Time‐Resolved Photoelectron Spectroscopy at Surfaces Using Femtosecond XUV Pulses

Abstract

Laser-based femtosecond light sources in the infrared, visible, and the nearultraviolet spectral regime have been successfully employed for a large number of experimental studies of real-time dynamics of ultrafast surface processes, as is discussed at length in the preceding chapters. Visible and near-ultraviolet wavelengths, however, are capable of probing only a relatively restricted range of energies in the surface electronic structure -specifically electronic states within a few electron volts around the Fermi energy. Most of the surface-related time-resolved photoemission studies to date are, for instance, based on a two-photon photoemission process (the first (static) two-photon photoemission work was performed by Teich et al. [1]) and focus on the ultrafast dephasing and decay of electronic excitations in this optical energy range [2]. The use of femtosecond pulsed light sources in the extreme ultraviolet (XUV) 1) spectral region, in contrast, allows direct access to much more deeply bound electronic levels. Furthermore, the range of electron momentum that can be studied can be substantially expanded by the use of these light sources. Both aspects are highly attractive in the study of ultrafast processes since they provide new and relevant system information, for instance, on the chemical or magnetic state, or the structure, of a surface -information that is often not accessible in time-resolved experiments employing sources at lower photon energy.The first experimental work showing that ultrafast light sources in the XUV and soft X-ray region of the spectrum were possible were conducted in the late 1980s. Studies by McPherson et al. [3] and Ferray et al. [4] were the first to clearly show the 1) The extreme ultraviolet regime covers the wavelength between 50 and 1 nm (20-1200 eV).

Exploring new avenues in high repetition rate table-top coherent extreme ultraviolet sources

“…This application-oriented approach has defined two major challenges in the field of HHG, which involved either increasing the energy per XUV pulse or the repetition rate 2,3 . The latter challenge is particularly important for photoelectron spectroscopy and microscopy in which space-charge effects need to be avoided 3,4 . Other possible applications are coherent diffractive imaging 5,6 , coincidence experiments 7 , or frequency metrology, in which multi-10 MHz lasers with a stabilized frequency comb are used 8 .…”

Coherent modulation of the electron temperature and electron–phonon couplings in a 2D material

“…Femtosecond laser excitation will very rapidly smear the spatial charge modulation which launches a coherent breathing mode within ~100fs. We then use time-and angle-resolved photoemission spectroscopy (trARPES) [26,27] and ultrafast electron calorimetry [19] to measure the dynamic electron temperature, which allows us to make two new observations. First, we observe a large coherent modulation of the electron temperature (between ~200 and ~1000 K) that is superimposed on the monotonic relaxation of the hot electrons -indicating a bi-directional exchange of energy between the electron and stronglycoupled phonon bath.…”

Ultrafast extreme ultraviolet photoemission without space charge

“…With the constraint of space charge setting the fundamental limits on the performance of photoemission experiments, this phenomenon has been extensively studied over a wide range of electron kinetic energies and pulse durations, both experimentally and theoretically 9,10,12,14–18 . For sub-ps pulses and electron kinetic energies in the ∼5–100 eV range produced from conductive samples, both shifts and broadening of the photoelectron spectra features are observed to scale with linear electron density $\rho \equiv N/D$, where N is the number of electrons emitted from the sample per pulse and D is the spot size of the light on the sample.…”

“…Working in the extreme-ultraviolet (XUV), space charge effects do not depend strongly on the sample studied or the photon energy since the photoelectric yield is dominated by secondary electrons and the full Brillouin zone of inner valence bands. It is common practice to use the results obtained for simple metals in general considerations of the problem, and reports of the slope parameters m s,b in the literature have varied by a factor of 2 9,10,12,14,18 . Recently, using Cu (001), Plötzing et al 12 have studied these effects for multiple spot sizes and determined m b  = 2.1 × 10 −6  eV mm and m s  = 3.2 × 10 −6  eV mm with an estimated systematic uncertainty of less than 20%.…”