Ultrafast molecular orbital tomography of a pentacene thin film using time-resolved momentum microscopy at a free-electron laser

Abstract

Understanding and control of photon-induced dynamics of molecules on solid surfaces, including atomic rearrangements as well as charge transfer and non-equilibrium electron dynamics, are of essential importance for surface chemistry but also for the development of new devices. We use time-resolved momentum microscopy at a free-electron laser (FEL) and extend orbital tomography to time-resolved imaging of electronic wave functions of excited molecular orbitals. This technique will provide unprecedented insight into the ultrafast interplay between structural and electronic dynamics. In this work we prove general applicability and establish the experimental conditions at FEL sources to minimize space charge effects and radiation damage. We investigate a bilayer pentacene film on Ag(110) by optical laser pump and FEL probe experiments. From the momentum microscopy signal, we obtain time-dependent momentum maps of the molecular valence states that can be related to the molecular initial states by simulations of the involved photoemission matrix elements. A state above the Fermi level is identified which is temporarily occupied after optical excitation.

Multispectral time-resolved energy-momentum microscopy using high-harmonic extreme ultraviolet radiation

“…The system thus specifically enables k z -selective probing of ultrafast electronic structure dynamics in 3D materials as well as ultrafast 3D orbital tomography of molecular layers. 32,63 Moreover, the setup complements time-resolved momentum microscopy in the XUV to soft x-ray regime at the FEL FLASH, for which the same ToF momentum microscope is used. 21 The two complementary probe photon sources particularly enable a unique merging of trARPES with time-resolved x-ray photoelectron spectroscopy [64][65][66] and diffraction 67 for combined investigations of ultrafast electronic, chemical, and geometric structure dynamics.…”

Hard x-ray photoelectron spectroscopy: a snapshot of the state-of-the-art in 2020

“…The ToF set-up has shown great promise in the VUV and soft X-ray regimes. [198][199][200] Medjanik et al demonstrated the first instance of applying this novel ToF kmicroscope to fully map the bulk electronic structure of tungsten in the soft X-ray regime. [141] In comparison to conventional ARPES with a dispersive analyser, this approach greatly simplified the process route to map the 4D spectral density function.…”

“…[129,136,288] Proof-of-principle experiments have been performed for a number of exciting emerging applications of time-resolved momentum microscopy, including ultrafast molecular orbital imaging and tracking transient changes of topological properties and orbital texture of out-of-equilibrium states of matter, where currently XUV excitation energies are used, but which could be translated into the hard X-ray regime in the future. [199,[390][391][392] A new powerful tool, that is already using hard X-rays, is emerging in the form of full-field photoelectron diffraction for structural analysis, which has been demonstrated in first static experiments using hard X-rays. [204,205] A serious obstacle for time resolved studies is the Coulomb interaction of electrons confined in a small spatio-temporal phase-space volume.…”