Radiative Association Process Research Articles

Formation of S2 species in different redox states by radiative association in atomic and ionic collisions

Radiative associations for formations of the S2, S2+ and S2- molecular species during atomic collisions S(3Pu) + S(3Pu), S(3Pu) + S+(4Su) and S(3Pu) + S-(2Pu) are investigated. The adiabatic potential energy curves (PECs) and spin-allowed transition dipole moments (TDMs) are obtained by the internally contracted multireference configuration interaction method with the Davidson correction (icMRCI+Q). A number of PECs and TDMs are chosen to calculate the corresponding cross-sections and rate coefficients of radiative associations. The calculated rate coefficients are valid for the temperatures from 100 to 16000 K and fitted to the analytical function according to the three-parameter Arrhenius–Kooij formula. These results indicate that transitions originating in the ΔΛ=0 selection rule are the main contributors for the radiative association process. The present study can elucidate the further understanding the radiative association, which plays an important role in the formation and evolution of the S2, S2+ and S2- molecules.

Radiative association for the formation of phosphorus monochloryl cation (PCl+) and aluminium monochloride (AlCl)

ABSTRACT Cross-sections and rate coefficients for the radiative association processes of Al(2P) with Cl(2P) for the formation of aluminium monochloride (AlCl) and P+(3P) with Cl(2P) to form phosphorus monochloryl cation (PCl+) have been estimated as a function of temperature. Rate coefficients have been estimated from cross-sections, which are calculated using a quantum mechanical method. They are found to vary from 4.95 × 10–21 to 4.24 × 10–16 and from 7.10 × 10–18 to 3.50 × 10–17 cm3 s–1 for AlCl and PCl+, respectively, for temperatures ranging from 10 to 15 000 K. The obtained rate constants are fitted with the Arrhenius–Kooij functions for incorporation into astrochemical reaction data bases.

Formation of SiO+ through radiative association of Si+(3s23p 2Pu) and O(2s22p4 3Pg)

We investigate the radiative association of SiO+ in the collision of a Si+(3s23p 2Pu) cation and an O(2s22p4 3Pg) atom using the quantum mechanical method, including the cross sections and rate coefficients. We consider 18 dipole-allowed radiative association processes of SiO+. The results show that the 2 2Π → A 2Π transition contributes most for the SiO+ radiative association at temperatures from 10 to 10 000 K. The 2 2Π → X 2Σ+ and 2 2Σ− → A 2Π transitions are also relatively significant at high temperatures. The total rate coefficient is found to vary from 7.72 × 10−18 to 4.92 × 10−17 cm3 s−1. Finally, an analytical function is fitted to the total rate coefficient for the convenience of astrochemical modelling. The obtained cross sections and rate coefficients are expected to be useful for modelling the Si chemistry in the diffuse interstellar medium.

A quantum mechanical calculation of the CN radiative association

ABSTRACT Radiative association of CN is investigated through the quantum mechanical method, including the cross sections and rate coefficients. The ab initio potential energy curves, transition dipole moments, and permanent dipole moments of CN are obtained by the internally contracted multireference configuration interaction method with Davidson correction and aug-cc-pwCV5Z-DK basis set. For the collision of the ground state C (3Pg) and N (4Su) atoms, except for the four previously studied processes including the A2Π → X2Σ+, X2Σ+ → A2Π, A2Π → A2Π, and X2Σ+ → X2Σ+ transitions, four other radiative association processes including b4Π → a4Σ+, a4Σ+ → b4Π, b4Π → b4Π, and a4Σ+ → a4Σ+ transitions are considered. We also considered the collision of the excited C (1Dg) and the ground N (4Su) atoms including the 24Π → 14Σ− process and the collision of the ground C (3Pg) and the excited N (2Du) atoms including 22Π → B2Σ+, 32Π → B2Σ+, and 42Π → B2Σ+ transitions. The temperature population factor is considered to describe the thermal population of the three different dissociation asymptotic energies. The results show that the contribution of the A2Π → X2Σ+ and b4Π → a4Σ+ transitions to the total rate coefficients is significant over the entire temperature range. While considering the collision of C and N involving excited states, the contribution of the 22Π → B2Σ+, 32Π → B2Σ+, and 42Π → B2Σ+ transitions to the total rate coefficients cannot be ignored at the temperature range larger than 10 000 K. Finally, the rate coefficients are fitted to an analytical function for astrochemical reaction modelling.

Rate coefficients of the aluminium monoxide formation by radiative association

ABSTRACT Radiative association may be a key pathway for the formation of aluminium monoxide (AlO) in diffuse interstellar clouds, especially for the oxygen-rich asymptotic giant branch (AGB) stars. In this work, we investigated the radiative association of AlO by the collision of the aluminium and oxygen atoms in their electronic ground states, which is thought to be the most probable radiative association process. First, the potential energy curves for 12 electronic states and the dipole moments between these states were calculated by the state-of-the-art ab initio methodology. Then, the cross-sections and rate coefficients for the radiative association of AlO were computed for the temperatures in the range of 10–10 000 K. The total rate coefficients are of the order of 2.73 × 10−19–1.14 × 10−16 cm3 s−1, and the A2Π → X2Σ+, 12Δ → A2Π, 12Π → X2Σ+, and 22Σ− → A2Π transitions play a key role. The calculated results can be used to investigate the chemical evolution of dust formation in the photospheres of the oxygen-rich AGB stars.

Radiative association of atomic and ionic nitrogen

ABSTRACT Radiative association for the formation of molecular nitrogen cation ${\rm{N}}_2^ + $ during the collision of an N(4Su) atom and an N+(3Pg) ion is investigated. The corresponding cross-sections and rate coefficients are computed by the quantum mechanical method based on ab initio potential energy curves and transition dipole moments, which are obtained by the internally contracted multireference configuration interaction method with the Davidson correction and aug-cc-pCV5Z-DK basis set. A number of low-lying doublet, quartet, and sextet states correlating to the N(4Su) + N+(3Pg) dissociation limit are considered. Hence, we investigate a number of dipole-allowed transitions and determine their contributions to the radiative association. The results show that transitions originating in the f4Πu, D2Πg, ${{\rm{B}}^2}\Sigma _{\rm{u}}^ + $, ${{\rm{1}}^4}\Sigma _{\rm{g}}^{\rm{ + }}$, and ${{\rm{1}}^6}\Sigma _{\rm{u}}^{\rm{ + }}$states are the main contributors for the radiative association process. The calculated rate coefficients are valid for temperatures from 100 to 10 000 K and fitted to the analytical function suitable for astrochemical reaction applications.

Radiative association for the formation of MgO

ABSTRACTThe radiative association process for the formation of magnesium oxide (MgO) may be of great importance due to its frequent occurrence in the low-density and dust-poor astrochemical environments. In this work, the cross-sections and rate coefficients for the A1Π → X1Σ+, ${\rm X}^1\Sigma ^+\, \rightarrow \, {\rm A}^1\Pi$, D1Δ → A1Π, a3Π → e3Σ−, ${\rm X}^1\Sigma ^+\, \rightarrow \, {\rm X}^1\Sigma ^+$, and A1Π → A1Π radiative association processes of forming MgO are theoretically estimated. The cross-sections for the transitions between the different states are obtained by using the semiclassical method for direct contributions and the Breit–Wigner theory as a complement for resonance contributions. For the transitions between the same states, the quantum mechanical method is used. The rate coefficients are then obtained from the cross-sections for the temperatures in the range of 10–10 000 K and the results are found to vary from 4.69 $\times \, 10^{-16}$ to 6.27 $\times \, 10^{-14}$ cm3 s−1. For temperatures lower than around 693 K, the rate coefficients for the A1Π → X1Σ+ process are dominant, which indicates this process is the most efficient way of producing MgO at low temperatures. However, the rate coefficients for the D1Δ → A1Π process go through a rapid increase with increasing temperature and become dominant at higher temperatures. For other processes, their rate coefficients are several orders of magnitude lower than those for the two processes mentioned above. The results can be used to further investigate the formation and evolution of MgO in low density and hot gas close to the photosphere of evolved oxygen-rich stars.

Radiative Association of Atomic and Ionic Carbon

Abstract We present calculated cross sections and rate coefficients for the formation of the dicarbon cation ( ) by the radiative association process in collisions of a atom and a ion. Molecular structure calculations for a number of low-lying doublet and quartet states of are used to obtain the potential energy surfaces and transition dipole moments coupling the states of interest, substantially increasing the available molecular data for . Using a quantum-mechanical method, we explore a number of allowed transitions and determine those contributing to the radiative association process. The calculations extend the available data for this process down to the temperature of 100 K, where the rate coefficient is found to be about . We provide analytical fits suitable for incorporation into astrochemical reaction databases.

Formation of the NH molecule and its isotopologues through radiative association

The rate coefficients and the cross-sections for the formation of imidogen (NH) molecule (and its isotopologues: 15NH and ND) through radiative association are determined by employing quantum mechanical perturbation theory, classical Larmor formula, and Breit–Wigner theory. We suggest the radiative association process as possible route for NH production in diffuse interstellar clouds.

A surface-hopping method for semiclassical calculations of cross sections for radiative association with electronic transitions.

A semiclassical method based on surface-hopping techniques is developed to model the dynamics of radiative association with electronic transitions. It can be proven that this method is an extension of the established semiclassical formula used in the characterization of diatomic molecule-formation. Our method is tested for diatomic molecules. It gives the same cross sections as the former semiclassical formula but, contrary to the former method, it allows us to follow the fate of the trajectories after the emission of a photon. This means that we can characterize the rovibrational states of the stabilized molecules. Using semiclassical quantization, we can obtain quantum state-resolved cross sections or emission spectra for the radiative association process. The calculated semiclassical state-resolved spectra show general agreement with the result of quantum mechanical perturbation theory. Furthermore, our surface-hopping model is not only applicable for the description of radiative association but it can be used for semiclassical characterization of any molecular process where spontaneous emission occurs.

Radiative charge transfer and association in slow Li−+ H collisions

Aims. The radiative charge transfer and association processes in Li − + H collisions are studied in the 10 -10 −10 eV center-of-mass energy range. Methods. we carried out total and ν -resolved state-selective cross sections have been carried out by using the fully quantum, optical potential, and semiclassical methods. Results. In the energy region below ~ 0.8 eV, the radiative association process is the dominant decay channel, while radiative charge transfer dominates at higher energies. Rich resonance structures are observed in the cross sections of both processes in the 0.1−1.5 eV energy range; These structures are associated with the quasi-bound states below the top of the centrifugal barrier of the effective potential in the entrance channel for specific vibrational and angular momentum states. It is found that with the increase of collision energy, the resonances occur for higher angular momentum states and lower vibrational states. Besides the cross sections for the studied processes we also present their reaction rate coefficients in the 10 -6 –10 6 K temperature range.

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.

SYNTHESIS OF FORMAMIDE AND RELATED ORGANIC SPECIES IN THE INTERSTELLAR MEDIUM VIA CHEMICAL DYNAMICS SIMULATIONS

ABSTRACT We show, by means of direct dynamics simulations, how it is possible to define possible reactants and mechanisms leading to the formation of formamide in the interstellar medium. In particular, different ion–molecule reactions in the gas phase were considered: NH3OH+, NH2OH , H2COH+, and NH4 + for the ions and NH2OH, H2CO, and NH3 for the partner neutrals. These calculations were combined with high level ab initio calculations to investigate possible further evolution of the products observed. In particular, for formamide, we propose that the NH2OH + H2CO reaction can produce an isomer, NH2OCH , that, after dissociative recombination, can produce neutral formamide, which was observed in space. The direct dynamics do not pre-impose any reaction pathways and in other reactions, we did not observe the formation of formamide or any possible precursor. On the other hand, we obtained other interesting reactions, like the formation of NH2CH . Finally, some radiative association processes are proposed. All of the results obtained are discussed in light of the species observed in radioastronomy.

Charge transfer and association ofLi+colliding with Na from very low to intermediate energies

The nonradiative charge-transfer processes of ${\mathrm{Li}}^{+}+\mathrm{Na}(3s)$ collisions have been investigated by using the fully quantum-mechanical molecular-orbital close-coupling method and the two-center atomic-orbital close-coupling method for the energy range of ${10}^{\ensuremath{-}4}--2\phantom{\rule{0.16em}{0ex}}\mathrm{keV}/\mathrm{u}$ and $0.2--10\phantom{\rule{0.16em}{0ex}}\mathrm{keV}/\mathrm{u}$, respectively. The radiative charge-transfer, radiative decay, and radiative-association processes have been studied by employing the fully quantum, optical-potential, and semiclassical methods for the energy range of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}--110\phantom{\rule{0.16em}{0ex}}\mathrm{eV}/\mathrm{u}$. The nonradiative charge-transfer processes dominate the collisions for energies above 0.2 eV/u while radiative decay processes dominate in the lower-energy region. Especially, we found that the radiative-association process is more important than the radiative charge-transfer process when $E<2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}\mathrm{eV}/\mathrm{u}$. The rate coefficients of nonradiative and radiative processes are also given for the temperature range of $3\ifmmode\times\else\texttimes\fi{}{10}^{4}\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${10}^{\ensuremath{-}6}\ensuremath{-}{10}^{3}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, respectively.

Radiative association processes to specific rovibrational levels in low-energyNa++Rb87collisions

The radiative association processes for ${\mathrm{Na}}^{+}$ colliding with $^{87}\mathrm{Rb}(5s)$ atoms have been investigated by using the quantum-mechanical method. The total and partial cross sections that associate to the specific rovibrational states are calculated rigorously in the energy range of ${10}^{\ensuremath{-}13}\ensuremath{-}1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. The association cross sections display rich resonance structures. The ratios of rotationally resolved cross sections to total association cross sections and the ratios of vibrationally resolved cross sections to rotationally resolved cross sections are calculated and presented. For energies less than ${10}^{\ensuremath{-}9}\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, association to $X\phantom{\rule{0.16em}{0ex}}{}^{2}{\ensuremath{\Sigma}}^{+}$ ($J=1$) dominates over all other association states. As the energy increases, each resonance of the total cross sections is associated with the given angular component of specific bound states. In the energy range of ${10}^{\ensuremath{-}13}\ensuremath{-}1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, the vibrational states $v\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}27$ and $v\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}28$ are the most probable final states. The probability for the formation of molecular ion ${\mathrm{NaRb}}^{+}$ in the rovibrational states is discussed. The existence of Cooper minima in the radiative association cross sections is found and illustrated.

Charge transfer and association of Na+with87Rb atoms from extremely low to intermediate energies

The nonradiative charge-transfer processes in Na${}^{+}$+${}^{87}$Rb(5$s$) collisions have been investigated by using the quantum-mechanical molecular-orbital close-coupling method and the two-center atomic-orbital close-coupling method for the energy range of 10${}^{\ensuremath{-}4}$--5 and 0.3--100 keV/u, respectively. The radiative charge-transfer, radiative-decay, and radiative-association processes have been investigated by using the fully quantum, optical-potential, and semiclassical methods for the energy range of 10${}^{\ensuremath{-}18}$--0.2 eV/u. The nonradiative charge-transfer processes dominate the collisions for energies above 0.2 eV/u and radiative-decay processes dominate in the lower-energy region. At the very low collision energies of 10${}^{\ensuremath{-}18}$--10${}^{\ensuremath{-}3}$ eV/u, the radiative-association process is more important than the radiative charge-transfer process. Most importantly, it is found that the radiative cross sections exhibit Langevin behavior as ${E}^{\ensuremath{-}1/2}$ for energies less than 10${}^{\ensuremath{-}2}$ eV/u.

Radiative association of LiHe+

Spontaneous radiative association of the molecular ion LiHe+ is investigated including three electronic states X1Σ+,A1Σ+, and a3Σ+. Cross sections for four processes of radiative association X→X, A→A, A→X, and a→a are calculated as functions of collision energy. The corresponding rates of formation are derived as functions of temperature. The A→X radiative association process exhibits the largest cross sections and rate coefficients because of the huge amount of the de-excitation energy from the A state to the X state.

Measurement of a Large Chemical Reaction Rate between Ultracold Closed-ShellCa40Atoms and Open-ShellYb+174Ions Held in a Hybrid Atom-Ion Trap

Ultracold 174Yb+ ions and 40Ca atoms are confined in a hybrid trap. The charge exchange chemical reaction rate constant between these two species is measured and found to be 4 orders of magnitude larger than recent measurements in other heteronuclear systems. The structure of the CaYb+ molecule is determined and used in a calculation that explains the fast chemical reaction as a consequence of strong radiative charge transfer. A possible explanation is offered for the apparent contradiction between typical theoretical predictions and measurements of the radiative association process in this and other recent experiments.

Radiative and collisional processes in space chemistry

The theoretical treatment of radiative association and charge transfer processes is developed using ab initio molecular calculations. Semi-classical and quantal dynamical approaches are presented for the determination of cross sections and rate constants which are important data for space chemistry models. Results are detailed for two specific reactions illustrating these important processes: the formation of H2O by radiative association in the atmosphere of evolved stars in the high temperature region and the C+(2P) + S(3P) → C(3P) + S+(4S) charge transfer process which is a fundamental reaction of the interstellar medium which drives the ionization balance of carbon and sulphur.

One-photon-assisted formation of ultracold polar molecules

Alkali-metal hydride molecules have large dipole moments in their ground electronic states. We explore the possibility of forming such molecules from a mixture of the ultracold atomic gases, employing a one-photon stimulated radiative association process. Using accurate molecular potential-energy curves and dipole moments, we have calculated the rate coefficients for populating each of the vibrational levels of the $X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}^{+}$ states of LiH and NaH. We have found that significant molecule formation rates into the upper vibrational levels can be realized with laser intensities and atomic densities that are easily attainable experimentally. We examine the spontaneous emission cascade which takes place from these upper vibrational levels on a time scale of milliseconds, and calculate the resulting rotational populations in $v=0$. We show that photon emission in the cascade process does not contribute to trap loss and that a large population of molecules in $v=0$ can be achieved.