  Using angle-resolved photoemission spectroscopy, we show the direct evidence
of charge transfer between adsorbed molecules and metal substrate, i.e.
chemisorption of CO on Pt(111) and Pt-Sn/Pt(111) 2x2 surfaces. The observed
band structure shows a unique signature of charge transfer as CO atoms are
adsorbed,revealing the roles of specific orbital characters participating in
the chemisorption process. As the coverage of CO increases, the degree of
charge transfer between CO and Pt shows clear difference to that of Pt-Sn. With
comparison to DFT calculation results, the observed distinct features in the
band structure are interpreted as backdonation bonding states of Pt molecular
orbital to the 2{\pi} orbital of CO. Furthermore, the change in the surface
charge concentration, measured from the Fermi surface area, shows Pt surface
has a larger charge concentration change than Pt-Sn surface upon CO adsorption.
The difference in the charge concentration change between Pt and Pt-Sn surfaces
reflects the degree of electronic effects during CO adsorption on Pt-Sn.
