Robin Hill experiment: He took the isolated chloroplasts of Stellaria and placed them in water having hydrogen acceptors (like chromates, ferricyanides, cytochromes, benzoquinones etc.) He illuminated water and the oxygen was liberated in the absence of carbondioxide.

The hydrogen acceptors are called Hill oxidants and the process of liberation of oxygen in the absence of carbon dioxide is called Hill oxidation. In plants hydrogen acceptor is NADP⁺