Citroni Margherita, Bini Roberto, Foggi Paolo, Schettino Vincenzo
LENS, European Laboratory for Non-Linear Spectroscopy, Via Nello Carrara 1, I-50019 Sesto Fiorentino, Florence, Italy.
Proc Natl Acad Sci U S A. 2008 Jun 3;105(22):7658-63. doi: 10.1073/pnas.0802269105. Epub 2008 May 27.
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.
高压方法越来越多地用于制备具有异常性质的新型致密材料。目前，人们越来越努力地从微观层面理解反应机理、建立并优化合成方法，这些努力主要针对碳基固体。一个基本但仍未解决的问题是分子体系的高压反应性中电子激发态是如何参与其中的。此类实验中的技术难题包括样品尺寸小以及由于吸收强激光场而可能对样品造成的损害。这些实验挑战使得通过线性和非线性光谱学直接表征高达几吉帕压力下电子性质随压力的变化成为一项艰巨的任务。我们在此报告了在压力下对分子晶体进行双光子激发光谱的测量，在苯（典型的芳香体系）中压力高达12吉帕。激子线的压力位移与单体荧光之间的比较为基态和第一激发态不同的压缩性提供了证据。随着压力增加，结构准分子形成，涉及等效晶体位点上排列成平行构型的分子。这些物种代表了苯向非晶态氢化碳转变的成核位点。目前的结果提供了苯在高压下化学反应性的统一图景。
