Initiation • Initial generation of free radicals is slow – Initiated by singlet oxygen (1O2) • metastable, excited energy state of O2 • two unpaired electrons in same orbital triplet oxygen ground state 2 electrons w/ same spin in 2 orbitals singlet oxygen excited state 2 electrons w/ different spin in 1 orbitalInitiation • Initial generation of free radicals is slow – Initiated by singlet oxygen (1O2) • metastable, excited energy state of O2 • two unpaired electrons in same orbital triplet oxygen ground state 2 electrons w/ same spin in 2 orbitals singlet oxygen excited state 2 electrons w/ different spin in 1 orbital