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Stability of Coronene at High Temperature and Pressure E. Jennings,† W. Montgomery,*,† and Ph. Lerch‡ Department of Earth Science, UniVersity of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ, United Kingdom, and Paul Scherrer Institute, Swiss Light Source, CH 5232 Villigen, Switzerland ReceiVed: June 1, 2010; ReVised Manuscript ReceiVed: September 28, 2010
The infrared response of coronene (C24H12) under pressure and temperature conditions up to 10 GPa and 300 °C is examined in situ using a diamond anvil cell and synchrotron-source Fourier transform infrared (FTIR) spectroscopy. Coronene is a polycyclic aromatic hydrocarbon that is present in the interstellar medium and meteorites which may have contributed to the Earth’s primordial carbon budget. It appears to undergo a reversible phase transition between 2 and 3.2 GPa at ambient temperature; new intramolecular bonds in the region 840-880 cm-1 result from compression. We document the shift of spectral features to higher wavenumbers with increasing pressure but find this change suppressed by increased temperature. By investigating the stability of coronene over a range of naturally occurring conditions found in a range of environments, we assess the survival of the molecule through various terrestrial and extraterrestrial processes. Coronene has previously been shown to survive atmospheric entry during Earth accretion; this can now be extended to include survival through geological processes such as subduction and silicate melting of the rock cycle, opening the possibility of extraterrestrial coronene predating terrestrial accretion existing on Earth. Introduction Coronene (C24H12), also known as superbenzene, is a polycyclic aromatic hydrocarbon (PAH) that has been observed in diverse natural environments, from mid-ocean-ridge basalts and volcanic bombs to the interstellar medium.1,2 At ambient conditions it is a yellow crystalline powder. We examined in detail the stability of coronene from ambient conditions up to 10 GPa and 300 °C, using synchrotron light and Fourier transform infrared (FTIR) spectroscopy in the resistively heated diamond anvil cell. At atmospheric pressure and high temperatures, peak assignments have previously been made both experimentally3-5 and theoretically by Langoff.6 During the early accretion of the solar system, meteoritic material, specifically carbonaceous chrondrites, would have experienced various pressure and temperature conditions. During accretion, material may be buried in the interiors of large bodies and released in subsequent violent collisions. Evidence for this is seen in the metamorphism and shock alterations of chondrites.7,8 Chondrites and interplanetary dust particles (IDPs), which would have been affected by such high pressure and temperature (PT) conditions, would have been brought to Earth during the Late Heavy Bombardment (LHB) of the Hadean (3.8-4.6 Gyr), experiencing further high PT conditions during impact. Carbonaceous chondrites especially are known to contain organic carbon including coronene9 and therefore are a significant contributor to the early Earth’s budget of organic matter. Matrajt et al.10 demonstrate that although the high-temperature conditions of atmospheric descent would have destroyed most organic molecules, 0.1% of the coronene could have survived to the Earth’s surface, making IDPs and micrometeorites a key source of organic material. High thermal and pressure stability would indicate potential for coronene to survive terrestrial processes such as subduction, burial, and volcanism and extraterrestrial * To whom correspondence should be addressed: e-mail: wren.
[email protected]. † University of Bristol. ‡ Swiss Light Source.
processes such as impacts, large body self-compression, and planetary differentiation. Coronene along with other PAHs has also been identified in the interstellar medium (ISM) through the analysis of emission features. Unidentified infrared emission bands (UIR bands) were observed in the ISM; many of these have been matched to peaks in the theoretical and experimental spectra of PAHs.2,11-13 The major UIR bands have been assigned to stretching and bending modes of PAHs, but some debate still exists around the assignment of weaker bands.14 We investigate the possibility that these bands could relate to high-pressure alteration of PAHs. In terrestrial systems, coronene, along with other PAHs, is created both naturally (for example, during fires) and anthropogenically (by the incomplete combustion or pyrolysis of fossil fuels) and is especially prevalent in motor exhaust.15,16 While it has been observed in volcanic ash and recovered from mantle xenoliths, terrestrial contamination is invoked rather than exploring the possibility of mantle-generated hydrocarbons.1,17 Previous high-pressure experimental work on PAHs has been carried out. For example, Davydov et al.18 determined the PT conditions of carbonization, graphitization, and diamondization using X-ray diffraction of samples generated in the pistoncylinder press. Increasing pressure drastically reduces the temperatures required for this process, and it was found that for coronene (and other PAHs) crystalline graphite was observed at 8 GPa and 1000 °C, with carbonization (increase in C:H ratio) becoming significant around 600 °C. Since high pressure decreases the temperature needed for carbonization, coronene more likely survives lower pressure and temperature processes such as burial and subduction as opposed to impacts and deep mantle processing. Yamamoto et al.19 used X-ray diffraction to determine that coronene crystals underwent several structural phase transitions at low pressures and temperatures of up to 1.8 GPa and 25 °C, from monoclinic at ambient conditions to orthorhombic above 1 GPa. Bernard et al.20 used FTIR to study coronene at ambient pressure over a variety of temperatures from 20 to 200 °C. They
10.1021/jp105020f 2010 American Chemical Society Published on Web 11/10/2010
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TABLE 1: Summary of Synchrotron-Source FTIR Experiments Carried out in the Diamond Anvil Cell expt no.
pressure range, GPa
temp range, °C
duration, h
1 2 3 4 5
0-10.3 5.3-8.8 3.1-3.2 1.2-2.6 6.6-7.4
25 25-200 25-202 25-301 25-279
2 3 2.5 4.5 6
found that peaks did not shift or change in intensity, implying no inter- or intramolecular changes had occurred in the studied range. In this work we measure the infrared response of coronene to varying pressure as well as temperature in a range relevant to geological processes. No work has previously been done at both high pressures and high temperatures. This parameter space extension allows us to investigate the stability of coronene across a range of pressures relevant to planetary science and the role of carbon chemistry throughout the solar system. Methods A membrane-type diamond anvil cell containing type II diamonds with 500 µm culets was used. A stainless steel gasket with a 200 µm diameter sample chamber and a preindented thickness of 30 µm depth was placed between the diamonds. This was loaded with a cesium iodide (CsI) window, a ruby, and a piece of crystalline coronene approximately 15 µm in thickness, obtained from the NASA Astrochemistry Group. CsI is transparent over the range of wavelengths used, so it will not add noise to the collected spectra. It acts as a pressure medium and fills the sample chamber, allowing a thinner sample to be used to prevent the saturation of absorbance spectra. It is stable over a greater PT range than PAHs.21 FTIR-microspectrometry in transmission was performed using synchrotron light at the Swiss Light Source operated in top-up mode. Thus the infrared intensity remains constant with time. The IR focus available at the end of the beamline was coupled to the side port of a Bruker Vertex 70 FTIR spectrometer with an f1 ) 34 mm/f2 ) 213 mm ellipsoidal mirror. The spectrometer, equipped with a KBr beamsplitter, was coupled to the input of a Bruker Hyperion IR microscope without any additional
optics. The microscope was equipped with two ×15 gold-coated Cassegrain objectives. The illuminated region of the sample was determined by an aperture of some 20-45 µm × 20-45 µm. In order to reduce the effect of scattered light and to enhance contrast fidelity, an identical conjugate aperture was placed between the collecting optics and the nitrogen-cooled mercurycadmium-telluride (MCT) detector. Resolution and number of coadded scans were 4 cm-1 and 256, respectively. Crystals were observed in parallel by use of white light and a charge-coupled device (CCD) camera. Experiments were each performed at a near-constant fixed pressure while the temperature was varied. As heating occurred, sample pressure was kept mostly constant by adjusting the membrane pressure and monitoring the shift in ruby fluorescence. A summary of experiments performed is provided in Table 1. A fresh sample was prepared for each pressure. Samples were first brought to a pressure and then heated at a rate of 2.5 °C/min. Spectra were obtained at intervals of approximately 50 °C. At each of these intervals, prior to taking a measurement, a fresh background spectrum was taken through the diamonds and CsI to adjust for an increase in noise and fringes caused by internal reflections through the transparent medium at higher temperatures. This background was subtracted from the sample spectrum during analysis. Following this, the sample was allowed to cool at the same rate with spectra obtained at the same temperature intervals. A separate experiment was also done at ambient temperature, by incrementally increasing the pressure to 10.3 GPa and then decreasing it again; spectra from this experiment are shown in Figure 1. Temperature was varied by steadily increasing (or decreasing) the voltage difference across a ring-shaped resistance heater set around the gasket and diamonds within the cell, and temperature was measured with a type-K thermocouple resting against one diamond positioned close to the culet. The total error budget in nonambient temperature measurements has been estimated at (20 °C and includes the precision of the thermocouples used ((4 °C) and temperature gradients within the gasket, as well as the temperature change that results from the integrated rate of 2.5 °C/min over the time necessary to coadd 256 spectra. Since the thermocouple is positioned at the edge of the diamond rather than within the sample, the temperature recorded is likely to be an underestimate. It should be noted that, for the 2.1 GPa
Figure 1. (a) Normalized infrared spectra of coronene at 300 °C and pressures up to 10.3 GPa (compression and decompression are shown) over the spectral regions of interest, 700-1800 and 2700-3200 cm-1. Microphotograph of in situ sample is shown at upper right; ruby crystal is seen at top right of sample. Peak formation is visible in the 840-880 cm-1 region; peaks at 3000 cm-1 show Fermi resonances affected by pressure. Data collected at ambient pressure are in agreement with previously published values.3 (b) Gaussian peak fits to ambient temperature compression data in the range 820-900 cm-1.
Coronene Stability at High Temperature and Pressure
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TABLE 2: Vibrational Frequency Assignmentsa frequency, cm-1 (this work)
assignment
description
767.5 811.4 847.5 852.9 1135.8 1312.9 1499.5 1608.3 2989.4 3018.6 3045.5 3054.9
C-H C-H C-H C-H C-H C-C C-C C-C C-H C-H C-H C-H
out-of-plane vibrations in-plane vibrations out-of-plane vibrations intramolecular coupling20 in-plane bend stretch stretch stretch stretch stretch stretch stretch
a After Cyvin et al.3 and Szczepanski and Vala.4 All major peaks measure fall within (12 cm-1 of these prior assignments.
experiment, temperatures are estimated from recorded voltages by use of the temperature-voltage relationship of other experiments, as the thermocouple failed. During heating, expansion led to a small pressure increase and the gas membrane was readjusted at each temperature interval to reduce pressure fluctuations. During the experimental cycle, color and texture changes in the sample were monitored. Pressure determination is based on the well-known ruby fluorescence scale.22 In order to position the sample in a reproducible manner and to increase efficiency, it is advantageous to measure the pressure within the DAC without having to remove the entire cell from its aligned position in the infrared path of the microscope. The optical path of our instrument is sufficiently flexible that a green laser source (543 nm) was coupled in between the FTIR spectrometer output and the microscope input and steered onto the sample focal plane by use of the optics of the microscope. Ruby and sample are usually close to each other, so that a movement of 10-100 µm is enough to have the laser light hit the ruby crystal. The R1 pressuredependent fluorescence line22 is collected by the reflection optics of the IR microscope. Light is collected at the end of one ocular and sent to a compact fixed-grating spectrometer unit (Ocean Optics HR4000) that includes a 800 line/mm grating read out by a linear CCD chip that has 4096 pixels. The position of some filters in the optical path can be adjusted with motorized filter changers, so that swapping from P mode to IR mode takes only a few seconds. A sketch of the setup can be found in ref 23. The error in the pressure calculation due to the precision of the spectrometer unit is calculated to be less than 0.01 GPa; the uncertainty of the pressure in the diamond anvil cell due to nonhydrostatic conditions is estimated to be (0.1 GPa. Results Following previous work, we have identified the major and many of the minor peaks in the spectrum collected at ambient conditions in Figure 1 and summarized them in Table 2.3,4 Six of the major peakss767.5, 811.4, 847.5, 852.9, 1312.9, and 1608.3 cm-1shave been fitted and analyzed in detail over the pressure and temperature ranges of our study. For clarity, peaks are referred to by their wavenumber at maximum amplitude at ambient conditions. At ambient pressure, our observations are consistent with previous work; the three highest amplitude peaks are at 852.9, 1312.9, and 1608.3 cm-1. For all modes, the peak value shifted to higher wavenumber as pressure increased, as shown in Figures 2 and 3. This trend is reversible as pressure is released to 2 GPa. Some peaks were
Figure 2. Peak center shifts and formation with compression (O) and decompression (b) at 300 °C of spectral features in the 750-880 cm-1 region. Dashed vertical line indicates phase transition at 0.4 GPa observed by Yamamoto et al.19 (not observed in this work), and broader dashed line indicates a possible new phase transition at ∼2.2 GPa determined by the appearance of new features and shifts in the slope of peak center displacement. These are most readily observed in the C-H vibrational bands. Gray solid lines track peaks; gray dashed lines indicate splitting due to intramolecular compression.
more susceptible to this pressure effect than others; features at higher wavenumbers (higher energy) exhibit greater displacement due to the increase of pressure (Figure 1). Below we use the term slope when discussing the pressure-induced wavelength change dνj/dP (where νj and P are wavenumber and pressure, respectively) of a specific absorption feature. With varying pressure, we observe a change in character of the 847 and 852 cm-1 C-H out-of-plane vibrations. Although previous studies reported only the 847 cm-1 peak, our spectra are in agreement with Bernard et al.,20 who attribute the doublet to intermolecular coupling in the solid phase. As pressure increases, an asymmetry of the peak becomes apparent, and at higher pressures, more absorption features develop (Figure 1b). Thus, these C-H vibrations respond differently to pressure than the C-C bond modes. With increasing pressure, the region from 840 to 880 cm-1 was best fit with four peaks as shown in Figure 1b. These spectral changes are completely reversible upon decompression to 2 GPa (solid markers in Figure 2). The formation of new absorption features together with a change in slope signals a phase transition. No change in the 1312.9 and 1608.3 peaks (C-C in-plane stretching) was observed in the spectra with increasing temperature up to 300 °C (Figure 4). This is in agreement with the findings of Bernard et al.,20 who found no change in the position or relative intensity of the spectral features. Specifically, even if the pressure of the sample increased during a heated experiment, the peak position remained fixed at the initial position, which was set by the initial pressure of the sample (Figures 3 and 4). This displacement is irreversible with cooling. Due to experimental constraints (i.e., the unavoidable mechanical tightening of the diamond anvil cell when held at high temperatures over longer periods of time), we were unable to determine the effects of decompression to ambient pressure. As with the 1312.9 and 1608.3 cm-1 bands, we see very little change in the 840-880 cm-1 region in behavior due to heating and cooling. The exception is in the 1.4 GPa experiment, where the formation of new peaks, which appears to be two peaks splitting into four, occurs ∼1 GPa lower than in the ambient
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Figure 3. Peak center shifts of the (a) 1312.9 and (b) 1608.3 cm-1 C-C stretching modes with pressure and temperature. Of specific interest is the softening effect of temperature; the slope dν/dP decreases with increasing temperature. Solid symbols indicate ambient temperature decompression data for comparison; there is some hysteresis in this system.
temperature experiment (Figure 5). Again, all shifts are reversible with temperature. At increasing pressures, there are substantial changes in the 3000 cm-1 region of the spectrum (Figure 1). At ambient conditions, this region has five Fermi resonances;20 the discussion of pressure and temperature of these changes is outside the scope of this paper and will be addressed elsewhere. Discussion Spectral Interpretation. The pressure and temperature dependence of the spectral features investigated are inconsistent with the previously reported phase transition occurring at 0.4 GPa19 and consistent with the previously reported insensitivity to high temperatures.20 By examining the slope of the 847.5 and 852.9 cm-1 modes between ambient pressure and 2.0 GPa and comparing it with that at higher pressures, we see a clear change, suggesting a further phase transition somewhere between 2.0 and 3.2 GPa (Figure 2). These absorption peaks are due to C-H vibrations and will be more sensitive to molecular rearrangement than the C-C vibrations at 1312.9 and 1608.3 cm-1, which show constant slope (Figure 3). This observed phase transition is completely reversible upon decompression to just under 2 GPa.
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Figure 4. Peak center shifts of (a) 1312.9 and (b) 1608.3 cm-1 peaks (C-C stretching modes) with temperatures up to 300 °C, for both heating (solid) and cooling (dashed). Ambient temperature peaks are shown for comparison; the spread in these values is due to the effect of pressure. Elevated temperature appears to suppress the effects of pressure shifts that occurred during previous unheated experiments.
In general, higher temperature has the effect of canceling the effect of pressure on the wavenumber at which maximum absorption is observed. Peak splitting does appear to begin at lower pressures under higher temperatures (Figure 5), suggesting that the second high-pressure phase of coronene dominates under these thermodynamic conditions. The phase transition remains fully reversible. Geologic and Astronomical Context. Coronene has a greater stability than most other PAH molecules, for example, chrysene. Recent experiments on chrysene show significant changes in the IR spectra over the same PT range discussed here.24 The compactness, aromatic nature, and high degree of symmetry of coronene allow a complete delocalization of p orbital electrons, similar to the electronic structure found in graphite and graphene.14 This stability is supported by our observations. Coronene exists in the ISM and so would make up a fraction of material accreting to form the early Earth. Its reversible behavior under these conditions would mean some of it could survive to the surface of the Earth and contribute to the total organic matter. This means that it could potentially play a role in the creation of primitive life. However, the stability of coronene also suggests that it will not polymerize or increase
Coronene Stability at High Temperature and Pressure
Figure 5. Peak center shifts of the 811.5, 847.5, and 852.9 cm-1 peaks (C-H vibrations) with pressure and temperature. Of specific interest is the stiffening effect of temperature; the slope dν/dP increases with increasing temperature. Solid symbols indicate ambient temperature decompression data; there is some hysteresis in this system. Note the appearance of new peaks in the 840-880 cm-1 region at a lower pressure than in experiments performed at ambient temperature. Light gray indicates reproduction of Figure 2 for comparison purposes.
in complexity as pressure and temperature increase (unlike cyanuric acid25) without interaction with other chemicals. The additional spectral features due to high pressures that we observe in this work are unlikely to be correlated to UIR bands as they are neither in the correct position nor quenchable to ambient conditions. Conclusions High-pressure and high-temperature experiments were performed on coronene, from ambient conditions up to 300 °C and 10.3 GPa. IR spectra were obtained in situ throughout the experiment. By analyzing the FTIR spectra collected in situ at these conditions, we have shown that coronone undergoes at least one reversible phase transition that is completely recoverable in this pressure-temperature region. This experimental work demonstrates clearly that coronene, due to a reversible phase transition, could survive through many extraterrestrial and terrestrial processes associated with planetary formation and evolution. These include the high pressures encountered in impacts during solar system accretion, the temperatures of entry through the Earth’s atmosphere, and passage through the high-pressure and high-temperature environments of the terrestrial rock cycle such as subduction and silicate melting (Figure 6). When considered in context with the work of Davydov et al.,18 our work suggests that coronene could be subducted into the mantle and returned as coronene, having undergone a reversible phase transition during this P-T path. Carbonisation occurs at pressures and temperatures far above those of subduction and burial, but complete conversion to graphite (i.e., graphitization) intersects geologically significant transitions such as the initiation of melt of peridotite, the dominant component of the Earth’s upper mantle, suggesting that the coronene that is not carbonized could persist in the molten rock. It is reasonable to assume that some residual coronene from the LHB could still be found on Earth today, locked up in ancient rocks and so excluded from biological interactions. If this surviving material could be isolated and analyzed, it could
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Figure 6. Previous investigations and geologic context: pressuretemperature ranges associated with geologic processes such as burial, subduction, and silicate melts. We have indicated the range of our experiments as well as previous high-pressure work on coronene together with the previously suggested carbonization and graphitization boundaries.18 Dry peridotite solidus is from Hirschmann,26 carbonated peridotite solidus is from Dasgupta et al.,27 and wet peridotite solidus is from Kawamoto and Holloway.28 Subduction values are from van Keken et al.29
carry isotopic information relating to its age and mode of formation. This residual coronene could be analyzed with the improvement of molecular extraction techniques and the sensitivity of isotopic measurements. It has been observed that the aromatic compounds in the Murchison meteorite have nonterrestrial δ 13C values.30 It is therfore reasonable to assume that any coronene surviving accretion to the Earth would have a similarly nonterrestrial δ 13C value. Investigating the stability of coronene under the pressure and temperature conditions associated with the early Earth has shown that the original coronene molecules from primitive precursors of the Earth would be unaltered by many of the processes of planetary accretion and subsequent terrestrial geological processes, provided no chemical reactions with other molecules have occurred. The observed reversible structural phase transition of coronene suggests that a fraction of the coronene found on Earth could be unchanged since its existence in primitive meteorites and possibly predate the Earth. It would have been preserved through conditions experienced during the assembly and differentiation of the Earth and also Mars: a variety of PAHs, including coronene, have been observed in Martian meteorites, including ALH84001, EETA79001, and Nakhla.31 These have been attributed variously to biogenic and abiogenic sources. Our research suggests that the coronene, at least, is potentially original material. The high chemical stability of coronene found in this study implies the survival of the molecule through many planetary processes such as subduction and burial. The chemical stability regime outlined by this and other work suggests that coronene could be cycled through the rock cycle and could be present in volcanic ejecta. Coronene has already been detected in volcanic bomb and volcanic ash.1 Coronene would have been brought to the Earth during the LHB; it exists in the ISM and has been shown to survive atmospheric entry. Coronene’s potential to survive processes occurring within the Earth since the LHB implies that coronene trapped in ancient rocks could outdate the Earth and contain isotopic information about its time and mode of formation.
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Acknowledgment. We thank M. J. Walter (University of Bristol) for the loan of equipment and helpful suggestions regarding Figure 6. We gratefully acknowledge helpful discussions with L. Allamandola (NASA Ames) regarding the finer points of PAH infrared spectra. W.M. is supported by a NERC Blue Skies postdoctoral fellowship. The funding agencies were not involved in the study design, collection, analysis and interpretation of data, the writing of the report, or the decision to submit the paper for publication. References and Notes (1) Zubkov, V. S. Geochem. Int. 2009, 47, 741–757. (2) Allamandola, L. J.; Sandford, S. A.; Wopenka, B. Science 1987, 237, 56–59. (3) Cyvin, S. J.; Cyvin, B. N.; Brunvoll, J.; Whitmer, J. C.; Klaeboe, P. Z. Naturforsch. 1982, 37a, 1359–1368. (4) Szczepanski, J.; Vala, M. Astrophys. J. 1993, 414 (1), 646–655. (5) Hudgins, D. M.; Sandford, S. A. J Phys. Chem. A 1998, 102, 344– 352. (6) Langhoff, S. R. J. Phys. Chem. 1996, 100, 2819–2841. (7) Wood, J. A. Icarus 1967, 6, 1–49. (8) Stffler, D.; Keil, K.; Scott, E. R. D. Geochim. Cosmochim. Acta 1991, 55, 3845–3867. (9) Yabuta, H.; Williams, L. B.; Cody, G. D.; Alexander, C. M. O.; Pizzarello, S. Meteorit. Planet. Sci. 2007, 42, 37–48. (10) Matrajt, G.; Brownlee, D.; Sadilek, A.; Kruse, L. Meteorit. Planet. Sci. 2006, 41, 903–911. (11) Allamandola, L. J.; Tielens, A.; Barker, J. R. Astrophys J. Suppl. Ser. 1989, 71, 733–775. (12) Salama, F.; Galazutdinov, G. A.; Krelowski, J.; Allamandola, L. J.; Musaev, F. A. Astrophys. J. 1999, 526 (1), 265–273.
Jennings et al. (13) Leger, A.; Puget, J. L. Astron. Astrophys. 1984, 137, L5–L8. (14) Tielens, A. G. G. M. Annu. ReV. Astron. Astrophys. 2008, 46, 289– 337. (15) Cachier, H.; Liousse, C.; Buatmenard, P.; Gaudichet, A. J. Atmos. Chem. 1995, 22, 123–148. (16) Baek, S. O.; Field, R. A.; Goldstone, M. E.; Kirk, P. W.; Lester, J. N.; Perry, R. Water, Air, Soil Pollut. 1991, 60, 279–300. (17) Podkletnov, N. E.; Markhinin, E. K. Origins Life 1981, 11, 303– 315. (18) Davydov, V. A.; Rakhmanina, A. V.; Agafonov, V.; Narymbetov, B.; Boudou, J. P.; Szwarc, H. Carbon 2004, 42, 261–269. (19) Yamamoto, T.; Nakatani, S.; Nakamura, T.; Mizuno, K.; Matsui, A. H.; Akahama, Y.; Kawamura, H. Chem. Phys. 1994, 184, 247–254. (20) Bernard, J. P.; Dhendecourt, L. B.; Leger, A. Astron. Astrophys. 1989, 220, 245–248. (21) Boehler, R.; Ross, M.; Boercker, D. B. Phys. ReV. B 1996, 53, 556–563. (22) Mao, H. K.; Xu, J.; Bell, P. M. J. Geophys. Res., [Solid Earth Planets] 1986, 91, 4673–4676. (23) Lerch, P. Spectrometry diagram, http://sls.web.psi.ch/view.php/ beamlines/ir/endstations/ Optics_Pressure_DAC.pdf, 2009. (24) Montgomery, W.; Jennings, E.; Lerch, P.; Sherman, D. Eos Trans. Fall Meet. Suppl. 2009, 90, P11A–1198. (25) Montgomery, W.; Crowhurst, J. C.; Zaug, J. M.; Jeanloz, R. J. Phys. Chem. B 2008, 112, 2644–2648. (26) Hirschmann, M. M. Geochem., Geophys., Geosyst. 2000, 1, 1–10. (27) Dasgupta, R.; Hirschmann, M. M.; Smith, N. D. J. Petrol. 2007, 48, 2093–2124. (28) Kawamoto, T.; Holloway, J. R. Science 1997, 276, 240–243. (29) van Keken, P. E.; Kiefer, B.; Peacock, S. M. Geochem., Geophys., Geosyst. 2002, 3, 1–10. (30) Sephton, M. A. Nat. Prod. Rep. 2002, 19, 292–311. (31) Zolotov, M.; Shock, E. J. Geophys. Res. 1999, 104, 14033–14049.
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