Homogeneous and inhomogeneous line shape of the electronic excitation of a single molecule on a surface

The homogeneous line profile of a single molecule adsorbed on a surface was measured by high resolution laser spectroscopy. For the prototype organic molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) adsorbed at step sites of a thin epitaxial film of KCl on Ag(100) we measure a zero phonon line of Lorentzian shape with a full width at half maximum of 460 +/- 25 MHz at a temperature of 6 K. The line is considerably narrower than those which have been recorded in fluorescence spectra of this and similar single molecules that were excited under the tip of a scanning tunneling microscope. The related time is, however, by a factor of similar to 15 smaller than the lifetime of PTCDA recorded in liquid He droplets, likely due to thermal dephasing. From this knowledge on the single molecule line profile, we also explain the inhomogeneous line profile of an ensemble of PTCDA molecules by a quantitative model based on surface defects.

Automated summary

High-resolution laser spectroscopy was used to measure the homogeneous line profile of a single molecule adsorbed on a surface. The study focused on the prototype organic molecule PTCDA adsorbed on KCl thin epitaxial film on Ag(100). The narrow zero-phonon line shape was observed, and by understanding the single molecule line profile, a quantitative model based on surface defects was used to explain the inhomogeneous line profile of an ensemble of PTCDA molecules.