INVESTIGATION OF LASER-INDUCED EFFECTS IN MOLECULAR LAYERS BY SCANNING-TUNNELING-MICROSCOPY

Aiming at the detection of laser-induced currents caused by resonant o ptical excitation of adsorbed molecules with a scanning tunneling micr oscope, we have developed a method that provides access to very small laser-induced effects normally hidden in the thermal background. An op tical compensation setup based on two lasers with different wavelength s for controlling very precisely the thermal signal, together with a s pecial scheme for signal averaging and interpolation, provides access to laser-induced signals down to < 100 fA. We apply this method to mol ecular films consisting of islands of the dye perylene-tetracarboxylic -dianhydride (PTCDA) embedded in the liquid crystal octylcyanobiphenyl (8CB). Under complete compensation of the background, a statistically significant residual laser-induced current has been observed on the P TCDA islands with a magnitude of 125 fA(rms) at a modulated laser inte nsity of 3.5 kW(rms)/cm(2).