X-ray Raman Spectroscopic Study of Benzene at High Pressure Michael Pravica,* ,† Ognjen Grubor-Urosevic, † Michael Hu, ‡ Paul Chow, ‡ Brian Yulga, † and Peter Liermann ‡ Department of Physics and Astronomy, UniVersity of NeVada Las Vegas and High Pressure Science and Engineering Center (HiPSEC), Las Vegas, NeVada 89154-4002, High Pressure CollaboratiVe Access Team (HP-CAT), Carnegie Institution of Washington, AdVanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: June 4, 2007; In Final Form: September 4, 2007 We have used X-ray Raman spectroscopy (XRS) to study benzene up to ∼20 GPa in a diamond anvil cell at ambient temperature. The experiments were performed at the High-Pressure Collaborative Access Team’s 16 ID-D undulator beamline at the Advanced Photon Source. Scanned monochromatic X-rays near 10 keV were used to probe the carbon X-ray edge near 284 eV via inelastic scattering. The diamond cell axis was oriented perpendicular to the X-ray beam axis to prevent carbon signal contamination from the diamonds. Beryllium gaskets confined the sample because of their high transmission throughput in this geometry. Spectral alterations with pressure indicate bonding changes that occur with pressure because of phase changes (liquid: phase I, II, III, and III′) and possibly due to changes in the hybridization of the bonds. Changes in the XRS spectra were especially evident in the data taken when the sample was in phase III′, which may be related to a rate process observed in earlier shock wave studies. Introduction. Benzene (C 6 H 6 ) is an important aromatic hydrocarbon that is a fundamental constituent of petroleum, particularly as a building block for larger molecules, e.g., in asphaltine, 1 which is a potpourri of aromatic and aliphatic molecules of varying sizes. Benzene also has a high degree of symmetry (C-C bond lengths are all 1.397 Å and the bond angles are all exactly 120°) that results from resonance behavior. 2 These attributes result in benzene’s fascinating chemistry toward formation of more complex molecules (in- cluding dyes and pharmaceutical compounds 3 ) and polymers. Recently 3-4 and in the past, 5-12 there has been interest in understanding the properties of benzene under high pressure and high-temperature, particularly in the spirit of inducing amorphization, 4 dimerization, 5 and other chemical transforma- tions 6 of benzene to develop high-pressure methods for creating novel compounds and polymers. 3-4 Shock wave studies of benzene have also been performed 13-14 and thus understanding the behavior of this compound under static high pressure would be useful to complement data of the material under dynamic loading conditions. The techniques used to interrogate benzene under pressure have been primarily X-ray diffraction, 4-7 Ra- man 6,9,12 and infrared (IR) 4,8 spectroscopy, and optical absorption spectroscopy. 11 It is in this spirit of better understanding the inter- and intramolecular changes to benzene at high pressure that we performed X-ray Raman spectroscopic measurements using a diamond anvil cell for pressure generation, 15 the first such measurement for benzene or any hydrocarbon that we are aware of under extreme conditions. X-ray Raman spectroscopy (XRS) 1,16-17 is a valuable technique for probing core electron bonding where a hard X-ray (2-10 keV 1 ) inelastically scatters from core electrons while exciting them into unoccupied states. At high pressures, X-ray Raman spectroscopy, although more or less equivalent to low-energy X-ray spectroscopies such as X-ray absorption spectroscopy (XAS) 1 and extended X-ray absorption fine structure (EXAFS), 1,16 is much preferable to these techniques as the probing soft X-rays utilized by these methods will not penetrate the confining diamonds and metallic gasket. These experimental methods also frequently require that the sample be placed in vacuum. 1 The only XRS studies on benzene of which we are aware were performed under ambient conditions. 1,17 XRS was found to be useful because of the better penetration of X-rays and thus no requirement for the samples to be placed in a vacuum system as well as better interrogating the pristine sample inside its possibly degraded/oxidized surface. 1 The technique, although still in its infancy, has become more feasible because of the high photon fluxes available (order of 10 12 photons/s) from third-generation synchrotrons. It has, for example, been found useful to qualitatively assess the aromatic fraction of carbons in asphaltenes. 1 Though this technique is extremely challenging to use for samples com- pressed by diamond cells that are typically 10 μg, where the diamonds can contaminate the carbon signal from the sample, it was our aim to observe the behavior of benzene under extreme conditions in the hope of better understanding the bonding chemistry of this interesting molecule with pressure. Experimental. We performed two separate experiments on benzene with two different samples. In both cases, HPLC-grade benzene (fw 78.11 g/mol, mp 5.5 °C, bp 80.1 °C) of purity >99.9%, purchased from Aldrich Chemical, was loaded in the liquid state into a beryllium gasket that had been drilled by * Corresponding author. E-mail: pravica@physics.unlv.edu. † University of Nevada Las Vegas and High Pressure Science and Engineering Center. ‡ Argonne National Laboratory. 11635 2007, 111, 11635-11637 Published on Web 09/20/2007 10.1021/jp074321+ CCC: $37.00 © 2007 American Chemical Society