valence electron hole. An ultrashort extreme ultraviolet probe pulse creates a second electron hole in the molecule. Information about the electron dynamics is accessed by analyzing angular distributions of photoemission probabilities at a fixed photoelectron energy. We demonstrate that a rigorous theoretical analysis, which takes into account the indistinguishability of transitions induced by the ultrashort, broadband probe pulse and electron hole correlation effects, is necessary for the interpretation of time-and angle-resolved photoelectron spectra. We show how a Fourier analysis of time-and angle-resolved photoelectron spectra from a molecule can be applied to follow its electron dynamics by considering photoelectron distributions from an indole molecular cation with coherent electron dynamics.

Imaging Electron Dynamics with Ultrashort Light Pulses: A Theory Perspective

“…describes the observation of a photoelectron with momentum Q el .ĉ † Q el ,σ andĉ Q el ,σ are creation and annihilation operators of an electron with momentum Q el and spin σ [32,44,45]. Since we consider spin-unresolved photoelectron spectra, the sum is over spin σ.…”

“…The difference between the semiclassical approach to obtain the time-resolved DSP according to Equation (32) and the correct expression in Equation (30) has been illustrated in Reference [27] by the calculation of the DSP from an ionized Br 2 molecule with coherent electron dynamics in the valence shell. The two highest occupied molecular orbitals of Br 2 are the πg and πu orbitals of Br 4p character.…”

“…Although generation of such pulses has been recently achieved [23], these constraints on the probe-pulse parameters are still quite demanding. These restrictions can be partially overcome with TRARPES technique [32], which we review in the next Section.…”

“…An advantage of this technique over X-ray scattering is that it allows achieving Ångstrom spatial resolution with light pulses of a much lower photon energy. An analysis of time-and angle-resolved photoelectron distributions obtained by ultrashort XUV probe pulses inducing single-photon ionization of a coherently evolving electron system within the QED framework has been performed in Reference [32]. This study provided a correct way to take into account the consequence of the broad probe-pulse bandwidth on a signal, which has not been taken into account in earlier studies of time-resolved photoelectron probability [29], but is critical for a correct interpretation of time-resolved photoelectron spectra.…”

“…Since electron binding energies, which determine photoelectron spectra, depend on the local chemical environment, one would think that photoelectron peaks would shift following a time-dependent electron density. However, an analysis of a time-resolved photoelectron probability within the QED framework has shown that this expectation is also incorrect [32]. Thus, an intuitive approach to interpret an outcome of a time-resolved measurement can lead to erroneous results.…”