[Reversible temperature dependence of surface-enhanced Raman scattering of adenine, purine and pyrene].

The surface-enhanced Raman scattering (SERS) spectra of adenine, purine and pyrene adsorbed on a SERS-active silver surface roughened by nitric acid have been measured over the -190 degrees C-30 degrees C surface temperature range. The results show that the Raman shifts and SERS intensity of the vibrations, which are enhanced by electromagnetic mechanism, are affected by temperature, those by chemical mechanism are not. However, it has been found that the observed temperature dependence can be explained by postulating the reorientation of the absorbates. Adenine molecules and purine molecules prefer to adopt a more perpendicular stance on the surface at high temperature, but for a planar symmetric molecule, such as pyrene, the preferred orientation is parallel and temperature has no effect on its orientation.