Bridgette: Laser Enabled Auger Decay in Atoms and Molecules: Probing Electron Correlation in Inner-valence ionised States

Auger type decay processes play a fundamental role in atomic/molecular spectroscopy, sur-
face analysis, radiation damage, etc. These transitions can be viewed as consisting of two steps.
Firstly, a high energy photon produces a hole in an inner electronic shell, emitting a photoelec-
tron with kinetic energy dependent on the incident photon. Then the hole is lled by a valence
electron and a second valence electron is emitted with energy dependent on the energy levels of
the singly and doubly charged ions and not the incident photon energy. Auger decay processes
are considered to be an important manifestation of electron correlation as they are only possible
because of electron-electron interactions.

Recently, Murnane and Kapteyn [1] have investigated Laser-Enabled Auger Decay (LEAD)
in the multi-photon regime for vacancies that are not energetic enough to undergo the normal
Auger decay. Here we show that if considered in the single-photon regime, the LEAD process
provides a valuable insight into electron correlation in the inner valence ionised states. Firstly,
we analyse the single photon LEAD for the 2s-ionised state in Ne . A detailed investigation of
the mechanism of this single-photon LEAD process reveals that it is only possible because the
initial 2s-1 one-hole state contains con gurations of the type two holes and an electron excited
to a high-energy orbital. The cross-section of the single-photon LEAD process becomes a direct
measure of this con guration mixing. We use the rst-principles algebraic diagrammatic con-
struction (ADC) scheme and the Stieltjes imaging technique[2] to evaluate the single-photon
LEAD cross-sections in ns-ionised states of Ne and Ar. The correlation in the inner valence
ionised states of trans-1,3 Butadiene are investigated as they are an example of a molecule with
strong con guration mixing resulting in the breakdown of the molecular orbital (MO) picture
of ionisation[3]. We show that the breakdown of the MO picture leads to a dramatic increase
of the single-photon LEAD cross-section relative to the atomic case. Finally, we propose that
single photon LEAD can be a sensitive experimental probe for the attosecond hole migration
triggered by the MO breakdown.


1 P. Ranitovic, X. M. Tong, C. W. Hogle, X. Zhou, Y. Liu, N. Toshima, M. M. Murnane,
H. C. Kapteyn, Phys. Rev. Lett. 106, 053002 (2011).
2 See e.g. K. Gokhberg, V. Vysotskiy, L. S. Cederbaum, L. Storchi, F. Tarantelli, V. Averbukh,
J. Chem. Phys. 130, 064104 (2009).
3 L. S. Cederbaum, W. Domcke, J. Schirmer, W. Von Niessen, Adv. Chem. Phys. 65, 115
(1986).

Amelle: Conical Intersection Dynamics in NO2 Probed by Homodyne High-Harmonic Spectroscopy

Conical Intersection Dynamics in NO₂ Probed by Homodyne High-Harmonic Spectroscopy

1. H. J. Wörner¹,²,*, 
2. J. B. Bertrand¹, 
3. B. Fabre³, 
4. J. Higuet³, 
5. H. Ruf³, 
6. A. Dubrouil³, 
7. S. Patchkovskii¹, 
8. M. Spanner¹,
9. Y. Mairesse³, 
10. V. Blanchet⁴, 
11. E. Mével³, 
12. E. Constant³, 
13. P. B. Corkum¹, 
14. D. M. Villeneuve¹

+Author Affiliations

1. ¹Joint Laboratory for Attosecond Science, National Research Council of Canada and University of Ottawa, 100 Sussex Drive, Ottawa, Ontario, Canada K1A 0R6.
2. ²Laboratorium für Physikalische Chemie, Eidgenössische Technische Hochschule Zürich, Wolfgang-Pauli-Strasse 10, 8093 Zürich, Switzerland.
3. ³Centre Lasers Intenses et Applications, Université de Bordeaux, CEA, CNRS, UMR5107, 351 Cours de la Libération, 33405 Talence, France.
4. ⁴Laboratoire Collisions Agrégats Réactivité (IRSAMC), UPS, Université de Toulouse, F-31062 Toulouse, France and CNRS, UMR 5589, F-31062 Toulouse, France

1. ↵*To whom correspondence should be addressed. E-mail: woerner@phys.chem.ethz.ch

ABSTRACT

Conical intersections play a crucial role in the chemistry of most polyatomic molecules, ranging from the simplest bimolecular reactions to the photostability of DNA. The real-time study of the associated electronic dynamics poses a major challenge to the latest techniques of ultrafast measurement. We show that high-harmonic spectroscopy reveals oscillations in the electronic character that occur in nitrogen dioxide when a photoexcited wave packet crosses a conical intersection. At longer delays, we observe the onset of statistical dissociation dynamics. The present results demonstrate that high-harmonic spectroscopy could become a powerful tool to highlight electronic dynamics occurring along nonadiabatic chemical reaction pathways.

http://www.sciencemag.org/content/334/6053/208

Thomas Gaumnitz: Chirped Auger emission at FLASH reveals long-range electron correlation

The energy of Auger electrons, i.e. secondary electrons emitted after a non-radiative decay of a deeply bound electronic core-hole, is an intrinsic property of the electronic structure of the excited atom and does not directly rely on the energy of the exciting photon, electron or ion. The electron emission is not mono-energetic, though; the fast - attosecond to femtosecond - decay times imply corresponding spectral widths in a range of a few tens of meV to a few eV. This time-energy correspondence suggests a uniform distribution of all energy components over time. In this work, we demonstrate that under spectroscopically relevant conditions, this presumption is wrong. Instead, our time-resolved experiments show evidence of an energetic chirp, i.e. a pronounced time-dependent variation of the Auger-electrons´ kinetic energy. It appears as a consequence of the correlated motion of both photo- and Auger electrons in the Coulomb field of the remaining ion. While the underlying mechanism - also known as ‘post-collision interaction’ (PCI) - has extensively been discussed in the literature, no attention has so far been paid to the consequence of the effect for the temporal properties of the escaping electron wave packets. We visualize this temporal energy variation by superimposing 13.5nm XUV pulses from FLASH with the oscillating electric field of a strong terahertz (THz) wave from the FLASH THz undulator in a xenon gas-target. Significantly, modified widths of kinetic energy spectra for opposite field gradients clearly indicate a chirp (see Fig. 1).

Figure 1: Kinetic energy spectra of xenon Auger lines formed after absorption of XUV photons in the presence of a THz field. The modified widths of the Auger lines for opposite field gradients indicate an energy chirp.


The experiments have been confirmed in the laboratory where a high harmonics XUV source and a laser based THz source were employed. The observed spectral modulations are reproduced with semi-classical as well as  quantum simulations, and are explained by an analytical model, which includes PCI in the presence of a time-dependent streaking field.

Stefan: High-energy density laboratory astrophysics studies of accretion shocks in magnetic cataclysmic variables

In this paper we present the experimental simulation of the accretion column in magnetic cataclysmic variables using high-power lasers. With an appropriate target and adapted diagnostics, the dynamics and the main physical properties of laboratory accreting plasma have been characterised. The results obtained validate the experimental design and prove the formation of a reverse shock which compresses and heats the accreting plasma. The data are compared to 2D radiation hydrodynamic simulations, confirming the experimental scenario.

http://www.sciencedirect.com/science/article/pii/S1574181811000875

Marco + Leszek: Electron VMI / Extraction of correlations from covariance mapping

(1) Electron VMI (discussion led by Marco)

(2) On the requirements for extraction of correlations from covariance mapping (Leszek).

Matle+Seb: Characterising and optimising impulsive molecular alignment in mixed gas samples

Impulsive molecular alignment has been fully characterized in linear molecules by matching
numerical simulations and experimental data of the Fourier transformed time-evolution of the
corresponding rotational wavepacket. For this, a qualitative measure of the alignment distri-
bution is sufficient, making this a versatile procedure in experiments where the molecular axis
distribution is not directly accessible. Seeding small molecules in Ar as a carrier gas has then
been employed to assist cooling and we systematically retrieve the molecule’s rotational temper-
ature and alignment distribution for different mixing ratios. It was found that seeding 10% N2
in Ar results in the best cooling. Compared to pure N2 the rotational temperature was reduced
from 24 ± 2 K down to 9 ± 2 K. This leads to an improvement of the alignment distribution from
〈cos2 θ〉 = 0.60 to 〈cos2 θ〉 = 0.71. For the same mixing ratio CO2 was cooled from 34 ± 3 K


to 9 ± 1 K improving the alignment distribution from 0.48 to 0.64. In O2 a cooling from 58 ± 2
K to 37 ± 4 K was observed, corresponding to an alignment distribution improvement from 0.49
to 0.58. The results demonstrate the wide applicability of the characterisation procedure and of seeded supersonic beams to optimise impulsive alignment of small molecules.

Felicity: High-harmonic generation in H2O

We demonstrate high-harmonic generation in H2O using 800 and 1300nm laser pulses up to a maximum intensity of 5×1014W/cm2. Under optimal phase-matching conditions, photon energies up to ~60 and ~87 eV are produced by using 800 and 1300nm light, respectively. The harmonic spectra in H2O, when compared with Xe with a similar ionization potential, exhibit significant extension of the cutoff region, indicating suppression of ionization arising from molecular orbital symmetry.



paper: http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-35-12-1947

slides: http://charles.qols.ph.ic.ac.uk/~twitting/consJCDM/material/20120228_felicity_waterHHG/High%20Harmonic%20Generation%20in%20HO2a.pptx