Abstract
The characteristic energy loss levels are derived phenomenologically for a target metal having an optical absorption band of similar energy to the free electron plasma frequency. The various relative positions of the band and plasma frequency are considered and it is shown that the presence of an optical band introduces further particle absorption levels and may shift in either direction or completely screen out the plasma line. The theory agrees with the general experimental picture, and predicts certain results which had hitherto been unsatisfactorily accounted for, but the lack of information concerning optical absorption bands in the relevant energy region precludes any detailed comparison for a particular metal.