Dissociative Excitation Of H2 Research Articles

The Generation of Saturn's Aurora at Lower Latitudes by Electrostatic Waves

AbstractIn the present paper we have modeled diffuse auroral emissions, which have been observed at lower latitudes as compared to the main auroral arc at Saturn. It is generally accepted that these emissions are generated by precipitation of hot population of magnetospheric electrons on the closed field lines. We have considered the pitch angle diffusion by electrostatic electron cyclotron harmonic waves as the mechanism to diffuse trapped electrons into the atmospheric loss cone. Electrons are thereby precipitated into the atmosphere and excite the neutral atmospheric constituents producing auroral emissions. Calculation of ultraviolet emission intensities has been performed at two L‐shells 5.35 and 7.0. Observed hot electron distribution functions and observed electrostatic electron cyclotron harmonic wave characteristics have been used in the study. Intensities of Lyman‐alpha emission from excitation of atomic H and Lyman‐alpha produced from dissociative excitation of H2 have been calculated, obtaining the values 20.1 R (1.8 R) and 1.6 kR (0.29 kR), respectively, for both L‐shells 7.0 (5.35).Further, intensities of Lyman and Werner bands of H2 are also calculated. These have the values 7.50 kR (1.43 kR) and 7.52 kR (1.49 kR), respectively, for L‐shells 7.0 (5.35). It is observed that the volume excitation rates of these four excitations peak at an altitude of about 2,000 km. From the range‐energy relation in H2 gas it is estimated that the electrons producing these excitations should have energies less than about 250 eV.

Simulation and optical spectroscopy of a DC discharge in a CH4/H2/N2 mixture during deposition of nanostructured carbon films

Two-dimensional numerical simulations of a dc discharge in a CH4/H2/N2 mixture in the regime of deposition of nanostructured carbon films are carried out with account of the cathode electron beam effects. The distributions of the gas temperature and species number densities are calculated, and the main plasmachemical kinetic processes governing the distribution of methyl radicals above the substrate are analyzed. It is shown that the number density of methyl radicals above the substrate is several orders of magnitude higher than the number densities of other hydrocarbon radicals, which indicates that the former play a dominant role in the growth of nanostructured carbon films. The model is verified by comparing the measured optical emission profiles of the H(n ≡ 3), C 2 * , CH*, and CN* species and the calculated number densities of excited species, as well as the measured and calculated values of the discharge voltage and heat fluxes onto the electrodes and reactor walls. The key role of ion–electron recombination and dissociative excitation of H2, C2H2, CH4, and HCN molecules in the generation of emitting species (first of all, in the cold regions adjacent to the electrodes) is revealed.

Cross sections for dissociative ionization and dissociative excitation of H 2 + by single photon absorption

The photo-absorption cross sections for the dissociative ionization and dissociative excitation of H 2 + are calculated for photon polarization parallel and perpendicular to the internuclear axis. The wavefunctions for the initial and final states are prepared using the Born-Oppenheimer approximation. For dissociative ionization, the cross section and angular distribution of photo-electrons are compared with those calculated with the fixed-nuclei approximation. For dissociative excitation, the cross sections for H (N = 1∼4) productions are shown.

Energy distributions of neutrals and charged species in hollow cathode H2 discharges: a study of fast H atoms

A joint study of atomic and molecular emission spectroscopy, ion mass spectrometry and Langmuir probes is presented for a hollow cathode H2 discharge at pressures 0.7–20 Pa, rendering energy distributions of the different groups of particles that span five orders of magnitude. The H Balmer line profiles can be decomposed into three contributions (‘narrow peak’, ‘plateau’ and ‘far wings’) with very different widths. The narrow peak and the plateau are largely attributed to electron impact dissociative excitation and ionization of H2, and display Doppler temperatures of ∼0.3 eV and ∼6 eV, respectively. The energy-resolved mass spectra of H+ confirm the assignment of the plateau region. The far wings give rise to emissions extending to Doppler shifts of 0.5 nm, which correspond to translational energies of ∼300 eV. The dependence of the relative intensities of these components on pressure is studied. With a grounded grid before the observation window, a structure is induced in the blue-shifted wing. Assuming that the structure is solely caused by dissociative excitation of H2 upon collisions with the plasma ions (H+, and ), their relative concentrations can be estimated. These estimations are in very good agreement with the values derived from independent mass spectrometric measurements.

Absolute densities of energetic hydrogen ion species in an abnormal hollow cathode discharge

We develop an optical measurement for densities of fast (units to tens of keV) hydrogen ions in an abnormal hollow cathode discharge in units to tens of mTorr pressure range. This method combines results from previous collisional-radiative models, comparing the intensity of Balmer H_{alpha} due to dissociative excitation of H2 by fast electrons to Doppler-shifted emission arising from charge exchange of energetic ions. The method requires only two inputs: the current density due to fast electrons and a single H_{alpha} spectrum of the characteristic emission channel at the anode. We model in particular the cylindrical interelectrode discharge of an inertial electrostatic confinement device. Experimentally, we find that the density of fast ions emerging from the cathode (bias -5 kV at 20 mTorr ) is in the order 10;{14}m;{-3} , increasing approximately linearly with current in the 10-30 mA range. Calculated densities agree with values obtained in similar apparatus using Langmuir probes and analysis of dust particle motion.

Fine‐structure physical chemistry modeling of Uranus H2 X quadrupole emission

A new hydrogen physical chemistry model has been developed at the fine‐structure level for application to the giant outer planet thermospheres. The model is applied to Uranus because observations of dayglow H2 X 1Σg+ (v) quadrupole and H3+ vibration‐rotation emission made at NASA IRTF and UKIRT provide critical constraints for thermospheric modeling. The observed H3+ vibration‐rotation emission infers an H3+ dominant ionosphere, predicted only for non‐LTE H2 X (v : J). Excitation mechanisms explored are solar and non‐solar electron energy deposition. Non‐solar electron forcing is constrained by the EUV H2 Lyman and Werner band emission measured by Voyager UVS. Analysis indicates that non‐solar electrons are dominant in the energy budget required to predict the observed thermospheric temperature profile. The modeled H2 X quadrupole emission infers that an additional mechanism is required to excite the H2 X (v = 1) population. Non‐thermal H produced in dissociative excitation of H2 X is a primary candidate.

Polarization ofBalmer-α radiation following electron impact on atomichydrogen

The polarization of Balmer-α radiation excited incollisions of electrons with atomic hydrogen is presented for anelectron energy range from threshold to 1000 eV. Measurementsare in good agreement with calculations carried out usingeither convergent-close-coupling or R-matrix withpseudo-states approaches. Cascade is demonstrated tohave a significant effect. Balmer-α excitation functiondata are also presented. A previous measurement of thepolarization of Balmer-α following dissociativeexcitation of H2 by electrons isconfirmed and extended.

Electron‐impact excitation and emission cross sections of the H2 Lyman and Werner Systems

Excitation functions of the H2 Lyman (Ly) ( B1∑∑u ‐ X1∑∑g) and Werner (W) (C1∏u ‐ X1∑∑g ) band systems have been reanalyzed using a combination of measurements and theoretical considerations. These systems are prominent emitters in outer planet atmospheric dayglow and auroral activity and can be used to infer energy deposition and heating rates. Earlier measurements of the cross sections reported by Shemansky et al. [1985a] have been found to be inaccurate in the threshold energy region. A combination of high‐resolution spectral and shape function measurements obtained at the Jet Propulsion Laboratory (JPL) in low‐ and medium‐energy regions have been used to obtain relative excitation shape functions of the two systems. At energies above 250 eV, we have used the measurements obtained earlier by De Heer and Carrière [1971] in order to define the first two terms of the Born electric dipole collision strength. The complete set of collision strength terms is obtained by combining the JPL and De Heer and Carrière [1971] data. We have found that, contrary to results of Shemansky et al. [1985a], the shape functions of the H2 Ly and W systems, expressed in units of threshold energy, are the same within experimental error. Absolute cross sections of the H2 Ly and W systems are established using the theoretical oscillator strengths of Abgrall et al. [1987, 1993a, b, c] and Abgrall and Roueff [1989]. The atomic hydrogen H Ly α emission cross section resulting from dissociative excitation of H2 at 100 eV obtained from relative intensities of H2 W emission lines is also derived and compared with other experimental measurements. At a gas temperature of 300 K and electron energy of 100 eV, the cross sections for the H Ly α, H2 Ly, and W emissions are (0.716 ± 0.095) × 10−17, (2.62 ± 0.34) × 10−17, and (2.41 ± 0.31) × 10−17 cm2, respectively. The accuracy of the absolute values and shape functions is limited primarily by electron gun performance, as was the case for Shemansky et al. The rotational dependence of the transition dipole matrix elements and perturbations between the B 1∑+u and C1∏+u states have a significant effect on the cross sections.

The middle ultraviolet‐visible spectrum of H2 excited by electron impact

The electron‐impact‐induced emission spectrum of H2 has been measured in the extended wavelength region 175–530 nm at a spectral resolution of 1.7 nm (full width at half maximum). The laboratory spectra observed in the middle ultraviolet (MUV) and visible spectral region are characterized by underlying H2 ( ) continuum emission, together with many strong lines assigned to the radiative decay of the gerade singlet states of H2, and to members of the H Balmer series resulting from dissociative excitation of H2. Our calibrated MUV spectral data, obtained at 14, 19, and 100 eV electron‐impact energies, provide absolute emission cross sections of these H2 lines and will assist in the interpretation of planned Galileo ultraviolet spectrometer observations of Jupiter's aurora in this wavelength region.

Line profile of H Lyman- beta emission from dissociative excitation of H2.

A high-resolution ultraviolet spectrometer was employed for a measurement of the H Lyman-Beta(H L(sub Beta)) emission Doppler line profile at 1025.7 A from dissociative excitation of H2 by electron impact. Analysis of the deconvolved line profile reveals the existence of a narrow central peak, less than 30 mA full width at half maximum (FWHM), and a broad pedestal base about 260 mA FWHM. Analysis of the red wing of the line profile is complicated by a group of Wemer and Lyman rotational lines 160-220 mA from the line center. Analysis of the blue wing of the line profile gives the kinetic-energy distribution. There are two main kinetic-energy components to the H(3p) distribution: (1) a slow distribution with a peak value near 0 eV from singly excited states, and (2) a fast distribution with a peak contribution near 7 eV from doubly excited states. Using two different techniques, the absolute cross section of H L(sub Beta)p is found to be 3.2+/-.8 x 10(exp -19)sq cm at 100-eV electron impact energy. The experimental cross-section and line-profile results can be compared to previous studies of H(alpha) (6563.7 A) for principal quantum number n=3 and L(sub alpha)(1215.7 A) for n=2.

Line profile of H Lyman α from dissociative excitation of H2 with application to Jupiter

Observations of the H Lyman α (Ly α) emission from Jupiter have shown pronounced emissions, exceeding solar fluorescence, in the polar aurora and equatorial “bulge” regions. The H Ly α line profiles from these regions are broader than expected, indicating high‐energy processes producing fast atoms as determined from the observed Doppler broadening. Toward understanding that process a high‐resolution ultraviolet (UV) spectrometer was employed for the first measurement of the H Ly α emission Doppler profile from dissociative excitation of H2 by electron impact. Analysis of the deconvolved line profile reveals the existence of a narrow central peak of 40±4 mÅ full width at half maximum and a broad pedestal base about 240 mÅ wide. Two distinct dissociation mechanisms account for this Doppler structure. Slow H(2p) atoms characterized by a distribution function with peak energy near 80 meV produce the peak profile, which is nearly independent of the electron impact energy. Slow H(2p) atoms arise from direct dissociation and predissociation of singly excited states which have a dissociation limit of 14.68 eV. The wings of H Ly α arise from dissociative excitation of a series of doubly excited states which cross the Franck‐Condon region between 23 and 40 eV. The profile of the wings is dependent on the electron impact energy, and the distribution function of fast H(2p) atoms is therefore dependent on the electron impact energy. The fast atom kinetic energy distribution at 100 eV electron impact energy spans the energy range from 1 to 10 eV with a peak near 4 eV. For impact energies above 23 eV the fast atoms contribute to a slightly asymmetric structure of the line profile. The absolute cross sections of the H Ly α line peak and wings were measured over the range from 0 to 200 eV. Analytic model coefficients are given for the measured cross sections which can be applied to planetary atmosphere auroral and dayglow calculations. The dissociative excitation process, while one contributing process, appears insufficient by itself to explain the line broadening observed at Jupiter.

Ion kinetic-energy distributions and Balmer-alpha (Hα) excitation in Ar-H2 radio-frequency discharges

Excited neutrals and fast ions produced in a 13.56 MHz radio-frequency discharge in a 90% argon −10% hydrogen gas mixture were investigated, respectively, by spatially and temporally resolved optical emission spectroscopy, and by mass-resolved measurements of ion kinetic energy distributions at the grounded electrode. The electrical characteristics of the discharge were also measured and comparisons are made with results obtained for discharges in pure H2 under comparable conditions. Measurements of Balmer-alpha (Hα) emission show Doppler-broadened emission that is due to the excitation of fast atomic hydrogen neutrals formed from ion neutralization processes in the discharge. Temporally and spatially resolved emission profiles of the Hα radiation from the Ar-H2 mixture are presented for the ‘‘slow’’ component produced predominately by electron-impact dissociative excitation of H2, and for the ‘‘fast’’ component corresponding to energies much greater than can be derived from dissociative excitation. For the Ar-H2 mixture, the fast component is significantly enhanced relative to the slow component. The measured kinetic-energy distributions and fluxes of predominant ions in the Ar-H2 mixture, such as H3+, H2+, H+, and ArH+, suggest mechanisms for the formation of fast hydrogen atoms. The interpretation of results indicate that H+ and/or H3+, neutralized and backscattered by collision with the powered electrode, are the likely sources of fast hydrogen atoms that produce Doppler-shifted Hα emission in the discharge. There is also evidence at the highest pressures and voltages of ‘‘runaway’’ H+ ions formed near the powered electrode, and detected with kinetics energies exceeding 100 eV at the grounded electrode.

Study of the phosphine plasma decomposition and its formation by ablation of red phosphorus in hydrogen plasma

Mass spectrometry and optical emission spectroscopy have been used to study the chemistry of PH3 plasma decomposition as well as its formation by ablation of red phosphorus in hydrogen plasma. It has been shown that PH3 decomposition easily equilibrates at low levels of PH3 depletion (15%–30 %), this depending mainly on the rf power. The ablation of red phosphorus in H2 plasma produces phosphine in significant amount, depending mainly on the total pressure but also on the rf power. It has also been found that H* and PH* emitting species originate not only by the dissociative excitation of H2 and PH3, respectively, but also by the direct excitation of the same species in the ground state. Considerations are developed on how to derive the H-atom and PH radical densities by actinometry, under specific experimental conditions. Besides, the linear dependence of PH3 formation rate, rPH3, on H-atom density, [H], leads to the definition of the kinetic equation rPH3=k[H], and to the hypothesis that the formation of PH radical on the surface or its desorption is the dominant mechanism for PH3 production.

A reexamination of important N2 cross sections by electron impact with application to the dayglow: The Lyman‐Birge‐Hopfield Band System and N I (119.99 nm)

The far ultraviolet emission spectrum (120 to 210 nm) of electron‐excited N2 has been obtained in a crossed‐beam laboratory experiment. The cross section of the Lyman‐Birge‐Hopfield (LBH) band system (a¹Πg → X¹Σg+) has been remeasured using experimental techniques we have previously developed for this metastable transition. The improved laboratory data set for the a¹Πg state allows a determination of the excitation, emission, and predissociation cross section from threshold to 200 eV for use in planetary atmosphere models of the dayglow and aurora. An analytic fit to the experimental cross section allows accurate estimates to arbitrarily high excitation energy. The close agreement in both energy dependence and absolute cross section values between the emission measurements, presented here, and published electron scattering results shows cascade is small (<5%). The total excitation cross section for the N2 a¹Πg state is estimated to be 6.22 ± 1.37×10−18 cm² at 100 eV. The absence of emission bands for υ′ ≥ 7 suggests the predissociation yield is unity. The excitation function of each vibrational level is found to have the same shape to within 5%. In the low‐energy region, ε < 20 eV, differences in excitation threshold lead to a significant departure of the relative vibrational cross sections from the Franck‐Condon distribution. Thus the relative LBH vibrational population distribution in a planetary dayglow or aurora is affected by the energy distribution of the electron flux; and we show that atmospheric models need to include this threshold effect. The N I (119.99 nm) cross section has also been remeasured and found to be 3.48 ± 0.77×10−18 cm² at 200 eV on the basis of a comparison with Lyman α emission from dissociative excitation of H2.

The cross section for production of the 3s state of atomic hydrogen in dissociative excitation of H2 by electrons

Several measurements have been made by various authors on the relative contribution to the Balmer-α emission of the 3s state of hydrogen atoms resulting from the dissociative excitation of H2 by electrons in the energy range 20 to 500 eV. These measurements, however, are mutually inconsistent except between two measurements, one of the decay of an equilibrium population of the 3s, 3p and 3d states, and the other based on analysis of a short electron-pulse technique. We have used the equilibrium method coupled with a time-delayed-observation technique to measure the contribution of the 3s state of hydrogen atoms. Our experimental results agree with those found previously by means of the short electron-pulse technique as well as with other determinations at 50 and 110 eV, thus confirming that less than 20% of the hydrogen atoms formed in the n = 3 energy level are formed in the 3s state.

Dissociative excitation of H2 by electron impact: Translational spectroscopy of long lived high Rydberg fragment atoms

Electron impact excitation of H2 leads to three distinguishable groups of high Rydberg fragment atoms. High Rydberg molecules are also detected. The measurements consist of (1) excitation functions and (2) time of flight distributions which are transformed to fragment translational energy distributions. For high Rydberg fragments from molecular high Rydberg states converging to the repulsive wall of the H+2 ground state, the measured and calculated kinetic energy distributions agree, in accordance with the prediction of the core ion model of high Rydberg dissociation. The remaining high Rydberg fragments result from dissociation of states of H2 with both electrons excited to low principal quantum numbers rather than from dissociation of molecular high Rydberg states.