Illustrated Glossary of Organic Chemistry

: To add energy via photons. From E = , where E = energy, h = Planck's constant, and ν = photon frequency.

Free radical bromination of an alkane such as methane begins with photolysis of molecular bromine via ultraviolet or visible light.

Ground state
Nuclear spin parallel to B0
Lower energy

Excited state
Nuclear spin antiparallel to B0
Higher energy
In NMR spectroscopy, absorption of an radio wave photon of energy ΔE causes excitation of a nucleus from its ground nuclear spin state to an excited nuclear spin state.