Two-photon absorption laser induced fluorescence measurement of atomic oxygen density in an atmospheric pressure air plasma jet

Abstract

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.

Progresses on the Use of Two-Photon Absorption Laser Induced Fluorescence (TALIF) Diagnostics for Measuring Absolute Atomic Densities in Plasmas and Flames

“…As an example, the quenching time of excited H atoms in flames can fall well below ns at atmospheric pressure [47], being faster than the pulse duration of ns lasers (typically few ns). In this case, to infer the quenching rate and effective lifetime of species from the fluorescence signal, the use of adequate models can be helpful on the correction of the TALIF signal [32,65,106]. However, the development of those models is based on different assumptions, limiting thus their accuracy.…”

Simultaneous quantification of atomic oxygen and ozone by full photo-fragmentation two-photon absorption laser-induced fluorescence spectroscopy

“…Although the fluorescence by the 226 nm TALIF laser alone corresponds to O atoms from the same two sources as the dual-laser case, namely the discharge and O 3 photo-fragmentation. However, the three-photon process, namely photo-dissociation of O 3 and subsequent two-photon excitation of O fragment, by the same 226 nm laser is complicated from point of quantitative analysis [28,30,39]. Because the ratios, of how much laser energy is consumed by O 3 photolysis, how many O 3 molecules are photo-dissociated, and how many ground-state O fragments are produced and contribute to the two-photon excitation process during the short laser pulse (10 ns here), are unknown.…”

Two-photon absorption laser induced fluorescence: rate and density-matrix regimes for plasma diagnostics

“…Photolytic phenomena are observed in TALIF experiments performed using ns-lasers [25,38,39]. For example, in oxygen containing plasmas, photodissociation of ozone or molecular oxygen due to the laser intensity at 225 nm, may occur at important rates.…”