Michael Greif

Energy-dependent photoemission delays from noble metal surfaces by attosecond interferometry

How quanta of energy and charge are transported on both atomic spatial and ultrafast timescales is at the heart of modern technology. Recent progress in ultrafast spectroscopy has allowed us to directly study the dynamical response of an electronic system to interaction with an electromagnetic field. Here, we present energy-dependent photoemission delays from the noble metal surfaces Ag(111) and Au(111). An interferometric technique based on attosecond pulse trains is applied simultaneously in a gas phase and a solid-state target to derive surface-specific photoemission delays. Experimental delays on the order of 100 as are in the same time range as those obtained from simulations. The strong variation of measured delays with excitation energy in Ag(111), which cannot be consistently explained invoking solely electron transport or initial state localization as supposed in previous work, indicates that final state effects play a key role in photoemission from solids.

Versatile attosecond beamline in a two-foci configuration for simultaneous time-resolved measurements

We present our attoline which is a versatile attosecond beamline at the Ultrafast Laser Physics Group at ETH Zurich for attosecond spectroscopy in a variety of targets. High-harmonic generation (HHG) in noble gases with an infrared (IR) driving field is employed to generate pulses in the extreme ultraviolet (XUV) spectral regime for XUV-IR cross-correlation measurements. The IR pulse driving the HHG and the pulse involved in the measurements are used in a non-collinear set-up that gives independent access to the different beams. Single attosecond pulses are generated with the polarization gating technique and temporally characterized with attosecond streaking. This attoline contains two target chambers that can be operated simultaneously. A toroidal mirror relay-images the focus from the first chamber into the second one. In the first interaction region a dedicated double-target allows for a simple change between photoelectron/photoion measurements with a time-of-flight spectrometer and transient absorption experiments. Any end station can occupy the second interaction chamber. A surface analysis chamber containing a hemispherical electron analyzer was employed to demonstrate successful operation. Simultaneous RABBITT measurements in two argon jets were recorded for this purpose.

Light-Matter Interaction at Surfaces in the Spatiotemporal Limit of Macroscopic Models.

What is the spatiotemporal limit of a macroscopic model that describes the optoelectronic interaction at the interface between different media? This fundamental question has become relevant for time-dependent photoemission from solid surfaces using probes that resolve attosecond electron dynamics on an atomic length scale. We address this fundamental question by investigating how ultrafast electron screening affects the infrared field distribution for a noble metal such as Cu(111) at the solid-vacuum interface. Attosecond photoemission delay measurements performed at different angles of incidence of the light allow us to study the detailed spatiotemporal dependence of the electromagnetic field distribution. Surprisingly, comparison with Monte Carlo semiclassical calculations reveals that the macroscopic Fresnel equations still properly describe the observed phase of the IR field on the Cu(111) surface on an atomic length and an attosecond time scale.

Following the molecular motion of near-resonant excited CO on Pt(111): A simulated x-ray photoelectron diffraction study based on molecular dynamics calculations

A THz-pump and x-ray-probe experiment is simulated where x-ray photoelectron diffraction (XPD) patterns record the coherent vibrational motion of carbon monoxide molecules adsorbed on a Pt(111) surface. Using molecular dynamics simulations, the excitation of frustrated wagging-type motion of the CO molecules by a few-cycle pulse of 2 THz radiation is calculated. From the atomic coordinates, the time-resolved XPD patterns of the C 1s core level photoelectrons are generated. Due to the direct structural information in these data provided by the forward scattering maximum along the carbon-oxygen direction, the sequence of these patterns represents the equivalent of a molecular movie.

Sensitivity of photoelectron diffraction to conformational changes of adsorbed molecules: Tetra-tert-butyl-azobenzene/Au(111)

Electron diffraction is a standard tool to investigate the atomic structure of surfaces, interfaces, and adsorbate systems. In particular, photoelectron diffraction is a promising candidate for real-time studies of structural dynamics combining the ultimate time resolution of optical pulses and the high scattering cross-sections for electrons. In view of future time-resolved experiments from molecular layers, we studied the sensitivity of photoelectron diffraction to conformational changes of only a small fraction of molecules in a monolayer adsorbed on a metallic substrate. 3,3′,5,5′-tetra-tert-butyl-azobenzene served as test case. This molecule can be switched between two isomers, trans and cis, by absorption of ultraviolet light. X-ray photoelectron diffraction patterns were recorded from tetra-tert-butyl-azobenzene/Au(111) in thermal equilibrium at room temperature and compared to patterns taken in the photostationary state obtained by exposing the surface to radiation from a high-intensity helium discharge lamp. Difference patterns were simulated by means of multiple-scattering calculations, which allowed us to determine the fraction of molecules that underwent isomerization.

Erratum: “Following the molecular motion of near-resonant excited CO on Pt(111): A simulated x-ray photoelectron diffraction study based on molecular dynamics calculations” [Struct. Dyn. 2, 035102 (2015)]

[This corrects the article DOI: 10.1063/1.4922611.].

Attosecond Transversal Streaking to Probe Electron Dynamics at Surfaces

The feasibility of attosecond transversal streaking to probe electron transfer dynamics at surfaces and interfaces has been studied. Our simulations suggest that the temporal resolution compares well to existing methods whereas the use of an s-polarized streaking field significantly reduces above-threshold photoemission (ATP) and thus also enables the detection of low-energy electrons.

Following attosecond photoemission from solids using interferometry

Implementing RABBITT on solids for the first time, we record energy-resolved absolute photoemission delays from noble metal surfaces. The structure of the observed delays in Ag(111) and Au(111) is inconsistent with previously promoted models.

Accessing Energy-Dependent Photoemission Delays in Solids

Our new detection scheme combines the RABBITT technique in solids with simultaneous measurements in a reference argon target. The experiment resolved attosecond delays in the photoemission from noble metal surfaces beyond simple ballistic transport.

Surface RABBITT for determination of absolute ionization phase: A novel route towards absolute photoemission delays

By extending the RABBITT technique to noble metal surfaces the authors observed, for the first time, sub-cycle resolved dynamics with attosecond pulse trains in condensed matter systems. In contrast to streaking, this method is applicable to low-energetic harmonics and the required IR field strength is significantly lower. A simultaneously recorded RABBITT trace in argon allowed the proponents to evaluate the absolute, surface specific contribution to the transition phase.