Radiative Association Of He Research Articles

Revisiting the Formation of HeH+ in the Planetary Nebula NGC 7027

Abstract From four independent calculations using three different theoretical approaches, rate coefficients for the formation of HeH+ via the radiative association of He+ and H were computed. Good agreement is found between our new calculations and prior results obtained two decades ago for kinetic temperatures between ∼800 and 20,000 K. This finding is inconsistent with a recent claim in the literature of a wide variation in published values and establishes the robustness of our knowledge of this process for the formation of HeH+. The implications of the current results to the first detection of HeH+ and its modeled abundance in the planetary nebula NGC 7027 are discussed.

Astrophysical detection of the helium hydride ion HeH.

During the dawn of chemistry1,2, when the temperature of the young Universe had fallen below some 4,000 kelvin, the ions of the light elements produced in Big Bang nucleosynthesis recombined in reverse order of their ionization potential. With their higher ionization potentials, the helium ions He2+ and He+ were the first to combine with free electrons, forming the first neutral atoms; the recombination of hydrogen followed. In this metal-free and low-density environment, neutral helium atoms formed the Universe's first molecular bond in the helium hydride ion HeH+ through radiative association with protons. As recombination progressed, the destruction of HeH+ created a path to the formation of molecular hydrogen. Despite its unquestioned importance in the evolution of the early Universe, the HeH+ ion has so far eluded unequivocal detection in interstellar space. In the laboratory the ion was discovered3 as long ago as 1925, but only in the late 1970s was the possibility that HeH+ might exist in local astrophysical plasmas discussed4-7. In particular, the conditions in planetary nebulae were shown to be suitable for producing potentially detectable column densities of HeH+. Here we report observations, based on advances in terahertz spectroscopy8,9 and a high-altitude observatory10, of the rotational ground-state transition of HeH+ at a wavelength of 149.1 micrometres in the planetary nebula NGC7027. This confirmation of the existence of HeH+ in nearby interstellar space constrains our understanding of the chemical networks that control the formation of this molecular ion, in particular the rates of radiative association and dissociative recombination.

Radiative association of He(23P) with lithium cations: [formula omitted] processes

The radiative association processes originating in the 13Π continuum of the He (23P)+Li+ collisional system are investigated in this study. The calculations of the dynamic collision processes are based on highly accurate state-of-the-art ab initio calculations of the potential energy functions for the 13Π and the three lowest 3Σ states of HeLi+ and the associated transition dipole-moment functions. Cross-sections for the spontaneous and stimulated radiative association processes are calculated as functions of collision energy. The corresponding rate coefficients characterizing the efficiency of the formation of the molecular ion in its a3Σ+, b3Σ+, and c3Σ+ states from the initial 13Π state are obtained over a wide range of temperatures. At very low temperatures the 1→b process has a maximum rate-coefficient value of about 7.9×10−13cm3s−1, whereas process 1→a reaches its maximum value of 2.0×10−13cm3s−1 at a temperature of about 500K. Altogether the three radiative association processes investigated here can be considered as the continuum-to-bound state radiative transition part of the total quenching of the initial collision channel.

Radiative association of He(23P) with lithium cations

Formation of HeLi+ molecular ion by radiative association of metastable helium atoms with lithium ions is investigated on the triplet-Sigma manifold. Energy dependent cross-sections are calculated for He(23P) colliding with Li+ in the continuum of the c3Σ+ electronic state and undergoing radiative transition into one of the rotational-vibrational states of the lower electronic states a3Σ+,b3Σ+, or c3Σ+ of HeLi+. Evaluation of the cross-sections is based on the fully quantal approach taking into account all possible state-to-state transitions at thermal energies (spontaneous association) or at higher background temperatures (stimulated association). The corresponding rate coefficients are presented as functions of temperature. Calculations show that the rate coefficients for two-state processes c→a and c→b are several orders of magnitude larger than the rate coefficient for the c-state formation. The rate coefficient for the c→a radiative association reaches 10-15cm3s−1 at low temperatures. As expected stimulation of the radiative association by black-body background radiation has a noticeable effect only on the one-state c→c process.

Radiative association of He$_{2}^{+}$: the role of quartet states

Radiative association of He$_{2}^{+}$: the role of quartet states

Radiative association of He+ with H2 at temperatures below 100 K

The paper presents a theoretical study of the low-energy dynamics of radiative association processes in the He+ + H2 collision system. Formation of the triatomic HeH2(+) ion in its bound rotation-vibration states on the potential-energy surfaces of the ground and of the first excited electronic states is investigated. Close-coupling calculations are performed to determine detailed state-to-state characteristics (bound <-- free transition rates, radiative and dissociative widths of resonances) as well as temperature-average characteristics (rate constants, photon emission spectra) of the two-state (X <-- A) reaction He+(2S) + H2(X1sigma(g)+) --> HeH2(+)(X2A') + h nu and of the single-state (A <-- A) reaction He+(2S) + H2(X1sigma(g)+) --> HeH2(+)(A2A') + h nu. The potential-energy surfaces of the X- and A-electronic states of HeH2(+) and the dipole moment surfaces determined ab initio in an earlier work [Kraemer, Spirko, and Bludsky, Chem. Phys. 276, 225 (2002)] are used in the calculations. The rate constants k(T) as functions of temperature are calculated for the temperature interval 1 < or = T < or = 100 K. The maximum k(T) values are predicted as 3.3 x 10(-15) s(-1) cm3 for the X <-- A reaction and 2.3 x 10(-20) s(-1) cm3 for the A <-- A reaction at temperatures around 2 K. Rotationally predissociating states of the He+-H2 complex, correlating with the upsilon = 0, j = 2 state of free H2, are found to play a crucial role in the dynamics of the association reactions at low temperatures; their contribution to the k(T) function of the X <-- A reaction at T < 30 K is estimated as larger than 80%. The calculated partial rate constants and emission spectra show that in the X <-- A reaction the HeH2(+)(X) ion is formed in its highly excited vibrational states. This is in contrast with the vibrational state population of the ion when formed via the (X <-- X) reaction He(1S) + H2(+)(X2sigma(g)+) --> HeH2(+)(X2A') + h nu.

Stimulated Radiative Association of He and H+

The enhancement of the rate coefficient for the radiative association of He and H+ to form HeH+ arising from stimulated emission due to a blackbody radiation field is calculated. The effects on the fractional abundance of HeH+ in the early universe, in supernova ejecta, and in planetary nebulae are small. There may occur some enhancement in the abundance of HeH+ in quasar broad-line clouds.

Formation of HeH + by radiative association of He + + H. An advanced ab initio study

Rate coefficients of all possible radiative association reactions for He + + H are calculated over a wide temperature range. For this purpose the potential energy and electric dipole moment functions of the X̃ 1σ + ground electronic and the first excited ã 3σ + and à 1σ + states of HeH + are determined at the configuration interaction level of theory and the electronic transition dipole moment function between the two 1 Σ states is evaluated using the state interaction method of Malmqvist and Roos. From the potential energy functions, all rotation-vibrational bound and rotationally quasi-bound states are obtained for each electronic state. Whereas formation of the ã 3Σ + and à 1Σ + excited states in He + + H collisions have small rates, the most efficient process leads to the formation of the HeH + ground electronic state. The temperature dependence of the corresponding rate coefficient shows a typical behavior with a maximum of 6 × 10 −15 cm 3 s −1 around 10 K and a monotonic decrease with increasing temperature. Accurate potential functions derived from previous directly correlated wavefunctions are used to assess the reliability of the calculations.

The radiative association of He+ and He and H(+) and H

view Abstract Citations (48) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Radiative Association of He + and He and H + and H Stancil, P. C. ; Babb, J. F. ; Dalgarno, A. Abstract The rate coefficient for the radiative association of He+ and He to form He(2+) is determined for temperatures T between 20 K and 500,000 K. It can be represented by a given relation to within 25 percent applicable up to 50,000 K. It is about an order of magnitude less than the rate coefficient for the radiative association of H(+) and H to form H(2+). Cross sections for the He(2+) and H(2+) reactions calculated using a fully quantum-mechanical method reveal weak resonance features at low energies. Implications of the He(+) + He radiative association process are discussed briefly in connection with the chemistry of supernova ejecta, planetary nebulae, and the early universe. Publication: The Astrophysical Journal Pub Date: September 1993 DOI: 10.1086/173113 Bibcode: 1993ApJ...414..672S Keywords: Association Reactions; Helium Ions; Interstellar Chemistry; Molecular Ions; Cosmology; Hydrogen Ions; Planetary Nebulae; Supernova Remnants; Supernova 1987a; Atomic and Molecular Physics full text sources ADS | data products SIMBAD (2)

Radiative association in planetary atmospheres

A summary account is presented of radiative association processes that may be significant in producing continuum emission in the atmospheres of the planets. The importance of the radiative association of N and O and of NO+O has been established by the presence in the airglow of the emission spectra. The possible contributions of O+O, O+O 2 and O ++O are discussed. The radiative association of He + and H may be an important source of HeH + in the Jovian ionospheres.

The radiative association of He(+) and H

view Abstract Citations (60) References (5) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Radiative Association of He + and H Zygelman, B. ; Dalgarno, A. Abstract An accurate evaluation of the rate coefficient for the radiative association of He(+) and H to form HeH(+) is described. The rate coefficient at 10,000 K is 1 x 10 to the -15th cu cm/s. The new value leads to an enhanced production of HeH(+) in astrophysical plasmas. Publication: The Astrophysical Journal Pub Date: December 1990 DOI: 10.1086/169475 Bibcode: 1990ApJ...365..239Z Keywords: Association Reactions; Helium Ions; Hydrogen; Interstellar Chemistry; Interstellar Gas; Planetary Nebulae; Molecular Ions; Radiation Spectra; Stellar Envelopes; Astrophysics; INTERSTELLAR: MOLECULES; MOLECULAR PROCESSES; NEBULAE: PLANETARY full text sources ADS | data products SIMBAD (2)

Radiative charge transfer and radiative association of helium ions with neon atoms

Ab initio calculations are carried out for the dipole moments of the X2Σ+, A2Π, and B2Σ+ states of HeNe+, and the transition dipole moments connecting them. The effects of spin-orbit interactions are explored briefly. The transition dipole moments are used in a calculation of the rate coefficients of radiative charge transfer and radiative association of He+ ions in neon and the associated spectra are obtained. Comparison with experimental data provides support for the conclusion that the radiation detected was emitted in the course of the collisions of He+ with Ne. Some quantitative discrepancies remain which may arise from intensity stealing by the A22Π1/2 state from the X2Σ+ state.