The photoemission threshold value for gold (Au) can be interpreted as the work function. The photoemission threshold value for the semiconductor silicon (Si) can be interpreted as the valence band maximum energy. It is often referred to as the ionization potential. The photoemission threshold value for copper phthalocyanine (CuPc) can be interpreted as the HOMO energy and is often referred to as the ionization potential.

Fig. 1 The photoemission threshold energy of gold (Au) ,
silicon (Si) and copper phthalocyanine (CuPc)