Elisabeth Schramm, Thomas Heindl, Jasper Hölzer, Alexander McNeish, Michael Puetz, Hermann Ries, Patricia Schall, Rasmus Schulte-Ladbeck, Rainer Schultze, Martin Sklorz, Gerd Spieker, Roman Trebbe, Andreas Ulrich, Jochen Wieser, Ralf Zimmermann
The objective of this accomplished project funded by the German BMBF was to develop a single photon ionization ion
trap mass spectrometer (SPI-ITMS) for detection of security relevant substances in complex matrices at low
concentrations. The advantage of such a soft ionization technique is a reduction of target ion fragmentation allowing
identification of signals from complex matrices and enabling MS/MS capability. To obtain low detection limits, the
applied photon energy has to be below the ionization potential (IP) of the bulk matrix components. Therefore, photon
energies between 8 eV (155 nm) and 12 eV (103 nm) are necessary which was achieved with newly developed electron
beam excimer lamps (EBEL). They generate light at different wavelengths depending on the selected rare gas emitting
wavelengths adapted to the analyzed substances. So, e.g. with a krypton-EBEL with 8.4 eV photon energy most narcotics
can be ionized without notable fragmentation. Due to their higher IPs, EBEL with higher photon energy have to be used
for most explosives. Very low false-positive and false-negative rates have been achieved using MS/MS studies. First
field tests of a demonstrator provided the proof of principle.
Elisabeth Schramm, Stephan Borrmann, Joachim Curtius, Andreas Goertler, Thomas Heindl, Andreas Kuerten, Alexander McNeish, Stefan Mitschke, Andrei Morozov, Fabian Muehlberger, Michael Puetz, Gerd Reichardt, Hermann Ries, Patricia Schall, Rasmus Schulte-Ladbeck, Rainer Schultze, Martin Sklorz, Roman Trebbe, Andreas Ulrich, Jochen Wieser, Ralf Zimmermann
The objective of this project funded by the German BMBF was to show that security relevant substances can be detected
in complex matrices at low concentrations using single photon ionization ion trap mass spectrometry (SPI-ITMS). The
advantage of such a soft ionization technique is a reduction of unwanted fragment ions in mass spectra allowing
identification of signals from complex matrices and enabling MS/MS capability. The MS/MS studies permit low false-positive
and false-negative rates. Additionally, the accumulation of the ions in the ion trap decreases the detection limit.
To obtain low detection limits the ionization potentials (IPs) of the relevant substances have to be below the IPs of the
bulk matrix components. That enables the utilization of a photon energy unaffecting the matrix components resulting in
increased sensitivity due to essentially non-existent background signals. As literature values for many ionization
potentials are unavailable, IPs of several security relevant substances were determined using monochromatized
synchrotron radiation from BESSY, Germany. All analyzed substances exhibited IPs significantly below the IPs of
common matrix molecules such as water, nitrogen and oxygen. First measurements with a pre-demonstrator show that it
is possible to shield matrix substances using a well chosen photon energy for soft ionization.
Low energy electron beam excitation of dense helium and neon was used for light production in the extreme ultraviolet (XUV) wavelength range. The entire system was completely filled with the working gas avoiding the use of vacuum equipment for light production, propagation and detection. Emission spectra from He and Ne are dominated by the second continua with peak intensities at 80 and 83 nm, respectively. The hydrogen Lyman-α line was observed as the dominant impurity line. This XUV light source was used for transmission measurements of LiF near its absorption edge.
Low energy electron beam excitation of dense gases is used for developing UV and VUV light sources. Emission spectra for various gases are shown and a first application for single photoionization in a time of flight mass spectrometer is presented.
The results of an experimental study on the temporal evolution of molecular-ion continua of rare gases using electron- and ion beam pumping are presented. The temporal evolution of radiation bands, which make up the molecular-ion continuum (MIC) has been measured. A hypothesis for a relaxation cascade of excitation energy transfer from doubly charged ions (Rg++n) to singly charged ions (Rg+n) has been analyzed. Results obtained using electron-beam pumping and a comparison with data obtained by other pumping methods allows us to draw the conclusion that for MIC in rare gases, the local maxima's wavelengths do practically not depend on pumping power and pumping method. The ratio of intensities in different maxima during plasma relaxation, however, and the behavior of each maximum with respect to pressure variations do depend on specific excitation power.
Time correlated optical emission spectra of argon, krypton and xenon in the wavelength range from 110 to 450 nm, and 0 to 6000 ns time interval, recorded at the Munich Tandem accelerator using heavy ion beam excitation with 2 ns beam pulses, were measured in order to clarify the origin of the so called third rare gas excimer continua. Experiments were performed at xenon and krypton pressures between 50 and 250 mbar, and argon pressures between 230 and 1500 mbar. All spectra clearly show different distinct peaks, emerging at different time delays after excitation. These spectral maxima are interpreted as arising from excimer emissions by separate radiating species, formed by gas kinetic processes. While the spectral shape of the components, forming the third continuum radiation in the heavy rare gases krypton and xenon turned out to be of complicate structure, in the case of argon all wavelength spectra could be reproduced by fitting a limited number of Gaussian functions with fixed center wavelengths and fixed widths to the data. Hereby, six distinct maxima, appearing at four different times after the excitation pulse, could be identified.
The fluorescence of dense rare gas targets has been produced and analyzed using X-ray flash, e-beam and ion beam as excitation techniques. For the first time to our knowledge an extended database of argon and krypton molecular UV emissions, i.e. the Molecular Ion Continua, has been obtained with a very good agreement between spectra whatever the pumping technics is used. The estimation of potential surface for both singly and doubly charged argon trimers and the consideration of previous ab initio calculation for the argon dimer enables us to confirm some former hypothesis but also to suggest new pathways likely to be at the origin of the population of the upper states radiating in the Molecular Ion Continua.
Low energy electron beams with particle energies of typically 10 to 20 keV are used for pumping gas lasers. Extremely thin (300 nm) ceramic (SiNx) membranes are used as entrance foils for the electron beam. Laser gas pressure up to several atmospheres is possible using this technique if the dimension of one side of the foil is restricted to about 1 mm. Energy loss of the electrons in the foil is less than 10%. The short range of the low energy electrons in the laser medium leads to a high specific power deposition. In transverse geometry the beam pumped volume is cylindrical with typically 1 to 3 mm diameter. This is well matched with the diameter of optical modes in stable optical cavities. The new pumping method is demonstrated using the 1.73 micrometers 5d[3/2]1-6p[5/2]2 XeI laser line in Ar-Xe laser gas mixtures at pressures between 130 and 650 mbar. Laser effect was observed for Xe concentrations between 0.1 and 1%. A low threshold pumping power of 5.5 W and a maximum output power of 6 mW at 13 W pumping power were measured. Scaling to higher power and shorter wavelength laser systems is discussed.
Excimer formation in dense gas targets is studied using a 10 to 20 keV electron beam for the excitation of rare gases. A SiNx ceramic foil, only 300 nm thick but strong and completely vacuum tight is used as an entrance window. The electrons loose only 4 to 6% of their energy in this foil. The low electron energy allows a table top setup without high voltage or x-ray radiation hazards. First applications of the system for excimer physics research and the development of novel vacuum ultraviolet light sources are described.
Efficient energy transfer between neon excimer molecules and hydrogen has been found. A small, high gap density light source has been developed, emitting entirely on the hydrogen 2p-1s transition at 121.567 nm (Lyman-(alpha) ). Light output densities of 10 W/cm2 are obtained. Electron beam energy conversion efficiencies of approximately equals 10% have been measured.
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