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# Probing molecular environment through photoemission delays

## Abstract

Attosecond chronoscopy has revealed small but measurable delays in photoionization, characterized by the ejection of an electron on absorption of a single photon. Ionization-delay measurements in atomic targets provide a wealth of information about the timing of the photoelectric effect, resonances, electron correlations and transport. However, extending this approach to molecules presents challenges, such as identifying the correct ionization channels and the effect of the anisotropic molecular landscape on the measured delays. Here, we measure ionization delays from ethyl iodide around a giant dipole resonance. By using the theoretical value for the iodine atom as a reference, we disentangle the contribution from the functional ethyl group, which is responsible for the characteristic chemical reactivity of a molecule. We find a substantial additional delay caused by the presence of a functional group, which encodes the effect of the molecular potential on the departing electron. Such information is inaccessible to the conventional approach of measuring photoionization cross-sections. The results establish ionization-delay measurements as a valuable tool in investigating the electronic properties of molecules.

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## Data availability

The data represented in Figs. 1c, 2d,e, 3 and 4e are available as Source Data. All other data that support the findings of this study are available from the corresponding authors upon reasonable request.

## Code availability

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

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## Acknowledgements

We acknowledge fruitful discussions with and support from F. Krausz. We are grateful for support from the King Saud University in the framework of the MPQ-KSU-LMU collaboration, and from the Researchers Supporting Project number RSP-2019/152. S.B. acknowledges support from the MULTIPLY fellowship program under the Marie Skłodowska-Curie COFUND Action and the Alexander von Humboldt Foundation. B.F. and J.S. acknowledge support from the Max Planck Society via the IMPRS-APS. M.F.K. is grateful for support from the Max Planck Society and the German Research Foundation via KL-1439/11-1.

## Author information

Authors

### Contributions

S.B., B.F. and L.O. contributed equally to this work. S.B., B.F., J.S., W.S., H.A.M., I.L., A.M.K., N.G.K., A.F.A., M.A., A.M.A. and M.F.K. conducted the experiments. S.B. and B.F. did the data analysis. G.H. worked on the ML code and extracted streaking traces. L.O., T.Z. and A.S.L. did the CWP calculations. L.P. performed the TDLDA simulations. D.B. and H.J.W. did the QST calculations. S.B., B.F., L.O., T.Z., D.B., H.J.W., A.S.L. and M.F.K. wrote the manuscript, which was reviewed by all the authors.

### Corresponding authors

Correspondence to Alexandra S. Landsman or Matthias F. Kling.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Physics thanks Renate Pazourek and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data

### Extended Data Fig. 1 Examples of streaking spectrograms.

(a)–(d), and (e)–(h) are measured streaking spectrograms for 80 eV and 93 eV XUV photon energy, respectively.

### Extended Data Fig. 2 Retrieval procedure of streaking delays.

In (a) and (b) the spectral amplitude and phase of the streaking traces, retrieved from the neon 2p and iodine 4d emission, respectively, for the case of 93 eV XUV central energy, are displayed in dependence of the angular frequency ω of the NIR laser field. Spectral intensities below the threshold of 0.1Imax(ω) (that is spectral amplitude threshold of $${\mathrm{A}}(\omega ) \le \sqrt {0.1} {\mathrm{A}}_{{\mathrm{max}}}\left( \omega \right)$$) are considered as noise level, for which the spectral phase ϕ(ω) is blanked. The Fourier filter, which confines the respected angular frequency range to the NIR laser spectrum, is displayed as black dash-dotted line. In (c) the streaking phase delay Δts(ω), green solid line, calculated using Eq. 1 as described in the main text, is shown together with the filtered spectral amplitude of the neon streaking trace. The latter is used as weighting factor to calculate the weighted mean value Δts. Here, the streaking phase delay is determined for the frequency region for which the amplitude is above the threshold.

### Extended Data Fig. 3 Quality of Gaussian fitting in GF analysis.

Independent Gaussian fitting for Ne and ethyl iodide peaks, used in GF analysis at 80 eV XUV photon energy.

### Extended Data Fig. 4 Relative streaking delays derived from Gaussian fitting analysis.

(a) and (b) Relative streaking delays, retrieved from Gaussian fitting analysis for results from measurements at 80 eV and 93 eV, respectively, as a function of NIR intensity calculated from the amplitude of the corresponding streaking curves. Error bars indicate the variation in relative streaking phase delay in individual measurements. This figure is complementary to Figs. 2(d) and (e) in the main text, which represent the results from machine learning analysis.

### Extended Data Fig. 5 Photoelectron spectra for XUV and NIR irradiation, and only XUV irradiation.

The shaded data represents the spectrum for XUV and NIR irradiation together. The black dashed line represents the spectrum for XUV irradiation only. By subtracting this from the combined XUV+NIR data set, the NIR contribution (red line) can be estimated, and is found to mainly originate from ATI electrons at low energies.

## Source data

### Source Data Fig. 1

Cross-section data in Fig. 1c.

### Source Data Fig. 2

Streaking delay from individual measurements at 80 eV and 93 eV in Fig. 2d and 2e, respectively.

### Source Data Fig. 3

Energy distribution of streaking delay and EWS delay in Fig. 3a and 3b, respectively.

### Source Data Fig. 4

Angular distribution of streaking delay for different molecules in Fig. 4e.

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Biswas, S., Förg, B., Ortmann, L. et al. Probing molecular environment through photoemission delays. Nat. Phys. 16, 778–783 (2020). https://doi.org/10.1038/s41567-020-0887-8

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