When chemical compounds of low-Z elements are excited by x-rays, the ionized core states are filled from the valence band. The molecular orbital character of the valence band is thus reflected in the x-ray emission spectrum from the compound. High resolution x-ray spectra from these compounds are useful both for the identification of the compound and for the scientific insight into their chemical bond structure. We have excited a series of energetic nitrogen compounds (NH4NO3, TNT, RDX) using intense tunable synchrotron radiation and have obtained x-ray emission spectra with a high-resolution grating spectrometer. When combined with the latest computational theoretical chemistry, we observe good qualitative agreement that gives insight into the bond structure of these compounds. The identification of chemical states is similar to what is commonly obtained by analytical techniques such as x-ray photoemission spectroscopy but without the absolute need of ultra-high vacuum.
X-ray spectrometry is an extremely important technique to analyze the chemical constituents of materials. The purpose of our metrology program is to push the analysis further to identify the actual chemical compounds present in a broad variety of applications. In parallel with our metrology program to develop new high-resolution x-ray spectrometers for chemical analysis, we need to understand the validity and basis of x-ray spectra that we are observing. To this end we have initiated a program of high-resolution x-ray spectroscopy using state-of-the-art techniques on a series of chemical compounds. Nitrogen compounds are highly suitable subjects for this study, since the N atom occurs in a wide variety of bonding situations, many compounds are technically important, and much bonding chemistry remains to be determined.
We chose to study a series of x-ray emission spectra from N compounds used as explosives. These compounds are interesting because a) they exhibit unusual chemical behavior, b) they incorporate multiple N atoms in equivalent sites, and c) they have sophisticated structures. In addition, recent crystallographic data give accurate coordinates for their structures, and computational chemistry techniques are just adequate to deal with the complexity of the molecules. It is possible that x-ray emission spectroscopy will be useful in the future detection and identification of these compounds.
We proposed to excite selectively the 1s atomic level of N atoms in a series of explosive compounds using intense x-ray radiation tuned just above the N K-edge. The hybridization that occurs in the molecular orbitals that combine to form bonds between N, C, and O atoms creates a continuum of electron states spread out over 30-40 eV in this region. Bond electrons from these states falling into the 1s core holes result in detailed spectra representing a wealth of chemical information. In order to observe features which reflect the structure in the valence band of the particular N compound, we needed to analyze the resulting N fluorescent emission spectra with a resolution of about 0.5 eV. It was important to establish an absolute calibration for the energy scale of the excitation and the emission (for use as reference spectra in the future, as well as for comparison with the theoretical calculations). Finally, we needed to understand the density of states and x-ray transition rates using advanced theoretical techniques in calculational chemistry.
We obtained spectra from powdered samples on the high-resolution spectroscopy beamline 8.0.1 at the Advanced Light Source (ALS). This incorporates both a stable soft x-ray grating monochromator and a Rowland circle grating spectrometer. The monochromator was calibrated using absorption of the N 1s vibrational spectrum of N2 gas, and the spectrometer was calibrated by elastic scattering of radiation from the monochromator. The resulting x-ray emission spectra were compared to theory using two techniques. The molecular orbital program StoBe incorporating density functional theory was used to identify the contributions from valence electrons in specific s and p bonds. The program FEFF8 was used to calculate an approximate valence band density of states and transition rates that qualitatively reproduced the observed x-ray spectra.
Calculated and measured x-ray emission spectrum from trinitrotoluene showing a contribution from the N-C pi bond and the molecular orbitals which contribute to it.
Lead Organizational Unit:mml
Source of Extramural Funding:
Some collaborators on this study were funded by the Office of Naval Research
Applied Physics Laboratory, University of Washington
Sample preparation: NIST Gaithersburg Advanced Light Source Measurements: Advanced Light Source, Lawrence Berkeley Laboratory, Berkeley, CA
Related Programs and Projects:
Microcalorimeter x-ray detector, ALS spectral comparisons with NIST x-ray spectrometry (Analytical Chemisry Division)