Balzani's first law of photochemistry is: "For a photochemical reaction to exist, the chemical in question must absorb light at the frequency used for irradiation." For example, green grass is green because chlorophyll (an Organometallic complex) absorbs other types of light but reflects green.
If you try to grow plants under green light (460nm to 600nm) - they will reflect it all and not be able to photosynthesise.
Photochemists use techniques such as:
- Quantum yield - what proportion of molecules react when a known flux of light is used to irradiate them.
- Flash Photolyis - using a high intensity Xenon flash lamp to initiate a reaction which can then be studied using absorption spectroscopy.
