Dissociative Excitation Of O2 Research Articles

The generation of Ganymede's diffuse aurora through pitch angle scattering

Abstract. Diffuse auroral intensities of neutral atomic oxygen OI λ1356 Å emission on Ganymede due to whistler mode waves are estimated. Pitch angle diffusion of magnetospheric electrons into the loss cone due to resonant wave–particle interaction of whistler mode waves is considered, and the resulting electron precipitation flux is calculated. The analytical yield spectrum approach is used for determining the energy deposition of electrons precipitating into the atmosphere of Ganymede. It is found that the intensities (4–30 R) calculated from the precipitation of magnetospheric electrons observed near Ganymede are inadequate to account for the observational intensities (≤ 100 R). This is in agreement with the conclusions reached in previous works. Some acceleration mechanism is required to energize the magnetospheric electrons. In the present work we consider the heating and acceleration of magnetospheric electrons by electrostatic waves. Two particle distribution functions (Maxwellian and kappa distribution) are used to simulate heating and acceleration of electrons. Precipitation of a Maxwellian distribution of electrons can produce about 70 R intensities of OI λ1356 Å emission for electron temperature of 150 eV. A kappa distribution can also yield a diffuse auroral intensity of similar magnitude for a characteristic energy of about 100 eV. The maximum contribution to the estimated intensity results from the dissociative excitation of O2. Contributions from the direct excitation of atomic oxygen and cascading in atomic oxygen are estimated to be only about 1 and 2 % of the total calculated intensity, respectively. The findings of this work are relevant for the present JUNO and future JUICE missions to Jupiter. These missions will provide new data on electron densities, electron temperature and whistler mode wave amplitudes in the magnetosphere of Jupiter near Ganymede.

Investigation of O-atom kinetics in O2, CO2, H2O and O2/HMDSO low pressure radiofrequency pulsed plasmas by time-resolved optical emission spectroscopy

In the present paper, O-atom kinetics is investigated in low pressure (around 0.7 Pa) O2/hexamethyldisiloxane pulsed plasmas by time-resolved optical emission spectroscopy. In order to distinguish direct O-atom excitation by electron impact from dissociative excitation of O2, CO, CO2 and H2O molecules formed in O2/HMDSO plasmas, measurements were first carried out in pure O2, CO2 and H2O plasmas. From these measurements it was concluded that direct O-atom excitation by electron impact was the dominant mechanism and that actinometry could be used to monitor O-atom kinetics provided molecular oxygen dissociative excitation is taken into account. Second, the surface O-atom loss coefficient measured in oxygen pulsed plasmas is in agreement with the literature (10−1) and is five times higher than the one measured in CO2 pulsed plasma (2 × 10−2). This latter result is attributed to competition effect at surface sites between CO and O. Finally, we point out the influence of water molecule wall adsorption on O-atom kinetics in H2O and O2/HMDSO pulsed plasmas. During the post-discharge, H2O and/or OH molecules adsorbed at the reactor walls strongly reduce the O-atom loss coefficient. During the discharge, ion bombardment enhances desorption of molecules leading to an increase in the O-atom loss coefficient.

Electron impact dissociative excitation of O2: 2. Absolute emission cross sections of the OI(130.4 nm) and OI(135.6 nm) lines

In this work, we report the OI(135.6 nm) absolute emission cross section resulting from the long‐lived (180 μs) OI(5S → 3P) transition from dissociative excitation of O2. From the ratio of the integrated intensities of the OI(135.6 nm) and OI(130.4 nm) features and from the absolute emission cross section for the OI(130.4 nm) emission feature from electron impact dissociative excitation of O2 at 100 eV, the absolute emission cross section for the OI(135.6 nm) feature was determined to be 6.4 × 10−18 cm2 at 100 eV. Electron impact‐induced optical excitation functions for optically allowed transitions at 115.2 nm and 130.4 nm and for an optically forbidden transition at 135.6 nm were also obtained over the electron impact energy range 0–600 eV. The OI(135.6 nm) emission cross section was measured in the laboratory utilizing a large collision chamber (1.5 m in diameter). Electrons were produced with an electrostatically focusing gun with a large focal length (50 cm). The OI(130.4 nm, 135.6 nm) excitation functions were put on an absolute scale as described in the text, and the OI(135.6 nm)/OI(130.4 nm) ratio was determined for the entire energy range (0–600 eV). The atomic O UV emission cross sections from dissociative excitation of O2 can be used to model the recent Hubble Space Telescope observations of OI(130.4 nm) and OI(135.6 nm) intensities from Ganymede [Feldman et al., 2000] and Europa [Hall et al., 1995, 1998].

Electron-impact studies of atomic oxygen:II. Emission cross section measurements of theO I 3So→ 3P transition(130.4 nm)

The optical excitation function of theO I 3So→ 3P transition (130.4 nm),produced by electron-impact excitation of atomic oxygen, hasbeen measured over an extended energy range from threshold to1.0 keV. Measurements were obtained in a crossed-beamexperiment using both magnetically confined andelectrostatically focused electrons in collision with atomicoxygen produced by a microwave discharge source. A 0.2 m vacuumultraviolet monochromator system was used to measure theemitted O I radiation at 130.4 nm. The relative O I(130.4 nm) emission intensity corresponding to the 3So→ 3P transition was then put on the absolute scale bynormalization to the O I (130.4 nm) cross sectionproduced by dissociative excitation of O2 at 30 eV(Kanik et al 2000).

Emission cross section of O I (135.6 nm) at 100 eV resulting from electron‐impact dissociative excitation of O2

In this Letter, we report for the first time, the ratio of the O I (135.6 nm)/O I (130.4 nm) absolute emission cross sections from electron‐impact dissociative excitation of O2 at 100 eV using facilities located at the University of Colorado, Laboratory for Atmospheric and Space Physics (LASP). The O I (135.6 nm) emission glow produced in the laboratory from the long‐lived transition O I g³P ← 5S° was measured in a large collision chamber (1m diameter) with a long focal length (50 cm) electrostatic electron gun. The ratio of the absolute emission cross sections of the O I (135.6 nm) and O I (130.4 nm) features resulting from electron‐impact dissociative excitation of O2 at 100 eV was found to be 2.3±0.6. From this ratio, the absolute emission cross section for the O I (135.6 nm) feature at 100 eV electron‐impact energy was determined to be (6.4±2.2)×10−18 cm². The O I (135.6 nm) cross section reported here can be used to better understand the recent Hubble Space Telescope observations of the O I (130.4 nm) and O I (135.6 nm) emission intensities from Ganymede [Feldman et al., 2000] and Europa [Hall et al., 1995].

Characterization of transformer coupled oxygen plasmas by trace rare gases-optical emission spectroscopy and Langmuir probe analysis

Trace rare gases-optical emission spectroscopy (TRG-OES) and Langmuir probe analysis have been used to measure the electron temperature, Te, in a high-density inductively (transformer) coupled (TCP) 10 mTorr oxygen plasma as a function of the 13.56 MHz radio frequency (rf) power. The oxygen atomic densities were estimated by O-atom optical emission (8446 Å), and rare gas actinometry (Ar, 7504 Å). In the H-(inductive)-mode, Te increases from 2.6 to 3.4 eV for the low-energy electrons sampled by the Langmuir probe and from ~3.5 to 6.0 eV for the high-energy electrons sensed by TRG-OES as the rf power is increased from 120 to 1046 W. In the E-(capacitive)-mode, below 50 W, Te measured by TRG-OES increases with rf power from ~4 eV at very low power (~7 W) to ~6.1 eV at 45 W. Between the highest E-mode power (~50 W) and lowest H-mode power (~120 W), the Te measured by TRG-OES drops from 6.1 to 3.5 eV, while Te derived from Langmuir probe measurements drops only slightly from 3.0 to 2.6 eV. In the H-mode, the electron energy distribution function (EEDF) is bi-Maxwellian from ~120 to 1046 W. In the E-mode, the EEDF changes from nearly Maxwellian (possibly Druyvesteyn) at low rf powers (~7 W) to bi-Maxwellian at the higher E-mode powers (~45 W). O2 dissociation is low (~2%) at the maximum rf power density of 5.7 W cm-2 (1046 W), and this low value is attributed to the high rate of O-atom recombination on the mostly stainless-steel walls. A detailed accounting of the sources of O (8446 Å) emission revealed significant contributions from electron impact excitation from O(1S) and dissociative excitation of O2.

Studies of atomic oxygen in O2+CF4 rf discharges by two-photon laser-induced fluorescence and optical emission spectroscopy

We have used two-photon laser-induced fluorescence to obtain quantitative measurements of the concentration of ground state O atoms in O2+CF4 rf discharges. Absolute calibration was achieved by generating a known concentration of atomic oxygen by UV laser photolysis of O2. Trace amounts of Ar were added to serve as an inert reference gas for concurrent optical emission measurements, in which the plasma-induced optical emission intensities from O* and Ar* lines were recorded. Emission line shapes were measured using a Fabry–Perot interfermoter to gain information on the mechanisms for formation of excited oxygen atoms in the plasma. Two excitation mechanisms were found to be important: (1) electron impact excitation of ground state atoms, e+O → O*+e, and (2) dissociative excitation of O2, e+O2 → O*+O+e. Evidence for both excitation mechanisms was obtained for O* (8446 Å) emission, with atomic excitation being dominant, whereas dissociative excitation appeared to be the dominant mechanism for O* (7774 Å) emission. Argon actinometry for the determination of ground state oxygen was directly tested. Because of the contribution from dissociative excitation, a strict proportionality, O*/Ar*∝[O]/[Ar], was not satisfied where O* (Ar*) is the intensity of an atomic oxygen (argon) emission line, and [O] ([Ar]) is the oxygen (argon) atom concentration. However, within certain limitations, the O* (8446 Å)/Ar* emission intensity ratio gives the right qualitative trends for the O atom concentration.

Dissociative Excitation of O2 in the Vacuum Ultraviolet by Electron Impact

The emission cross section of the 1304-Å resonance triplet (3S0→3P) of atomic oxygen produced by dissociative excitation of O2 by electron impact was measured from threshold to 300 eV. The peak cross section is at 100 eV with a value of 3.00×10−18 cm2. The relative emission cross section of the 1356-Å multiplet (5S0→3P) of atomic oxygen was also determined. The peak cross section for the 1356-Å multiplet at 100 eV is at least 1.65×10−18 cm2.

Photoelectron excitation of atomic-oxygen resonance radiation in the terrestrial airglow

Recent laboratory measurements of the absolute cross sections for the excitation of the O I (3S) resonance state by electron impact on O and O2 when combined with in situ measurements of the photoelectron energy distribution from 120 to 300 km show that photoelectron impact is the principal excitation mechanism for λ1304-A resonance radiation in the dayglow; dissociative excitation of O2 is found to play a minor role. The laboratory measurements indicate that the 3S state is strongly populated by cascade processes and imply that the O I(3p3P → 3s3S; λ8446 A) transition should be a prominent dayglow emission feature. These experiments also show that the excitation of atomic oxygen by low-energy electron impact cannot account for the λ1304- or 1356-A emission observed in the tropical ultraviolet airglow.