Analysis of complex thermal desorption spectra: PTCDA on copper

Organic molecules often show very complex thermal desorption spectra. If there is an ordered structure the activation energy for desorption Edes will decrease within the first few layers because of the decreasing van der Waals interaction between molecules and substrate. This is specially true for the system 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on Cu(1 1 1). The thermal desorption spectra for this particular system consist of three different signals which can be attributed to the second layer, the multilayer and a phase consisting of nanocrystals. We did not see any desorption from the first layer of PTCDA on Cu(1 1 1). In the first part of this paper, we will outline a numerical algorithm to evaluate the spectra with respect to the desorption energy of the second layer (Edes,secondlayer=2.35eV) and the multilayer (Edes,multilayer=2.2eV). In agreement with the transition state theory, we found a pre-exponential factor of 4×1019s-1. Furthermore, we will show that nanocrystals have a contribution to the thermal desorption spectra different from the one of the multilayer. By evaluation of the third, high temperature signal it is possible to get parameters which describe the distribution of the nanocrystals and therefore gain further understanding of their growth.