Extreme Ultraviolet Spectroscopy

The chemical environment of an active site in a molecule reveals information about its role and functionality in chemical reactions. Because of their short wavelength, and consequently atomic-selectivity, X-ray-based spectroscopic techniques are ideally suited to provide chemical insights. Short-wavelength  light  carries advantages  of (i) direct single-photon excitation and/or ionization for spectroscopy; (ii) nano-scale spatial resolution, thanks to a tighter spot, for imaging and microscopy. X-ray radiation is conventionally classified between “hard” and “soft”, depending if the photon energy is higher or lower than approx. a keV or 1 nm in wavelength. Both types of radiation can give insight into the local atomic environment. However, due to their much higher absorption cross-section, soft X-rays or even extreme ultraviolet (EUV, 10-100eV or 12-120nm in wavelength) are desirable when the number of the scattering species is low, like in the case of thin films, nano-particles and interfaces. In this case, EUV combines the advantages of photon methods with those of electron beams.

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In the last decades, giant beamlines have provided invaluable insights for proof-of-principle research (see Fig.) , which eventually have led to significant progress in many areas of science and technology. Beamlines are unmatched for proof-of-principle spectroscopy and imaging, but are only discontinually accessible for the single researcher or industry. For the scientific progress, student training, the full-duty industrial productions, and also the preparation of beamtimes, complementary sources of 24/7 access are needed. Complementary means that beamlines ("Giant Facilities") and tabletop XUV sources are not mutually exclusive, as much as "airplanes" did not make "cars" obsolete! Once N. Bohr wrote "Contraria non contradictoria, sed complementa sunt".

Plasma-based photon sources are indeed of great potential because of the demonstration of EUV pulses as short as a few picoseconds and with peak brightness as high as six orders of magnitude above 3rd generation synchrotrons. Investigating laser-produced plasmas is of long tradition in many research fields, such as inertial fusion,  matter under extreme conditions, astrophysics, next generation lithography at 13.5nm, and water-window bio-microscopy.

We self-developed and operate several instrumental architectures for both EUV spectroscopy and/or imaging at nano-scale, namely:

  • Self-Triggered Spark-Gap (STSG or "Pseudospark") Extreme UV Incoherent Source. A multiline spectrum for a gap-filling gas or mixture is generated, with a few mJ pulse energy and 100ns pulse duration, at repetition rates of approx. 20Hz.
  • CPA Plasma-Laser (CPA-PL) Extreme UV Coherent Source. A highly monochromatic line (Δλ/λ< 1e+5) is produced as a superradiant emission across a plasma column of approx. 10mm, with tens of uJ pulse energy an a few ps pulse duration, at single shot mode. The wavelength is discretely tunable depending on the target material, where a chirped-pulse amplification (CPA) laser of several J pulse energy is shone.
  • Capillary Ar Discharge Plasma-Laser (CAD-PL) Extreme UV Coherent Source. A highly monochromatic line (Δλ/λ< 1e+5) is produced as a superradiant emission across a plasma column of approx. 200mm, with approx. 100 uJ pulse energy an a few ns pulse duration, at repetition rate of 10Hz. The wavelength is fixed to 46.9nm (26eV) as given by the Ar medium.
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Qualitative comparison of state-of-the-art short-wavelength source specifications and requirements for materials science applications, with respect to three analytical parameters. Legend: XRL = XUV Radiation & Laser (our self-developed source!), HHG = High Harmonic Generation, xFEL = X-ray Free-Electron Laser.

 

Laser microanalysis profited from the sensitivity of ICP-MS, achieving detection limits in the low mg/kg across the mid and high mass range. The flexibility and rapidity of the method have been discussed in hundreds of papers. Remaining challenges are the non-stoichiometric sampling (“fractionation”) and the redundant sampled mass, and heterogeneity in particle size, with respect to the fraction that is eventually atomized/ionized in the plasma and measured at the detector. Further, the laser diffraction prevents the achievements of spot sizes at nano-scale.

XUV using laser-produced or discharge-produced hot/dense plasmas can close such gaps, for 24/7 operation in the university or industry "home-lab". XUV laser-action has been also accomplished on table-top systems, which in the time of large beamlines with limited access, helps bridging the gap between the user and the tools.

High-res microanalysis was performed using XUV radiation at 10—50nm. The high photon energy generates ions, from H onwards, directly from the sample, without an ICP intermediate. The online detection was performed with  self-developed TOFMS with a reflectron or a novel “transmistron”. XUV-PIMS (Photo Ionization Mass Spectrometry) allowed nano-destructive analysis, and results on single-shot chemical imaging are being populated.

References