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

Simon: Controlling the XUV Transparency of Helium Using Two-Pathway Quantum Interference

Atoms irradiated with combined femtosecond laser and extreme ultraviolet (XUV) fields ionize through
multiphoton processes, even when the energy of the XUV photon is below the ionization potential.
However, in the presence of two different XUV photons and an intense laser field, it is possible to induce
full electromagnetic transparency. Taking helium as an example, the laser field modifies its electronic
structure, while the presence of two different XUV photons and the laser field leads to two distinct
ionization pathways that can interfere destructively. This work demonstrates a new approach for coherent
control in a regime of highly excited states and strong optical fields.

http://prl.aps.org/pdf/PRL/v106/i19/e193008

sildes here: http://charles.qols.ph.ic.ac.uk/~twitting/consJCDM/material/20120221_Simon/Journal%20Club%20IC%20Feb21%202012.pptx

Richard: Atomic inner-shell X-ray laser at 1.46 nanometres pumped by an X-ray free-electron laser

Atomic inner-shell X-ray laser at 1.46 nanometres pumped by an X-ray free-electron laser

http://www.nature.com/nature/journal/v481/n7382/abs/nature10721.html

Since the invention of the laser more than 50 years ago, scientists have striven to achieve amplification on atomic transitions of increasingly shorter wavelength1, 2, 3, 4, 5, 6, 7. The introduction of X-ray free-electron lasers8, 9, 10makes it possible to pump new atomic X-ray lasers11, 12, 13 with ultrashort pulse duration, extreme spectral brightness and full temporal coherence. Here we describe the implementation of an X-ray laser in the kiloelectronvolt energy regime, based on atomic population inversion and driven by rapid K-shell photo-ionization using pulses from an X-ray free-electron laser. We established a population inversion of the Kα transition in singly ionized neon14at 1.46 nanometres (corresponding to a photon energy of 849 electronvolts) in an elongated plasma column created by irradiation of a gas medium. We observed strong amplified spontaneous emission from the end of the excited plasma. This resulted in femtosecond-duration, high-intensity X-ray pulses of much shorter wavelength and greater brilliance than achieved with previous atomic X-ray lasers. Moreover, this scheme provides greatly increased wavelength stability, monochromaticity and improved temporal coherence by comparison with present-day X-ray free-electron lasers. The atomic X-ray lasers realized here may be useful for high-resolution spectroscopy and nonlinear X-ray studies.

Additional Info:

Jon's News&Views: http://www.nature.com/nature/journal/v481/n7382/full/481452a.html