The evolution in pressure of electronic states of benzene was recently studied with non-linear optic spectroscopic techniques, allowing the identification of the primer mechanism of the high-pressure transformation of benzene in amorphous hydrogenated carbon.
The results of the research done by Margherita Citroni, Roberto Bini, Paolo Foggi and Vincenzo Schettino, for the Laboratorio Europeo di Spettroscopie Non lineari (Lens) of the University of Florence, were published in the online version of the Pnas journal on May 26, 2008 and in the hard copy version on June 3.
The high-pressure engagement (10-1000 Kbar) over the last few years brought the discovery of new materials that the preparation of already noted substances but not characterized as making these new synthetic approaches interesting.
In this experiment carried out at Lens by Margherita Citroni, the florescence that the absorption profile of two crystal photons of benzene under 200 Kbar of pressure was measured.
The measure of the absorption was particularly interesting as it was the first time it was carried out in conditions of such drastic pressure.
The comparison of the two data sets permitted the observation of the different behaviour of the compression of the electronic states of the system, bringing about an alteration of photophysical and photochemical processes.
In particular, the high-pressure formation of an excimer, most likely the highly reactive type that triggers the transformation of benzene into amorphous hydrogenated carbon.
The formation of this type, a sort of structural dimer, comes about due to the effect of the pressure under the action of the collection motions of the crystal that allow the realization between prime molecules close to the threshold distance of 2.6 A, under which excimers form.
Otherwise, the same types can be formed at low pressure, with the consequential reduction of the reaction threshold, following the absorption of sufficient radiation to populate, in catalytic quantity, in electronic states excited by the crystal.
