Complex sentence: In 1918, Walther Nernst proposed that the photochemical reaction of hydrogen and chlorine is a chain reaction in order to explain the large quantum yield, meaning that one photon of light is responsible for the formation of as many as 10 6 molecules of the product HCl.
Split into simpler sentences: In 1918, Walther Nernst proposed that the photochemical reaction between hydrogen and chlorine is a chain reaction in order to explain what's known as the "quantum yield" phenomena. This means that one photon of light is responsible for the formation of as many as 10 6 molecules of the product HCl.