Photolysis:• A laser is used to dissociate O 2 molecules producing a known atomic oxygen population. The laser pulse also interacts with these O atoms over the course of the pulse causing two-photon excitation. The resulting TALIF signal is recorded. The amount of [O] produced depends on the wavelength, photon fluence and molecular O 2 density [O 2 ] the laser beam interacts with. The dissociation energy of O 2 is 5.17 eV so once the photon energy of the laser exceeds this value [O] will be generated.• Single laser shot:  = 225.6 nm (E photon = 5.5 eV) t Laser = 8 ns  Single photon absorption in the Herzberg continuum of O 2 ( = 3.21  10 -24 cm 2 at 225.6 nm ) Photo-dissociation of O 2  2 O( 3 P) atoms. Fast process (~10 -13 -10 -14 s)  Laser then excites these O atoms via 2-photon absorption  TALIF signalWhere : Photo-dissociation cross section at  Laser : Photon fluence.• Laser pulse energy E pulse : 1.2 mJ • Laser Frequency f: 1.33  10 15 Hz.• Focal spot area A: 6.03  10 -9 m 2 .As the resulting atomic oxygen density is a function of the laser energy, [O] will vary over the course of the laser pulse. The laser pulse has a Gaussian temporal behaviour so the resulting [O] generated will also vary in a Gaussian manner. Using atmospheric oxygen as our calibrating gas (atm. Pressure = 1.013  10 5 Pa, temperature = 300 K, humidity 0.45) the overall atomic oxygen density value at the end of laser pulse [O] Cal is 7.08  10 19 m -3 .• The atomic O produced by photolysis are "hot". E O(hot) = E photon(225.6 nm) -E dissociation = 5.5 -5.17 = 0.33 eV  O fragments have velocities of up to 1411 m/s so some may leave the laser focal zone before twophoton absorption occurs.• t Laser = 8 nm  O hot atoms travel ~ 11.3 m while laser on so ~ 45 % of O hot potentially lost from the focal zone over the duration of the laser pulse.However due to the small mean free path at atmospheric pressures (~ 66 nm) the root mean square diffusion distance x RMS = 4 = 9.95  10 -7 m so only ~ 4.5 % of the atomic O leave the focal zone over the duration of the laser pulse.O atoms in the red zone can potentially leave the laser focal zone and so will not contribute to the TALIF signal recorded• The hot oxygen atoms produced during photolysis will experience enhanced quenching due to their high velocities when compared with O atoms produced in the plasma. This affects the quenching factor Q which will affect the branching ratio a ij = A ij /(A + Q) for the excited state of these "hot" O atoms.