The 3D orbital is not completely occupied in the 3D metals and leads to the sharp peak in the EELS spectra. In metallic copper it I completely occupied again and cannot be excited anymore since no electron can go there in recombination.

Have a look at http://www.webelements.com/copper/atoms.html for the configurations and compare for examplewith  the iron configuration where the 3D orbital is not completely occupied. http://www.webelements.com/iron/atoms.html

If you oxidize Cu one electron is leaving the shell and an unoccupied state is generated again. This explains why Cu does not and CuO has white lines.

Bert