Dresden 2006 – scientific programme
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O: Oberflächenphysik
O 9: Organic films I
O 9.11: Talk
Monday, March 27, 2006, 17:30–17:45, TRE Phys
Temperature effect in branching of photochemical reactions — •Zharnikov Michael1, Andrey Shaporenko1, Andrea Baumer2, Dietrich Menzel3, and Peter Feulner3 — 1Angewandte Physikalische Chemie, Universität Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg — 2Walter Schottky Institut, Am Coulombwall 3, D-85748 Garching — 3Physikdepartment E20, Technische Universität München, D-85747 Garching
Radiation-induced damage represents a severe constraint for the characterization of organic and biological materials by advanced electron or x-ray spectroscopy and microscopy. A possibility to reduce damage is cooling of the samples down to cryogenic temperatures. The protective effect of cooling is commonly related to hindrance of mass transport in the object, whereas the basic irradiation-induced bond cleavage is believed to be unaffected. To prove this hypothesis we studied radiation damage of self-assembled monolayers, which are prototypes of thin organic layers and highly organized biological systems. We demonstrate that the effect of cooling is twofold. It freezes the structure, but by decreasing the mobility of fragments it also changes the branching of various photochemical reactions, thereby strongly modifying the cross sections as well as the products of irradiation induced processes. Two limiting cases could be identified. Reactions involving transport of single atoms and small fragments proceed nearly independent of temperature. Reactions requiring transport of heavy fragments are, however, efficiently quenched by cooling. We speculate that bonds can recombine if the fragments are forced to stay in place due to their reduced mobility at low temperatures.