LASER INDUCED HYDROGEN MOTION IN ORGANIC CRYSTALS AT LOW TEMPERATURE

Abstract

We study here a laser induced reaction leading to a rearrangement of the acid protons in benzoic acid crystals at very low temperature. This hydrogen motion results from the electronic excitation of pentacene molecules substitutionally included into a benzoic acid host matrix, protonated or deuterated. The growth of a deuterated benzoic acid crystal doped with pentacene leads to a partial deuteration of pentacene molecules, resulting in several origin bands in the absorption spectrum. The dynamics of the system following non selective and selective laser irradiations of the differently deuterated pentacene molecules is analyzed in terms of hydrogen Introduction. Hydrogen motion in solids at low temperature is intensively studied with a wide variety of methods. Among them, optical spectroscopy is a very sensitive one, specially when performed on crystals giving narrow spectral lines. Carboxylic acid crystals represent a class of model systems commonly used for studies on proton transfer 11-51, In such a system where two molecules associate to form a dimer bound by two hydrogen bonds, the hydrogen motion consists in a double exchange of the acid protons along the two hydrogen bonds giving rise to a tautomer form. For an isolated dimer the two tautomer forms are identical and the potential energy may be represented by a symmetrical double well. In the crystal the two forms are no longer equivalent and the double well becomes asymmetric, modifying the protons dynamics. An optical way of monitoring the proton movement is to replace a dimer by a dye impurity: any change in the dye environment, a tautomerization in the neighboring dimer for example, is reflected in its absorption spectrum. A particularly interesting feature of such a system is that the optical excitation of the dye molecule induces the desired environmental change which is then probed using the same dye molecule. By such a method, it was possible to study the protons tunneling mechanism in mixed thioindigolbenzoic acid crystals at very low temperature 161. In the system studied here, composed of benzoic acid (BA) as the matrix and pentacene (PC) as the impurity, it has been proved that the induced modification of the neighborhood was not a double proton exchange but a more severe rearrangement 171. This conclusion was drawn from the fact that the induced changes were very long-lived (10 orders of magnitude longer than in the tautomerization case), and that the energy difference between the two states was at least 30 times larger than the one reported in the literature for a tautomerization in a BA crystal 181. A proposed mechanism was an