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# Absolute timing of the photoelectric effect

## Abstract

Photoemission spectroscopy is central to understanding the inner workings of condensed matter, from simple metals and semiconductors to complex materials such as Mott insulators and superconductors1. Most state-of-the-art knowledge about such solids stems from spectroscopic investigations, and use of subfemtosecond light pulses can provide a time-domain perspective. For example, attosecond (10−18 seconds) metrology allows electron wave packet creation, transport and scattering to be followed on atomic length scales and on attosecond timescales2,3,4,5,6,7. However, previous studies could not disclose the duration of these processes, because the arrival time of the photons was not known with attosecond precision. Here we show that this main source of ambiguity can be overcome by introducing the atomic chronoscope method, which references all measured timings to the moment of light-pulse arrival and therefore provides absolute timing of the processes under scrutiny. Our proof-of-principle experiment reveals that photoemission from the tungsten conduction band can proceed faster than previously anticipated. By contrast, the duration of electron emanation from core states is correctly described by semiclassical modelling. These findings highlight the necessity of treating the origin, initial excitation and transport of electrons in advanced modelling of the attosecond response of solids, and our absolute data provide a benchmark. Starting from a robustly characterized surface, we then extend attosecond spectroscopy towards isolating the emission properties of atomic adsorbates on surfaces and demonstrate that these act as photoemitters with instantaneous response. We also find that the tungsten core-electron timing remains unchanged by the adsorption of less than one monolayer of dielectric atoms, providing a starting point for the exploration of excitation and charge migration in technologically and biologically relevant adsorbate systems.

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## Data availability

The data that support the findings of this study are available from the corresponding authors upon request.

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## Acknowledgements

We acknowledge discussions with M. Schultze, experimental support by A. Kim and A. Schiffrin and infrastructural support by F. Krausz. This work was supported by the Max Planck Society, the Deutsche Forschungsgemeinschaft Cluster of Excellence, Munich Centre for Advanced Photonics, a Consolidator Grant from the European Research Council (ERC-2014-CoG AEDMOS), LASERLAB-EUROPE (grant agreement number 654148, European Union’s Horizon 2020 research and innovation programme), FWF Austria (SFB-041 ViCoM, SFB-049 NextLite) and COST Action CM1204 (XLIC). Calculations were performed using the Vienna Scientific Cluster (VSC).

### Reviewer information

Nature thanks M. Chini and T. Fennel for their contribution to the peer review of this work.

## Author information

### Author notes

1. These authors contributed equally: M. Ossiander, J. Riemensberger

### Affiliations

1. #### Physik-Department, Technische Universität München, Garching, Germany

• M. Ossiander
• , J. Riemensberger
• , M. Mittermair
• , M. Schäffer
• , A. Duensing
• , M. S. Wagner
• , R. Heider
• , M. Wurzer
• , M. Gerl
• , M. Schnitzenbaumer
• , J. V. Barth
• , P. Feulner
•  & R. Kienberger
2. #### Max-Planck-Institut für Quantenoptik, Garching, Germany

• M. Ossiander
• , J. Riemensberger
• , M. Schäffer
• , M. Gerl
•  & R. Kienberger

• S. Neppl
4. #### Institute for Theoretical Physics, Vienna University of Technology, Vienna, Austria

• F. Libisch
• , C. Lemell
•  & J. Burgdörfer

### Contributions

M.O., J.R., S.N., M.M., M. Schäffer, A.D., M.S.W., R.H., M.W., M.G. and M. Schnitzenbaumer carried out the experiments. M.O. and J.R. analysed the experimental data. F.L., C.L. and J.B. performed the electron transport and DFT calculations. M.O. wrote the initial manuscript. J.V.B., P.F. and R.K. supervised the study. All authors discussed and reviewed the manuscript.

### Competing interests

The authors declare no competing interests.

### Corresponding authors

Correspondence to M. Ossiander or R. Kienberger.

## Extended data figures and tables

1. ### Extended Data Fig. 1 I/W(110) and W(110) measurements.

a, Main panel, XUV I/W(110) photoelectron spectra taken at 105 eV central photon energy for different adsorbate surface coverages (key in top left inset). Three effects are observable with decreasing iodine surface coverage: a decrease of the I4d peak intensity, an increase of the W4f peak intensity due to reduced inelastic scattering, and a shape change of the valence-electron peak towards a clean tungsten spectrum. The iodine surface density was calibrated by taking a full monolayer I/W(110) photoelectron spectrum as a reference before thermal desorption and comparing the I4d photoelectron flux before and after the thermal desorption of iodine. Because the iodine surface coverage saturates, this allows for a reliable coverage calibration. Top centre inset, illustration of the employed Shirley background (BG) subtraction scheme; top right inset, the magnified valence photoelectron spectrum. b, Relative photoemission delays for I/W(110). Both W4f to I4d (blue) and valence to I4d (red) delays are shown. All individual measurements are depicted by crosses, averages for individual coverages are depicted by circles. Vertical error bars mark 95% confidence assuming a Student’s t-distribution and horizontal error bars mark maximum errors. The blue line represents a linear regression to the W4f to I4d delay (blue line), the shaded area represents the 95% confidence interval of this model. c, Attosecond streaking delay measurements on a pristine W(110) surface. The W4f−CB emission delay (blue circles and histogram) as a function of the time of measurement after the preparation of a pristine surface (yellow) reveals a small deviation of the centre (red, dashed line) of the normal distribution (red, solid line) fit to all measurements from the extrapolation to an instantaneous measurement due to surface contamination. The large number of measurements allows the extraction of the photoemission timing of the clean surface by extrapolation.

2. ### Extended Data Fig. 2 Typical streaking spectrogram measured for I/W(110) at 32% of the saturated iodine surface coverage and its reconstruction.

The first column shows the experimental data, the second column the results of the reconstruction algorithm and the third column their difference. The residual mainly consists of background independent of the XUV−NIR-delay, which is cancelled during the retrieval by differentiating along the delay axis. The three rows focus, from top to bottom, on the streaking of the valence, W4f and I4d photoemission peaks. The XUV−NIR-delay difference between consecutive spectra in the spectrogram is 200 as. The photoemission delay results extracted from this sample spectrogram are $${\rm{\Delta }}{\tau }_{{\rm{W}}4f-{\rm{I}}4d}=81\hspace{1pt}{\rm{as}}$$ and $${\rm{\Delta }}{\tau }_{{\rm{Valence}}-{\rm{I}}4d}=14\hspace{1.5pt}{\rm{as}}$$.

3. ### Extended Data Fig. 3 Gas-phase iodine/helium measurements.

a, Unstreaked XUV photoelectron spectra of iodoethane (red) and an iodoethane/helium mixture (blue) recorded at 105 eV central photon energy. Electrons emitted through the N4,5VV Auger process are spectrally separated from all timed photoelectron peaks. b, Relative I4d−He1s photoemission delay for different mixture compositions of iodoethane and helium. Shown are individual measurements (blue circles), linear regression (red line) and the 95% confidence interval of the regression (shaded area). c, Histograms (blue) of the individual relative photoemission delay measurements (yellow) between He1s and I4d electrons in iodomethane and iodoethane at 105 eV photon energy and normal-distribution fits to the data (red).

4. ### Extended Data Fig. 4 Results of DFT calculations.

a, DFT-derived electron density in the surface bandgap of a clean W(110) surface. A 2D cut through the $$(1\bar{1}0)$$ plane, perpendicular to the (110) surface, is shown. Tungsten atoms are indicated as black circles. Second-layer atoms are projected onto the plane of the cut. The inelastic mean free path λIMFP for conduction-band photoelectrons is marked as a guide to the eye. b, Energy-resolved density of states for an iodine-covered W(110) surface. The red dashed line represents the density of states in the proximity of the top-layer tungsten atoms and the blue dashed line represents the density of states near the iodine adsorbates. The full lines are folded with the spectrum of the experimental XUV pulse. The yellow line represents the full density of states of the iodine covered tungsten surface folded with the experimental XUV spectrum. See Methods for details of ‘jellium edge’ and EF.

### DOI

https://doi.org/10.1038/s41586-018-0503-6