Role of excited electronic states in the high-pressure amorphization of benzene

High-pressure methods are increasingly used to produce new dense materials with unusual properties. Increasing efforts to understand the reaction mechanisms at the microscopic level, to set up and optimize synthetic approaches, are currently directed at carbon-based solids. A fundamental, but still...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2008-06, Vol.105 (22), p.7658-7663
Main Authors: Citroni, Margherita, Bini, Roberto, Foggi, Paolo, Schettino, Vincenzo
Format: Article
Language:English
Subjects:
Absorption spectra
Chemical reactions
Crystals
Electronics
Excimers
Experiments
Fluorescence
High pressure
Hydrocarbons
Methods
Molecules
Monomers
Physical Sciences
Reaction mechanisms
Reactivity
Spectrum analysis
Water pressure
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Summary:High-pressure methods are increasingly used to produce new dense materials with unusual properties. Increasing efforts to understand the reaction mechanisms at the microscopic level, to set up and optimize synthetic approaches, are currently directed at carbon-based solids. A fundamental, but still unsolved, question concerns how the electronic excited states are involved in the high-pressure reactivity of molecular systems. Technical difficulties in such experiments include small sample dimensions and possible damage to the sample as a result of the absorption of intense laser fields. These experimental challenges make the direct characterization of the electronic properties as a function of pressure by linear and nonlinear optical spectroscopies up to several GPa a hard task. We report here the measurement of two-photon excitation spectra in a molecular crystal under pressure, up to 12 GPa in benzene, the archetypal aromatic system. Comparison between the pressure shift of the exciton line and the monomer fluorescence provides evidence for different compressibilities of the ground and first excited states. The formation of structural excimers occurs with increasing pressure involving molecules on equivalent crystal sites that are favorably arranged in a parallel configuration. These species represent the nucleation sites for the transformation of benzene into amorphous hydrogenated carbon. The present results provide a unified picture of the chemical reactivity of benzene at high pressure.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.0802269105