Search for dynamical charge transfer excitations of CO and pyridine on copper by EELS

There are many indications, that the so-called chemical effect in surface enhanced Raman scattering (SERS) is caused by dynamic charge transfer excitations between metal and adsorbate. Consequently there have been attempts to observe these excitations by independent spectroscopies. Here we report electron energy loss spectroscopy (EELS) results on Cu(1 1 1) and cold-deposited copper with an energy resolution below 100 meV. We have not found any structure which may be assigned to these excitations, in contrast to some literature reports. On the other hand charge transfer excitations have been clearly observed by 2-photon photoemission by others. We propose an explanation, why EELS and linear optical spectroscopies are not well suited to observe the charge transfer excitations, which are so crucial in the “first layer effect” of SERS.