Hot Atoms Research Articles

Energy transfer in O collisions with He isotopes and Helium escape from Mars

[1] Accurate data on energy-transfer collisions between hot oxygen atoms and the atmospheric helium gas on Mars, are reported. Anisotropic cross sections for elastic collisions of O(3P) and O(1D) atoms with helium gas have been calculated quantum mechanically and found to be surprisingly similar. Cross sections, computed for collisions with both helium isotopes, 3He and 4He, have been used to construct the kernel of the Boltzmann equation describing the energy relaxation of hot oxygen atoms. Computed rates of energy transfer in O+He collisions have been used to evaluate the flux of He atoms escaping from the Mars atmosphere. Atmospheric layers mostly responsible for production of the He escape flux are identified. Our results demonstrate that strong angular anisotropy of scattering cross sections increases the collisional ejection of light atoms and is critical in the evaluation of He escape from Mars, Venus and Earth.

Multiply-charged ions and interstellar chemistry

Gaseous molecules and ions, and even dust grains, can accumulate charge in the interstellar medium (ISM) by harvesting the energy of UV photons, cosmic rays, helium ions and metastable atoms. This Perspective views the various modes of gas-phase formation of multiply-charged cations and the possible impact of their reactions on the chemical and ionization structure of the ISM, in the light of what is still very limited knowledge. Emphasis is given to gas-phase reactions of multiply-charged cations with atoms, molecules and electrons that lead to charge reduction, charge separation and chemical bond formation and these are examined for multiply-charged atoms, small molecules, hydrocarbons, polycyclic aromatic hydrocarbons and fullerenes, primarily as dications but also as a function of charge state. The increased electrostatic interaction due to multiple charge is seen to promote bonding to individual charge sites on large molecules (e.g. fullerenes) and allow ensuing "surface" chemistry under the influence of Coulomb repulsion. The unique ability of multiply charged cations to undergo charge separation reactions, either unimolecular or bimolecular, can feature in the production in the ISM of internally cold, but translationally hot, cations of lower charge state or hot atoms that may provide the driving force for subsequent chemical reactions in what is otherwise an ultracold environment. Available chemical kinetic models that account for the role of multiply-charged ions in the ISM are few and of limited scope and the observation of these ions in the ISM has remained elusive.

Femtosecond coherent control of thermal photoassociation of magnesium atoms

We investigate femtosecond photoassociation of thermally hot atoms in the gas phase and its coherent control. In the photoassociation process, formation of a chemical bond is facilitated by light in a free-to-bound optical transition. Here, we study free-to-bound photoassociation of a diatomic molecule induced by femtosecond pulses exciting a pair of scattering atoms interacting via the van-der-Waals-type electronic ground state potential into bound levels of an electronically excited state. The thermal gas of reactants is at temperatures in the range of hundreds of degrees. Despite this incoherent initial state, rotational and vibrational coherences are observed in the probing of the created Mg2 molecules.

Venus' atomic hot oxygen environment

We present a Monte Carlo study of the atomic hot oxygen corona of Venus. In this model we consider elastic, inelastic, and quenching collisions between the traced hot particle and the ambient neutral atmosphere as well as differential cross sections to determine the scattering angle in the collisions. We also include rotational and vibrational excitation energies for the calculation of the initial energy of the produced hot oxygen atoms. Our results indicate that the differential cross sections and the fraction between elastic, inelastic, and quenching collisions are the most sensitive parameters which effect the corona density. We found that the hot O densities inferred from PVO observations can only be reproduced during high solar activity based on a forward scattering model but without inelastic and quenching collisions. The corona densities for low solar activity (VEX solar conditions) are about a factor of 2–3 smaller than for high solar activity.

Efficient frequency conversion induced by quantum constructive interference

We demonstrate in experiment an efficient cw four-wave mixing scheme with maximal intensity conversion efficiency up to 73% in a double-Λ system of hot rubidium atoms. Relevant theoretical analysis shows that this high conversion efficiency benefits greatly from the constructive interference between two four-wave mixing channels, characterized by two different space-dependent phases.

Possibility of nonexistence of hot and superhot hydrogen atoms in electrical discharges

Recently, the existence of extremely energetic hydrogen atoms in electrical discharges has been proposed in the literature with large controversy, from the analysis of the anomalous broadening of hydrogen Balmer lines. In this paper, the velocity distribution of H atoms and the profiles of the emitting atom lines created by the exothermic reaction H2++H{2}→H{3}{+}+H+ΔE are calculated, as a function of the internal energy defect ΔE . The shapes found for the non-Maxwell-Boltzmann distributions resulting in non-Gaussian line profiles raise serious arguments against the existence of hot and superhot H atoms as it has been proposed, at least with those temperatures.

ChemInform Abstract: Chemical Reactions Induced and Probed by Positive Muons

The application of μ+ science, collectively called μSR, but encompassing a variety of methods including muon spin rotation, muon spin relaxation, muon spin repolarization, muon spin resonance and level-crossing resonance, to chemistry is introduced emphasizing the special aspects of processes which are “induced and probed” by the μ+ itself. After giving a general introduction to the nature and methods of muon science and a short history of muon chemistry, selected topics are given. One concerns the usefulness of muonium as hydrogen-like probes of chemical reactions taking polymerization of vinyl monomers and reaction with thiosulphate as examples. Probing solitons in polyacetylene induced and probed by μ+ is also an important example which shows the unique nature of muonium. Another important topic is “lost polarization”. Although this term is particular to muon chemistry, the chemistry underlining the phenomenon of lost polarization has an importance to both radiation and hot atom chemistries.

Dissociation of oxygen on Ag(100) induced by inelastic electron tunneling

Scanning tunneling microscopy (STM) is used to study the dissociation ofmolecular oxygen on Ag(100) induced by inelastic electron tunneling (IET)at 5 K. This dissociation is possible above 3.3 V with a yield of(3.63 ± 0.47) × 10 − 9 per electron. Dissociation leads to three different types of hot atom motion: lateralmotion, a cannon ball mechanism, and abstractive dissociation. Analysis of theI–t characteristics during dissociation suggests that the dissociation is proceeded by anadsorption site change.

Influence of hydrocarbons on vibrational excitation of H 2 molecules

The influence of light hydrocarbons on vibrational excitation of H 2 generated in a special source of excited molecules has been studied. Molecule dissociation on the hot tungsten filament and atom recombination on cooled walls is used for the production of molecular excitation in the source. Specific influence of CH 4, C 2H 4 and C 2H 6 on vibrational distribution of hydrogen molecules from the source is presented here. Production of vibrationally excited H 2 molecules due to the thermal dissociation on the hot filament is observed when C 2H 4 (ethene) and C 2H 6 (ethane) are introduced alone in the source. These molecules appear to be produced by direct process on the hot filament in the case of ethene while in the case of ethane H 2 ( v ) is produced by the similar process as for H 2. Production of H 2 ( v ) is not observed when only CH 4 is introduced into the source. In this case vibrational relaxation of H 2 ( v ) is observed when CH 4 is introduced into the source in addition to H 2. Studied processes are relevant for the modelling of edge plasma in tokamaks.

Magneto-optical control of the group velocity of light in an inhomogeneously broadened medium

We investigate the use of a magnetic field, in conjunction with an elliptically polarized laser field, to control the group velocity of light in an inhomogeneously broadened medium. We show both theoretically and experimentally that the dispersion and hence, the speed of light in an atomic system, having V-configuration with Zeeman sublevels as their excited states, can be effectively controlled magneto-optically. We present approximate analytical solutions for a homogeneously broadened medium and numerical solutions for both a homogeneously and an inhomogeneously broadened medium. We show that while one can tune the speed of light from subluminal to superluminal with the magnetic field in a static or cooled medium with V-type atoms, in hot atoms one can achieve an efficient magneto-optical tunability in the subluminal regime. We experimentally demonstrate such control of light speed using a single elliptically polarized light and a moderately strong magnetic field in a 87Rb vapor cell. We propose to use the optical field for coarse control and the magnetic field for finer control of the light speed in an atomic medium.

Suprathermal hydrogen produced by the dissociation of molecular hydrogen in the extended atmosphere of exoplanet HD 209458b

The ionization and dissociation of molecular hydrogen by the ultraviolet (UV) radiation of the parent star lead to the formation of hydrogen atoms with an excess of kinetic energy and, thus, are an important source of suprathermal hydrogen atoms in the upper atmosphere of exoplanet HD 209458b. Contemporary aeronomical models did not investigate these processes because they assumed the fast local thermalization of the hot atoms of hydrogen by elastic collisions. However, the kinetics and transfer of these atoms were not calculated in detail, because they require the solving of the Boltzmann equation for a nonthermal atom population. This work estimates the effect of the UV radiation of the parent star and the accompanying photocleacton flux on the production of the suprathermal fraction of atomic hydrogen in the H2 → H transition region. We also consider the formation of the escaping flux of Hatoms created by this effect in the upper atmosphere of HD 209458b. We calculate the production rate and energy spectrum of the hydrogen atoms with excess kinetic energy during the dissociation of H2. Using the numerical stochastic model created by Shematovich (2004) for a hot planetary corona, we investigate the molecular-scale kinetics and transfer of suprathermal hydrogen atoms in the upper atmosphere and the emergent flux of atoms evaporating from the atmosphere. The latter is estimated as 3.4 × 1012 cm−2 s−1 for a moderate stellar activity level of UV radiation, which leads to a planetary atmosphere evaporation rate of 3.4 × 109 g s−1 due to the process of the dissociation of H2. This estimate is close to the observational value of ∼1010 g s−1 for the rate of atmospheric loss of HD 209458b.

Electromagnetically induced photonic bandgap in hot Cs atoms

Three-level Λ-type thermal Cs atoms are used to demonstrate the phenomenon of a photonic bandgap induced by quantum coherence with a standing wave (SW). We observed the transmitted signals of probe field driven by several kinds of SW, which are formed by a strong forward-traveling field and a backward-traveling field when a mirror reflects the forward-traveling beam. Considering Doppler inhomogeneous broadenings with a SW drive, we employ Fourier transformation to solve density-matrix equations for simulation results. The simulation results are found to be consistent with the experimental results.

ON THE EXISTENCE OF ENERGETIC ATOMS IN THE UPPER ATMOSPHERE OF EXOPLANET HD209458b

Stellar irradiation and particles forcing strongly affect the immediate environment of extrasolar giant planets orbiting near their parent stars. Here, we use far-ultraviolet emission spectra from HD209458 in the wavelength range (1180-1710)A to bring new insight to the composition and energetic processes in play in the gas nebula around the transiting planetary companion. In that frame, we consider up-to-date atmospheric models of the giant exoplanet where we implement non-thermal line broadening to simulate the impact on the transit absorption of superthermal atoms (HI, OI, and CII) populating the upper layers of the nebula. Our sensitivity study shows that for all existing models, a significant line broadening is required for OI and probably for CII lines in order to fit the observed transit absorptions. In that frame, we show that OI and CII are preferentially heated compared to the background gas with effective temperatures as large as T_{OI}/T_B~10 for OI and T_{CII}/T_B~5 for CII. By contrast, the situation is much less clear for HI because several models could fit the Lyman-a observations including either thermal HI in an atmosphere that has a dayside vertical column [HI]~1.05x10^{21} cm^{-2}, or a less extended thermal atmosphere but with hot HI atoms populating the upper layers of the nebula. If the energetic HI atoms are either of stellar origin or populations lost from the planet and energized in the outer layers of the nebula, our finding is that most models should converge toward one hot population that has an HI vertical column in the range [HI]_{hot}(2-4)x10^{13} cm^{-2} and an effective temperature in the range T_{HI}(1-1.3)x10^6 K, but with a bulk velocity that should be rather slow.

Ab InitioMolecular Dynamics Study of Hot Atom Dynamics after Dissociative Adsorption ofH2on Pd(100)

The relaxation of hot hydrogen atoms upon the dissociative adsorption of H2 on Pd(100) was studied by ab initio molecular dynamics simulations based on density functional theory, modeling the full dissociative adsorption process in a consistent manner. In spite of the nonlinear dependence of every single trajectory on the run conditions, on the average it is the energy dissipation to the substrate that determines the mean distance of the two H atoms after adsorption which amounts to three to four lattice units and provides an upper bound for heavier species such as oxygen.

Hot and super-hot hydrogen atoms in microwave plasma

“Super-hot” (kinetic energy ∼4–8 eV) and “hot” (kinetic energy ∼0.3 eV) H atoms were detected in a surface wave (500 MHz) generated H2 plasma column, at pressure p=0.01 mbar, from the analysis of the Hβ, Hγ, Hδ, and Hε emission line profiles. These profiles were found to evolve from single Gaussian to bi-Gaussian toward the plasma column end. Population inversion between the levels 5→4 and 6→4 was detected. At pressure p=0.2 mbar, super-hot atoms were not detected and the temperature of the hot atoms was found to increase with the upper level principal quantum number.

Off-resonance absorption and dispersion in vapours of hot alkali-metal atoms

We study the absorptive and dispersive properties of Doppler-broadened atomic media as a function of detuning. Beginning from the exact lineshape calculated for a two-level atom, a series of approximations to the electric susceptibility are made. These simplified functions facilitate direct comparison between absorption and dispersion, and show that dispersion dominates the atom–light interaction far from resonance. The calculated absorption and dispersion are compared to experimental data for Rb vapour on the D1 transition, showing the validity of the approximations.

Energy relaxation in collisions of hydrogen and deuterium with oxygen atoms

Collision energy transfer processes between hydrogen, deuterium, and oxygen atoms in the upper atmospheres of the terrestrial planets are studied. A new set of interaction potentials has been constructed using an accurate ab initio method. Full orientation‐dependent scattering cross sections have been obtained quantum mechanically and have been incorporated into the construction of the linear Boltzmann kinetic equation describing the energy relaxation process. The isotope and temperature dependence of the energy relaxation parameters have been analyzed. Distributions of the secondary energetic recoil atoms have been computed and the fractions of hot atoms capable of escaping from the atmospheres of the terrestrial planets have been determined. For applications to atmospheric physics and astrophysics, we have computed effective hard sphere cross sections for O + H and O + D collisions that closely reproduce the energy relaxation kinetics obtained from the linear Boltzmann equation. These effective cross sections, which are functions of the laboratory frame collisional energy and the temperature of the bath gas, may be used in simulations of the thermalization of hot O, H, and D atoms and their escape from planets.

Theoretical treatment of hydrogen abstraction by hot tritium atoms

Extended Huckel calculations using Cusachs's modification were performed for the methane–tritium system with the objective of determining how the overlap populations (and energies) varied as a function of GHT distances. The model chosen was axial attack of the hot tritium atom along the CH(1) bond direction which leads to abstraction of the H(1) atom. A position of maximum H(1)T overlap population was found at about the same place as the position of minimum CH(1) overlap population, thus setting certain limits on geometric configurations which would be expected to undergo the H(1) abstraction reaction. Attention is drawn to the pertinence of overlap population results to the bond energy–bond order method of Johnston for obtaining potential energy surfaces. The overlap populations also indicate why in compounds containing very electronegative substituents, the abstraction reaction occurs at a lower energy than would be predicted by the simple bond dissociation energy dependence found experimentally by Rowland for all other systems. A potential energy surface was also traced out for the CH4–T reaction and a basin was found indicating a stable CH4T (or CH5) molecule. Examination of activation energies calculated previously by Eyring and Polyani for axial attack compared to direct attack shows clearly why at low kinetic energies of T the abstraction reaction is favored over the substitution reaction; and why only the abstraction reaction occurs at T kinetic energies lower than ∼1.8 eV.

Resonance line radiative transfer for hot atom coronae using Kappa distributions

Hot atomic populations are an important component of the planetary exospheres. Usually, radiative transfer models describing the scattering of light by moving atoms assume that these populations have a Maxwellian velocity distribution. However, the velocity distributions of the hot populations could actually have some more extended wings. Popular velocity distributions often used in plasma physics and recently proposed to describe neutral planetary environments are Kappa velocity function distributions. In this paper, following the work of Hummer [Non-coherent scattering: I The redistribution functions with Doppler broadening. R Astron. Soc Month Not 1962;125:21] and Cranmer [Non-Maxwellian redistribution in solar coronal Lyα emission. Astrophys J 1998;508:925–39], we calculate the frequency redistribution functions of radiation scattered by moving atoms with Kappa velocity distribution. We also present a detailed study of a radiative transfer model taking into account Kappa velocity distribution functions, for integer and semi-integer values of κ. We apply this theory to a model of Jupiter hydrogen corona containing 0.1% column density of hot hydrogen to quantify the spectroscopic and imaging differences between Kappa velocity distributions and bi-Maxwellian velocity distributions. When assuming a Kappa velocity distribution with κ=2 for the hot population, intensity increases of ∼40% occur at the bright limb and ∼15% on the disk compared with the same calculations done using a Maxwellian velocity distribution. The line profile differs slightly from a Maxwellian distribution on the disk and at the bright limb, but the difference is larger above the limb. Kappa distributions used to study the Jovian atmosphere are speculative and further studies are needed to link the formation of the hot exospheric populations to the Kappa velocity distributions.

Translational and internal states of hydrogen molecules produced from the ultraviolet photodissociation of amorphous solid methanol

Translationally and internally hot H(2) molecules are produced from the 157 nm photodissociation of amorphous solid methanol at 90 K by two distinct mechanisms: exothermic recombination of two H-atom photoproducts bound to the surface and unimolecular molecular elimination of H(2) from the photoexcited methanol. The vibrationally hot H(2)(v=2-5) products are characterized by high translational and rotational temperatures. A third mechanism, the almost thermoneutral abstraction of a hydrogen atom from methanol parent molecule by the photolytically produced hydrogen atom, yields translationally and rotationally cold H(2)(v=0 and 1) products. Comparison with the results of the vacuum ultraviolet photolysis of water ice is discussed. Production of translationally hot and cold hydrogen atoms is also confirmed.