Hot Hydrogen Atoms Research Articles

The role of intrinsic magnetic fields in planetary evolution and habitability: the planetary protection aspect

AbstractThe widely used definition of a habitable zone (HZ) for planets as a circumstellar area, where the star's luminosity is sufficiently intense to maintain liquid water at the surface of a planet, is shown to be too simplified. The role of a host star's activity and the intrinsic magnetic field of a planet with respect to their influence on mass loss processes of close-in gas giants and a definition of a HZ for the terrestrial-type exoplanets are discussed. The stellar X-ray/EUV radiation and the stellar wind result in ionization, heating, chemical modification, and slow erosion of the planetary upper atmospheres throughout their lifetime. The closer the planet is to the star, the more efficient are these processes, and therefore, the more important becomes the magnetic protection of a planet as a potential habitat. Different ways for planetary magnetic dipole moment estimation, based on existing magnetic dynamo scaling laws as well as on the recent measurements of hot atomic hydrogen clouds around close-in ‘Hot Jupiters’ are considered, and the predictions of these estimations are compared to each other.

Consequences of the Heliopause Instability Caused by Charge Exchange

The heliopause (HP) is a tangential discontinuity that divides the interacting streams of the solar wind (SW) and local interstellar medium (LISM). It has been known for some time that the heliopause is subject to Rayleigh-Taylor instabilities driven and mediated byinterstellar neutralatoms. Here we identify anewform of instabilityontheflanks of HP driven by hot neutral hydrogen atoms created by charge exchange of interstellar neutrals with hot heliosheath plasma. To investigate the instabilities and the consequences of these on heliospheric structure we performed highresolution, shock-capturing, adaptive-mesh-refinementcalculations of the SW-LISMinteraction. Low numerical dissipation allows us to analyze the fine structure of the heliosheath resulting from the HP instability. We show that secondary neutrals play an essential role in destabilizing the flanks of the HP. We analyze the time-dependent location of the HP and the termination shock and the influence of their excursions on the plasma distribution in the inner heliosheath. It is found that perturbations generated by the HP instability can affect the distribution of plasma in the inner heliosheath on shorter timescales than the timescale of the instability as it develops near the stagnation axis. Finally, by way of application, we estimate the intensity of the soft X-ray emission generated in different regions of theHPandshowthatitsinstabilityleadstoasubstantialenhancementintheX-rayemissionwhichbearsanimprintof the HP shape. Subject headingg ISM: kinematics and dynamics — magnetic fields — MHD — shock waves — solar wind — X-rays: stars

Hyperthermal hydrogen atoms in argon-hydrogen atmospheric pressure microplasma jet

An argon-hydrogen atmospheric pressure microplasma jet was constructed for the treatment of materials. The microplasma jet device operating at 50 W produced long plasma jet of 30 mm with gas temperatures measured, using OH emissions, from 1600 to 2600 K as a function of distance. Excitation temperature was found to be from 7000 to 10 000 K. Through the analysis of Hα line broadening mechanisms, surprising hot hydrogen atoms H (n=3) were found with temperatures ranging from 12 000 to 19 600 K.

Evidence of catalytic production of hot hydrogen in RF generated hydrogen/argon plasmas

This is the third in a series of papers by our team on apparently anomalous Balmer series line broadening in hydrogen containing RF generated, low-pressure ( < 600 mTorr ) plasmas. In this paper the selective broadening of the atomic hydrogen lines in pure H 2 and Ar / H 2 mixtures in a large general electronics conference (GEC)-type cell (36 cm length × 14 cm ID) was mapped as a function of position, H 2 / Ar ratio, time, power, and pressure. Several observations regarding the selective line broadening were particularly notable as they are unanticipated on the basis of earlier models. First, the anomalous broadening of the Balmer lines was found to exist throughout the plasma, and not just in the region between the electrodes. Second, the broadening was consistently a complex function of the operating parameters, particularly gas composition (highest in pure H 2 ) position, power, and pressure. Clearly, not anticipated by earlier models were the findings that under some conditions the highest concentration of “hot” ( > 10 eV ) hydrogen was found at the gas entry end, and not in the high-field region between the electrodes, and that in other conditions, the hottest H was at the (exit) pump (also grounded electrode) end. Third, excitation and electron temperatures (measured optically) were less than 1 eV in all regions of the plasma not directly adjacent ( > 1 mm ) to the electrodes, providing additional evidence that the energy for broadening, contrary to standard models, is not obtained from the electric field. Fourth, in contrast to our earlier studies of hydrogen/helium and water plasmas, we found that in some conditions 98% of the atomic hydrogen was in the “hot” state throughout the GEC-type cell. Virtually every operating parameter studied impacted the character of the hot H atom population. Clearly, second and third order effects exist, indicating a need for experimental design. Some non-field mechanisms for generating hot hydrogen atoms are outlined.

Divalent Metal Ion-Peptide Interactions Probed by Electron Capture Dissociation of Trications

Electron capture dissociation (ECD) of the peptide Substance P (SubP) complexed with divalent metals has been investigated. ECD of [SubP + H + M]3+ (M2+ = Mg2+ -Ba2+ and Mn2+ -Zn2+) allowed observation of a larger number of product ions than previous investigations of doubly charged metal-containing peptides. ECD of Mg-Ba, Mn, Fe, and Zn-containing complexes resulted in product ions with and without the metal from cleavage of backbone amine bonds (c' and z* -type ions). By contrast, ECD of Co and Ni-containing complexes yielded major bond cleavages within the C-terminal methionine residue (likely to be the metal ion binding site). Cu-containing complexes displayed yet another behavior: amide bond cleavage (b and y'-type ions). We believe some results can be rationalized both within the hot hydrogen atom mechanism and mechanisms involving electron capture into excited states, such as the recently proposed amide superbase mechanism. However, some behavior, including formation of (cn 'M - H)+ ions for Ca-Ba, is best explained within the latter mechanisms with initial electron capture at the metal. In addition, the ECD behavior appears to correlate with the metal second ionization energy (IE2). Co and Ni (displaying sequestered fragmentation) have IE2s of 17.1 and 18.2 eV, respectively, whereas IE2s for Mg-Ba, Mn, and Fe (yielding random cleavage) are 10.0 to 16.2 eV. This behavior is difficult to explain within the hot hydrogen atom mechanism because hydrogen transfer should not be influenced by IE2s. However, the drastically different fragmentation patterns for Co, Ni, and Cu compared to the other metals can also be explained by their higher propensity for nitrogen (as opposed to oxygen) binding. Nevertheless, these results imply that directed fragmentation can be accomplished via careful selection of the cationizing agent.

Loss of hydrogen and oxygen from the upper atmosphere of Venus

Atmospheric escape from the upper atmosphere of Venus is mainly influenced by the loss of hydrogen and oxygen caused by the interaction of solar radiation and particle flux with the unprotected planetary environment. Because one main aim of the ASPERA-4 particle/plasma and VEX-MAG magnetic field experiments on board of ESA's forthcoming Venus Express mission is the investigation of atmospheric erosion processes from the planet's ionosphere–exosphere environment, we study the total loss of hydrogen and oxygen and identified the efficiency of several escape mechanisms involved. For the estimation of pick up loss rates we use a gas dynamic test particle model and obtained average loss rates for H + , and O + pick up ions of about 1 × 10 25 s - 1 and about 1.6 × 10 25 s - 1 , respectively. Further, we estimate ion loss rates due to detached plasma clouds, which were observed by the pioneer Venus orbiter and may be triggered by the Kelvin–Helmholtz instability of about 0.5 – 1 × 10 25 s - 1 . Thermal atmospheric escape processes and atmospheric loss by photo-chemically produced oxygen atoms yield negligible loss rates. Sputtering by incident pick up O + ions give O atom loss rates in the order of about 6 × 10 24 s - 1 . On the other hand, photo-chemically produced hot hydrogen atoms are a very efficient loss mechanism for hydrogen on Venus with a global average total loss rate of about 3.8 × 10 25 s - 1 , which is in agreement with Donahue and Hartle [1992. Solar cycle variations in H + and D + densities in the Venus ionosphere: implications for escape. Geophys. Res. Lett. 12, 2449–2452] and of the same order but less than the estimated H + ion outflow on the Venus nightside of about 7.0 × 10 25 s - 1 due to acceleration by an outward electric polarization force related to ionospheric holes by Hartle and Grebowsky [1993. Light ion flow in the nightside ionosphere of Venus. J. Geophys. Res. 98, 7437–7445]. Our study indicates that on Venus, due to its larger mass and size compared to Mars, the most relevant atmospheric escape processes of oxygen involve ions and are caused by the interaction with the solar wind. The obtained results indicate that the ratio between H/O escape to space from the Venusian upper atmosphere is about 4, and is in a much better agreement with the stoichiometrically H/O escape ratio of 2:1, which is not the case on Mars. However, a detailed analysis of the outflow of ions from the Venus upper atmosphere by the ASPERA-4 and VEX-MAG instruments aboard Venus Express will lead to more accurate atmospheric loss estimations and a better understanding of the planet's water inventory.

Quantum study of Eley-Rideal reaction and collision induced desorption of hydrogen atoms on a graphite surface. II. H-physisorbed case

Following previous investigation of collision induced (CI) processes involving hydrogen atoms chemisorbed on graphite [R. Martinazzo and G. F. Tantardini, J. Chem. Phys. 124, 124702 (2006)], the case in which the target hydrogen atom is initially physisorbed on the surface is considered here. Several adsorbate-substrate initial states of the target H atom in the physisorption well are considered, and CI processes are studied for projectile energies up to 1 eV. Results show that (i) Eley-Rideal cross sections at low collision energies may be larger than those found in the H-chemisorbed case but they rapidly decrease as the collision energy increases; (ii) product hydrogen molecules are vibrationally very excited; (iii) collision induced desorption cross sections rapidly increase, reaching saturation values greater than 10 A2; (iv) trapping of the incident atoms is found to be as efficient as the Eley-Rideal reaction at low energies and remains sizable (3-4 A2) at high energies. The latter adsorbate-induced trapping results mainly in formation of metastable hot hydrogen atoms, i.e., atoms with an excess energy channeled in the motion parallel to the surface. These atoms might contribute in explaining hydrogen formation on graphite.

Possibilities of atomic hydrogen storage by carbon nanotubes or graphite materials

Atomic hydrogen storage by carbon nanotubes (CNTs) and highly oriented pyrolytic graphite (HOPG) has been studied using a flow catalytic reactor and an ultra-high vacuum surface science apparatus including scanning tunneling microscope (STM), respectively. Defect sites on CNTs as adsorption sites of atomic hydrogen are introduced by oxidation pretreatment using La catalyst. Pd catalysts are then deposited on CNT surfaces for dissociation of H2 into atomic hydrogen, which spills over to the defect sites. In the best case, 1.5 wt% of hydrogen is stored in the defective CNT with Pd particles at 1 atm and 573 K. In temperature programmed desorption (TPD) experiments, H2 starts to desorb at 700–900 K depending on the annealing temperatures of CNTs prior to hydrogen storage. On the HOPG surface, hot atomic hydrogen produced by dissociation of H2 using tungsten wire desorbs from graphite terraces at 400–700 K, which is much lower than that on CNTs. It is possible that one can decrease the desorption temperature by changing the method of H2 dissociation.

Martian hydrogen exosphere charge exchange with solar wind

Charge exchange of solar wind with the Martian exosphere has been shown to play a crucial role in the formation of the magnetic pileup boundary observed by Mars Global Surveyor. However, the question of how the Martian exospheric structure is quantitatively altered by charge exchange of the solar wind remains open. To answer this question, a three‐dimensional Monte Carlo exosphere model was developed. In this model the effects of planetary rotation, photoionization, Lyman α scattering, and charge exchange with the solar wind are included. Although the change of exospheric density profile is indistinctive, simulation results show that the existence of the hot atomic hydrogen population produced by charge exchange alters the exospheric temperature structure greatly and the symmetry of the exospheric structure is totally destroyed at high altitude. Another noticeable phenomenon is that charge exchange and Lyman α scattering combine to make the nightside exosphere be cooler than and expand more slowly than the ambient exosphere. In the end we use our simulation results to estimate the contribution of the hot component to Ly α emission. Furthermore, output of our model can be used to predict particle observations made by future missions to Mars.

Can the (M• — X) Region in Electron Capture Dissociation Provide Reliable Information on Amino Acid Composition of Polypeptides?

It has been suggested that small losses from reduced peptide molecular species in electron capture dissociation (ECD) could indicate the presence of certain amino acids [H.J. Cooper, R.R. Hudgins, K. Håkansson and A.G. Marshall, J. Am Soc. Mass Spectrom 13, 241 (2002)], similarly to immonium ions in high-energy collision-activated dissociation. The diagnostic value in ECD of the (M•–X) region (1 Da ≤ X ≤ 130 Da) was tested on several synthetic peptides. The insufficiency of the existing knowledge for making correct conclusions on the amino acid composition is demonstrated and new suggestions of the origin of losses are presented based on the “hot hydrogen atom” ECD mechanism. Generally, it is shown that not only protonation but also charge solvation is responsible for the small losses. The origin of 17 Da and 59 Da losses is revisited and a new mechanism for the 18 Da loss is suggested. The loss of a side chain plus a hydrogen atom is found to be a rather reliable indicator of the presence of histidine, tryptophan, tyrosine and, to a lesser degree, threonine. The overall conclusion is that the (M• - X) region does contain information on the amino acid composition, but extraction of this information requires additional studies.

Hydrogen-exchange reactions via hot hydrogen atoms produced in the dissociation process of molecular hydrogen on Ir{111}

Adsorption and reaction of hydrogen (deuterium) on the Ir{111} surface has been studied with temperature-programmed desorption and direct measurements of desorbing molecules using a quadrupole mass spectrometer at ∼100 K. H2 exposure of the D-precovered Ir{111} surface was found to induce the desorption of HD and D2 molecules. This result suggests that energetic H atoms (hot H atoms) produced in the dissociation process of incident H2 molecules react with preadsorbed D atoms and desorb as HD molecules or produce secondary energetic D atoms via energy transfer. Secondary energetic D atoms (secondary hot D atoms) also induce the associative reactions with preadsorbed D atoms and desorb as D2 molecules. We will discuss the hot-H-atom-mediated reaction based on both empirical and steady-state approximation models for interpreting the present experimental results.

A quasiclassical trajectory study of reactivity and product energy disposal in H+H2O, H+D2O, and H+HOD

The dynamics of the H+H2O→H2+OH, and some isotopic counterpart reactions has been investigated by quasiclassical trajectory (QCT) calculations, and using a recently developed potential energy surface [Wu et al., J. Chem. Phys. 113, 3150 (2000)] that was derived from high quality ab initio calculations. We make an extensive comparison with QCT and quantum scattering results based on other surfaces, particularly that from Ochoa and Clary, as well as with experimental results. Our results show that, in agreement with earlier theoretical results, the cross sections for the reaction of translationally hot hydrogen atoms with ground state H2O (yielding H2+OH) and with ground state D2O (yielding HD+OD) are significantly smaller than experiment. Our results are in agreement with accurate quantum results on comparably accurate surfaces, thereby showing that the disagreement with experiment is not a problem with either the dynamics method or the potential surfaces. In contrast to this, other properties of the reaction dynamics are in generally excellent agreement with experiment. For example, the role of stretch excitation on the H+D2O cross sections follows the trends observed in the experiments. Bend excitation is found to be more active than was previously thought in enhancing reactivity, but is still within experimental uncertainty. Water rotation is found to play an important role in experiments that sample j(H2O) values of 5 or greater. Our studies of the H+D2O and H+HOD→H2+OD,HD+OH reactions yield isotope branching ratios and product distributions (for both spectator and newly formed diatoms) that are generally in good agreement with experiment. The only exception to this arises with the HD rotational distributions in H+D2O, where the observed distributions show less excitation and broader distributions. The internal distributions of experimentally unresolved products are also discussed. We conclude that the new potential energy surface used here is very accurate for describing the H+H2O→H2+OH reaction and isotopic counterparts, providing significant improvement over previously published results.

Sorption of athermal hydrogen atoms

Sorption–desorption processes during interaction of hot hydrogen atoms with metals are considered. Non-activated penetration and primary accommodation of incident atoms at on-the-surface and under-the-surface sites was considered. Desorption of ad- and absorbed atoms was supposed due to recombination of atoms from both the same and different sites. Analytical expressions for concentrations were obtained and contribution of various factors analyzed.

Hot Neutral H in the Heliosphere: Elastic H-H, H-p Collisions

Williams et al. (1997) have suggested that a population of hot hydrogen atoms is created in the heliosphere through elastic H-H collisions between energetic `solar' atoms (neutralized solar wind) and interstellar atoms. They used a BGK-like approximation (Bhatnagar et al., 1954) for the Boltzmann collision term and the collision cross sections suggested by Dalgarno (1960). We show that both assumptions result in a significant overestimation of the the H-H collision effect. On the basis of calculated momentum transfer cross-sections for elastic H-H collisions, we argue that elastic H-H and H-p collisions cannot produce hot H atoms in the heliosphere.

Vibration–rotation excitation of CO by hot hydrogen atoms: Comparison of two potential energy surfaces

Collision cross sections for rotational and vibrational excitation of CO by fast H atoms are calculated for two potential energy surfaces, the older Bowman–Bitman–Harding potential and the recently constructed surface of Werner, Keller, and Schinke. Both quantum mechanical and classical calculations are performed. The results obtained with the new potential energy surface are very similar to those obtained with the older potential; in particular, they do not rectify the discrepancies between the experimental and theoretical cross sections for vibrationally elastic transitions into small rotational states of CO.

The distribution of hot hydrogen atoms produced by electron and proton precipitation in the Jovian aurora

The energy distribution functions of nonthermal thermospheric hydrogen atoms are calculated for electron and proton precipitation in the Jovian aurora. A numerical model taking into account the production, elastic and inelastic relaxation and transport processes for hot H atoms is developed. This model is based on a Monte Carlo solution of the nonlinear Boltzmann equation for hot H atoms produced by electron and proton impact on H and H2 and exothermic chemical reactions. The distribution functions show a much higher energy tail for proton than electron precipitation. It is shown that the steady state flux of hot atoms (E ≥ 2 eV) is essentially isotropic. The peak and column hot H densities are about 3 × 105 cm−3 and 1 × 1014 cm−3 for a 100 erg cm−2 s−1 precipitation combining hard (22 keV) and soft (0.2 keV) electrons mixed with a 10 erg cm−2 s−1 flux of soft (0.3 keV) protons. These column densities, coupled with the wide range of hot H atom energies, may play an important role in the formation of the Lyman α line profile. Multiple scattering in the wings of the Ly α line by the fast H atoms is shown to partly account for the broad Ly α profile observed in the Jovian aurora with the Hubble space telescope.

Formation of Hot Atomic Hydrogen Beam for Surface Diffraction Experiments

An attempt to generate a monoenergetic hot atomic hydrogen (H) beam for surface diffraction experiments was performed with photo-dissociation of hydrogen iodine (HI) molecules at 266 nm. The velocity and angular distributions of recoiling H fragments generated via two laser photolysis channels corresponding to the parallel and perpendicular transitions were investigated by means of resonantly enhanced multi-photon ionization (REMPI)-Doppler spectroscopy with the aid of tunable coherent VUV light around the Lyman-α line. Monoenergetic H beams with the velocities of 1.12×104 m/s for parallel transition and 1.75×104 m/s for perpendicular transition, having a sufficiently narrow velocity spread for surface diffraction experiments, were obtained.

Quantum scattering calculations for vibrational and rotational excitation of CO by hot hydrogen atoms

Collision cross sections were calculated for vibrational and rotational excitation of CO by H atoms at collisional kinetic energies from 0.7 to 1.9 eV. The BBH [J. M. Bowman, J. S. Bittman, and L. B. Harding, J. Chem. Phys. 85, 911 (1986)] potential energy surface was used and collision dynamics were treated within the quantum coupled states approximation, which is shown to be quite accurate for this system, and also using the infinite order sudden approximation for the rotational degree of freedom, which is shown to be less accurate than expected. Results are compared with experimental data and with quasiclassical trajectory values.

Experimental and theoretical velocity profiles for pure rotational scattering in carbon dioxide-hot hydrogen atom collisions

Rotational and translational excitation of CO{sub 2} molecules by hot hydrogen atoms has been studied via time dependent diode laser absorption spectroscopy. This high resolution infrared laser probe technique has been used to measure Doppler recoil profiles of CO{sub 2} molecules as they undergo translational and rotational excitation due to collisions with translationally energetic hydrogen atoms. The detailed CO{sub 2} absorption line shapes show clear evidence of scattering both into and out of each rotational state. Classical scattering calculations were performed using a hard shell potential and the infinite order sudden approximation to simulate the experimentally observed Doppler profiles for each CO{sub 2} rotational state. The widths of the Doppler profiles for molecules scattered into each rotational state were well fit by this simple potential model, but the scattering amplitudes (number of molecules scattered into each state) were not predicted as accurately by the calculations. Correlations between translational and rotational energy transfer, in which translational recoil is found to increase with increasing changes in angular momentum {Delta}J, are clearly observed in both the experimental results and the calculations. 22 refs., 6 figs.

Experimental and theoretical velocity profiles for pure rotational scattering: CO–hot hydrogen atom collisions

Time dependent diode laser spectroscopy was used to measure Doppler profiles of P-branch lines in the fundamental vibrational band of CO shortly after collisions with H atoms having a translational energy of 2.3 eV. Observed spectral line profiles reflect two Doppler components, a negative room temperature component from molecules scattered out of a given v=0, J state, and a hot component from molecules scattered into the same state. Observed profiles are explained qualitatively using state-to-state differential cross sections calculated from the ab initio potential energy surface of Bowman, Bittman, and Harding and a rigid rotor coupled states quantum scattering approximation.