Abstract Listing

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Astrophysics and Ionospheres
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Last Modified: 2001-04-20

Abstract
In low temperature interstellar clouds and in circumstellar envelopes, many molecules are synthesized by chains of ion-molecule reactions. The final step in the production of neutral species is likely to be a rapid dissociative recombination reaction between a positive molecular ion and an electron. Although large numbers of dissociative recombination reactions are included in model networks of the chemistry of interstellar clouds, laboratory information concerning the neutral product branching fractions of such reactions has been slow in coming. In recent years, a variety of important systems have been studied using storage rings. Modellers must still extrapolate these results to the large number of systems yet unstudied. Current ideas on how to perform this extrapolation and how sensitive the results are to it will be discussed. The importance of radiative recombination channels for large molecules will also be touched upon.

450607: Help!!! Theory for H₃⁺ recombination badly needed

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Dissociative Recombination of H₃⁺
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Last Modified: 2001-04-18

Abstract
The recent discovery of large column densities of H₃⁺ in the diffuse interstellar medium has revealed an enigma in its chemistry which is closely related to dissociative recombination of H₃⁺. The currently accepted values of cosmic ray ionization rate and dissociative recombination rate lead to a very low H₃⁺ number density and thus an unconventionally long column length. An accurate value of the recombination rate is imperative in this analysis. Although experimental values of the recombination rate measured by several different techniques are converging, theoretical values are lower than the experimental values by orders of magnitude. H₃⁺ recombines via Rydberg states which are very sensitive to stray electric fields. Since such fields cannot be completely eliminated in the laboratory experiments we need independent theoretical values. Since H₃⁺ is the most fundamental species, this presents a rare occasion in which a new development in the molecular theory is crucial for an important general astrophysical problem.

451854: Quantum dynamical study of H₃⁺ recombination

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Dissociative Recombination of H₃⁺
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Preferred Presentation Format: Oral
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Last Modified: 2001-04-13

Abstract
We have performed three-dimensional quantum dynamical study on dissociative recombination of H₃⁺. Since the lowest Rydberg state, 2sA₁^' state, is rather isolated from other Rydberg states of H₃, and has crossing with upper dissociative surface, we pay attention on the transition from 2sA₁^' state to dissociative states. First, using hyperspherical coordinates, we have calculated three components of non-adiabatic couplings between 2sA₁^' state and dissociative states over wide range of geometry. The results shows large couplings between 2sA₁^' state and upper dissociative state at compact geometry. Next, we have calculated hundreds of vibrational states on 2sA₁^' surface. Then we have calculated predissociation rates of these vibrational states using calculated couplings, where we have utilized time-dependent version of the Fermi's golden rule. Based on these rates and other calculated quantities, we will discuss how H₃⁺ recombination proceeds.

452347: Mode-dependent vibrational autoionization in ammonia

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Ionization
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Last Modified: 2001-04-20

Abstract
Double-resonance excitation has been used to prepare Rydberg states of ammonia in which two different vibrational modes are excited, and photoelectron spectroscopy has been used to study the vibrational autoionization of these states. Spectra for Rydberg states converging to levels of the ion in which both the symmetric stretch and umbrella mode are excited indicate that autoionization via the umbrella mode is significantly faster than via the symmetric stretching mode. This observation is rationalized in terms of Walsh diagrams for the two modes. Rotationally resolved photoelectron spectra provide additional information on the decay mechanism via the umbrella mode. This information is consistent with the decay of ns or nd Rydberg series through the ejection of odd partial waves. The potential relevance of this work to dissociative recombination will be discussed. This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract W-31-109-ENG-38.

453458: Electron collision studies on CN⁺, HCN⁺/HNC⁺, CN^- and C₄^-

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Storage Rings
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Last Modified: 2001-04-20

Abstract
Absolute cross sections and rate coefficients were measured for the dissociative recombination of CN⁺(X¹S⁺ and a³P, v=0). Branching fractions over the final product states were also determined. Unusually large dissociative excitation cross sections are reported. The dissociative recombination of isomeric HCN⁺/HNC⁺ target ions was investigated. A HCN⁺/HNC⁺ mixture was first studied, and HNC⁺ was then isolated. The two sets of measurements exhibit differences.

Electrons were scattered on CN^- ions. Pure detachment completely dominates over fragmentation channels. The cross section rises from threshold at 7 eV up to a plateau with a 5 Å² cross section value. Electron collisions on C₄^- anions were also investigated. Total cross sections for the C₄, C₃, C₂ and C productions were measured. Then a grid was inserted in front of the detector. We deduced cross sections for the pure detachment (found overwhelmingly dominant), but also for the fragmentation into C₃+C, 2C₂ and C₂+2C. A near-threshold resonance is attributed to the short lived C₄^-2.

453598: Analytical treatment of the K-matrix integral equation

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Theory
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Last Modified: 2001-04-20

Abstract
First, general theoretical procedure to investigate dissociative recombination processes is presented. This is composed of quantum chemical calculations, MQDT analysis of spectroscopic experiments, evaluation of electronic coupling functions, non-perturbative solution of K-matrix integral equation, and MQDT analysis of dynamic processes. Then, it is demonstrated that the K-matrix equation can be solved analytically under the good approximation of separable electronic coupling function. Numerical test is provided for the dissociative recombination of hydrogen molecular ion.

453657: Wave packet studies of dissociative recombination

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Theory
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Special Equipment Needs: overhead projector
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Last Modified: 2001-04-20

Abstract
Wave packet methods, that is, numerical solutions of the time-dependent Schrödinger equation, are standard techniques in the study of the dynamics of chemical reactions. We have used these techniques with great success in the calculation of cross sections for dissociative recombination and dissociative excitation of molecular ions by electron impact. We will outline the basic method and the approximations involved in its implementation, both in the case of diatomic and polyatomic systems. We will illustrate the method with a number of examples, including ion-pair formation in HD⁺ and dissociative recombination of the water ion.

454123: Three body kinematical correlation in the dissociative recombination of H₃⁺

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Molecular dissociation dynamics
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Last Modified: 2001-04-19

Abstract
From its conceptual simplicity, H₃ occupies a special place in molecular physics and quantum chemistry, in particular with regard to studies of nuclear motion on molecular potential surfaces. Particular interest in the nuclear dynamics on the H₃ ground-state potential surface stems also from the dissociative recombination of the H₃⁺ion -- a process where it captures an electron and dissociates into neutral fragments. The H₃⁺ ion acts as a key species in the chemistry of the interstellar medium, and notably its abundance in diffuse interstellar clouds, strongly affected by the dissociative recombination with low-energy electrons, is still is a matter of vivid discussion. We have measured the fragmentation patterns for the dissociative recombination of the triatomic hydrogen molecular ion in the vibrational ground state using the storage ring technique and molecular fragment imaging. A broad distribution of vibrational states in the molecular fragment after two-body dissociation, and a large predominance of nearly linear momentum geometries after three-body dissociation were found. We also compare our results to a simple model based on phase-space consideration.

454823: Electron induced vibrational cooling of the hydrogen molecular ions H₂⁺ and D₂⁺

Abstract
Rate coefficients for the vibrational de-excitation and dissociative recombination in collisions between H₂⁺ and D₂⁺ ions and low-energy electrons (kT < 30 meV) have been measured at the storage ring TSR, Heidelberg. The recombination of the molecular ions with electrons proceeds through a doubly excited neutral state and can either lead to dissociation (DR) or to autoionization. The latter process can lead to vibrational cooling by transferring internal energy to the escaping electron (superelastic collision, SEC). Mainly through SEC, vibrational cooling of stored H₂⁺ and D₂⁺ ion beams was observed using the foil-induced Coulomb explosion imaging (CEI) technique. Combining the measured time evolution of the relative vibrational population with the vibrational state specific contribution to the DR - obtained by imaging of the recombination fragments - relative DR rate coefficients for the lower vibrational states have been obtained. Taking into account the influence of DR on the time dependence of the vibrational populations, SEC rate coefficients were derived. Lately the measurements have been extented to determine also the absolute DR rate coefficents for vibrationally cold H₂⁺ and D₂⁺ ions (more than 99.9 % in v=0 after 30 s).

455271: Absolute cross sections for state-selected reactions of O⁺ (⁴S, ²D, ²P)

Abstract
We have obtained reliable absolute cross sections for the ion-molecule reactions O⁺(⁴S, ²D, ²P) + N₂ (H₂, CO₂, H₂O, O₂), which are recognized as the most important set of reactions in planetary ionospheres. A novel technique, which combines the radio frequency octopole ion guide and the dissociative charge transfer reactions He⁺ (Ne⁺, Ar⁺) + O₂, has been successfully demonstrated and used for preparing state-selected reactant O⁺(²P), O⁺(²D), and O⁺(⁴S) ions with high purities. We have also developed a differential retarding potential method for improving the center-of-mass kinetic energy (E_c.m.) resolution. These developments have made possible the measurement of absolute cross sections for the reactions involving state-selected O⁺(⁴S, ²D, ²P) at kinetic energies down close to thermal energies. Notably, charge transfer product O₂⁺ ions formed in the O⁺(⁴S, ²D, ²P) + O₂ reaction are known to undergo rapid dissociative recombination reactions with electrons, giving rise to excited oxygen atoms, which are the source of sky aurora.

455332: Dissociative recombination in planetary and cometary aeronomy

Abstract
The dissociative recombination of molecular ions plays an important role in the ionospheres of the planets, satellites, and comets. This is the main loss process for ionospheric plasma and is a key ingredient in the determination of electron densities and ion composition. Consequently, this process also has an important effect on the dynamics of the plasma. Examples of the dynamical and chemical role of dissociative recombination will be presented for the ionospheres of Venus, Titan, Jupiter, and comets. The hot atoms and molecules produced by dissociative recombination reactions are also important for the neutral atmospheres of solar system bodies. For example, the exospheres of both Venus and Mars are largely populated by superthermal oxygen atoms produced by the dissociative recombination of molecular oxygen ions.

455430: Reactions of electrons with hydrocarbon cations: From linear alkanes to aromatic species

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Afterglow Experiments
Invited: Y
Preferred Presentation Format: Oral
Consider for Sci-Mix: N
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Last Modified: 2001-04-18

Abstract
Our knowledge of the recombination of complex ions is derived almost entirely from experimental investigations. The work done at Rennes has been performed at 300 K, using the FALP-MS technique. We have studied the recombination kinetics of branched and cyclic carbocations, formed from saturated and unsaturated aliphatic hydrocarbons or aromatic species like benzene and methyl-substituted benzene. When not gaseous, these species are liquid at room temperature and have been injected by means of a bubbler. In order to study heavier cations whose neutral parent are solid, an oven has been constructed and several parts of the apparatus have been modified by adding heating devices. It should be noted that the study of the dissociative recombination of a positive ion is possible only if its neutral parent does not attach electrons. Phenanthrene, Fluoranthene, and Triphenylene fulfill this condition, but very little is known about the electron attachment of other affordable PAHs. Our latest results will be presented.

455642: Impulsive photodissociation dynamics

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Molecular dissociation dynamics
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Last Modified: 2001-04-20

Abstract
Impulsive optical excitation is a general method which allows to manipulate and control molecular dynamics. The timescale of the process is chosen to be short on the time scale of nuclear motion and long relative to typical electronic excitation time. The method can be employed to control the ground state dynamics of a molecule. The case of the tri-iodide molecular ion will be used to demonstrate the principle. The process can be also employed to control photodissociation dynamics using pump-probe techniques. A controlled pump probe technique has also been used for the ionization of Li₂ demonstrating how this simple process can be employed as a quantum computer. Finally the same principle can be used for the control of photoassociation reactions. An impulsive probe can be used to locate in phase space the position and momentum of the molecule.

455855: Optical spectroscopy of recombining ions in flowing afterglow plasmas

Abstract
The recombination of molecular ions with electrons often results in excited molecular fragments that can be observed by molecular emission spectroscopy. Flowing afterglow plasmas are highly suitable for such studies since the resulting emissions are strong and readily observable, but the interpretation of the spectra in terms of yields of particular states require a well-calibrated spectroscopic system, extensive modeling of the ion-chemical processes to identify the recombining species, and corrections for cascading from higher states into the observed states. In addition, isomeric forms of the recombining ion species may complicate the analysis. In this progress report, we discuss the experimental methods and present, in part preliminary, yields for several radiating states that arise from the dissociative recombination of N2O+, N2OH+/ HN2O+, CO2+ , CO2H+, and HCO+/COH+. No emissions were detected from recombination of the astrophysically important H3+ ion, which may be surprising since its recombination has been invoked as one mechanism for producing H3* spectral emissions.

455930: Dissociative electron attachment in gas and condensed phases

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Electron Attachment
Invited: Y
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Consider for Sci-Mix: N
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Last Modified: 2001-04-19

Abstract
Dissociative electron attachment is a resonance process which can be investigated by using the one-pole approximation of the R matrix theory. We apply a quasiclassical version of this approach to dissociative attachment to methyl halides in gas and condensed phases. Many interesting observations, particularly a strong temperature effect for methyl chloride, vibrational Feshbach resonance for methyl iodide, and condensed-matter effects in dissociative attachment to methyl- and perfluoromethyl chloride are explained by our theoretical calculations. The same approach is being developed for description of electron attachment to clusters.

455989: Importance of Jahn-Teller coupling in the dissociative recombination of H₃⁺ by low energy electrons

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Dissociative Recombination of H₃⁺
Invited: Y
Preferred Presentation Format: Oral
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Special Equipment Needs: laptop projector (for laptop running Windows 98)
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Last Modified: 2001-04-17

Abstract
The dissociative recombination (DR) rate of an H₃⁺ ion bombarded by a low energy electron has been measured in numerous experiments. We have formulated this problem within a theoretical framework that recasts the low-energy reaction as a conventional "curve crossing problem". This treatment of the full Jahn-Teller coupling physics results in a low energy recombination rate much larger than previous theory. Other observables like the fragmentation branching ratios have also been calculated and compared with experiment, and they show encouraging agreement.

456373: Molecular photodissociation and dissociative recombination

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Molecular dissociation dynamics
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Last Modified: 2001-04-19

Abstract
The theory of photodissociation processes using quantum wavepacket dynamics will be reviewed. Applications of the theory will be presented for several molecular systems (HF, HCl, HOBr and N₂O) and comparisons will be made with experimental results. Particular attention will be paid to the prediction of product quantum state distributions and to the role of electronically non-adiabatic transitions. New work on the dissociative recombination of H₃⁺ and on attempts to reconcile theoretical predictions and experimental observations for this process will be presented.

456399: Dissociative electron attachment at low temperatures with molecules and clusters

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Electron Attachment
Invited: Y
Preferred Presentation Format: Oral
Consider for Sci-Mix: N
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Last Modified: 2001-04-19

Dissociative electron attachment to molecules is very similar to the dissociative recombination of molecular ions with electrons. When attachment occurs to a Van der Waals dimer or higher oligomer, a significant chemical re-arrangement happens. There are a variety of situations where attachment does not occur with a monomer M and yet is efficient with clusters involving M_n starting at a given size n, often due to thermodynamic reasons. The role that such processes could play in interstellar chemistry has recently been identified for carbon clusters.

We have used the CRESU experiment in order to study attachment to clusters of HCl, HBr, C₃F₆, H₂O and C₆H₆ down to 50 K. The importance of processes such as:

has been demonstrated and a lower limit for the rate coefficient has sometimes been obtained.

456847: State-selected associative ionization as a probe of the molecular dissociative channels

Abstract
Associative ionization and dissociative recombination share the same reaction channels, although the precise dynamics depends on the actual collision taking place. The large angular momentum put into the system by heavy particle collisions does indeed obliterate the subtle role of closed Rydberg channels, as these resonances are displaced by the centrifugal energy. However, the systematic study of associative ionization processes at low energy in hydrogen atom collisions provides detailed information on the branching ratios and preferred symmetries of dissociative recombination. The effect of isotopomers on total cross sections, together with the recent development of a new diagnostic tool to measure the internal energy of the associative ionization products, give an unambiguous identification of the reaction pathways. Such an analysis is in progress for the HeH system.

456932: Extension of the quantum defect theory and its application to electron and molecular ion collisions

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Theory
Invited: Y
Preferred Presentation Format: Oral
Consider for Sci-Mix: N
Special Equipment Needs: projector with RGB input (for power point)
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Last Modified: 2001-04-20

The dissociative states can be included into the framework of the quantum defect theory (QDT) if we regard the dissociative state as a highly excited vibrational state above the dissociation limit. By extending the QDT to include the frame transformation of such discretized dissociative states, we can uniformly represent the dissociative processes. This method enables one to investigate the dissociative recombination (DR) affected by the dissociative intermediate states, the dissociative excitation (DE) and the DR producing electronically highly excited atoms. After confirming the validity of that method for the HeH⁺ DR, we shall report on the DR, DE, and rot-vibrational transition by electron impact for the hydrogen molecular ions of various rot-vibrational states. Finally we shall discuss an application of this method to the DR of H₃⁺.

457586: Coupling bound states to continua in neutral triatomic hydrogen

Abstract
Coincident multi-particle imaging [U. Müller et al., Phys. Rev. Lett. 83 2718 (1999)] is applied to investigate the dissociation of state-selected Rydberg states of neutral triatomic hydrogen, H₃. This technique yields the branching ratio into H₂+H and H+H+H channels, the ro-vibrational distribution of the H₂ fragments, as well as the momentum vector correlation in the event of three-particle breakup. The correlation experiments reveal complex phase-space images of the nonadiabatic coupling, which opens the bound excited state to the dissociative continuum. This correlation has been studied for excited s-, p-, and d-manifolds for various vibrational and rotational levels in H₃ and in the heavy homonuclear isotope, D₃. A second experiment [I. Mistrik et al., Phys. Rev. A 68 042711 (2001)] investigates the competition between predissociation and autoionization of triatomic hydrogen at the ionization threshold. These experiments capture intermediates active in dissociative recombination. We will discuss our results in the spirit of dissociative recombination of slow electrons with H₃⁺.

457587: An electrostatic storage ring for the research of electron-ion collisions at KEK

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Storage Rings
Invited: Y
Preferred Presentation Format: Oral
Consider for Sci-Mix: N
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Last Modified: 2001-04-20

Abstract
The cooler ring TARN II that had been used for the research of the dissociative recombination of molecular ions with electrons since 1990 has shut down in 1999 with closing of Tanashi campus of KEK. Instead of the magnetic storage ring TARN II, an electrostatic storage ring was newly built at Tsukuba campus of KEK and ion beams were first stored in May, 2000. The ring has a circumference of 8 m and can store light to heavy ions independent of their masses up to an E/q of 30 keV. Light ions are produced with an ECR ion source. Furthermore, an electrospray ion source will be added to produce macromolecular ions. Measured lifetimes of stored light single-charge-ions are from 10 to 50 s at a vacuum pressure in the order of 10e-11 Torr. In order to study electron-ion collisions, a compact electron target/cooler has been designed, which will be installed in a straight section of the ring. The status of the electrostatic ring including the ion source and the electron target is presented.

457731: Dissociative recombination of electrons and ions: The early experiments

Abstract
The discovery in the late 1940s of a new, very rapid electron-ion recombination process is described. Microwave and fast electronic techniques that became available in the post-WW2 period were applied to afterglow studies of electron removal in weakly ionized noble gases. Surprisingly, the electron loss was controlled not by ambipolar diffusion, as expected, but by very fast recombination with ions (at ~100,000 times the theoretical radiative recombination rate). Shortly before this, D. R. Bates and colleagues had postulated a new process, dissociative recombination [DR] of molecular ions and electrons, to explain rapid electron loss in the Earth's ionosphere. In 1950 he suggested that DR was responsible for our results. Follow-on laboratory experiments demonstrated the required presence of molecular noble gas ions for the occurrence of the large electron loss. Afterglow spectroscopic studies indicated that excited atoms were produced by the molecular ion recombination. The clinching evidence for DR was provided by interferometric studies of the afterglow line shapes. These showed that the excited atoms emitted a characteristic DR-broadened line profile.

457750: Recombination of H₃⁺ and D₃⁺ with electrons: Low limit of the recombination rate coefficient

From the decay of plasma in the mixture of He, Ar and H₂ (or D₂) we determined the overall rate constant (a) of the recombination of H₃⁺ and D₃⁺ ions with electrons. We observed pronounced dependence of a on partial pressure of hydrogen (deuterium). The dependence of a on the H₂ (or D₂) indicates that observed recombination proceeds most probably via formation of long lived Rydberg states that are stabilised against the reverse autoionisation by collision with neutral molecule. From our study it follows that binary dissociative recombination of H₃⁺ ions with electrons is very slow process (at 270 K) - with rate coefficient a < 3×10^-9 cm³s^-1.

This work was supported in part by Grant Agency GACR No. 202/99/D061, 202/00/1689, by Charles University under project No. 146/2000/B FYZ MFF. The studies were carried out with support of EU in frame of the ETR network (HPRN-CT-2000-00142).

457913: Dynamics of three-body dissociative recombination of dihydrides

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Molecular dissociation dynamics
Invited: Y
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Consider for Sci-Mix: N
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Last Modified: 2001-04-19

Abstract
Dissociative recombination of tri-atomic dihydrides, e.g., H2D+, CH2+, NH2+ and H2O+ show a large propensity for break up into three atoms. The three-body yields for these cases ranges from 60 to80%. In the cases of CH2+, NH2+, and OH2+ sufficient energy is released to yield the first excited electronic states of C, N, and O. We determine (1) the fraction going to the ground and excited states; (2) the distribution of recoil energies; and (3) the angular distribution of the two H atoms for each state of the center atom. The work was done at CRYRING at Manne Siegbahn Laboratory in Stockholm and used modified imaging technique that will be described.

458058: Advances in the calculation of electron affinities

Abstract
During the past few years there have been remarkable advances in the quantitative calculation of atomic and molecular electron affinities. It is now possible to calculate many electron affinities with an accuracy better than 1 kcal/mol (0.05 eV). These advances are a result of developments in coupled cluster theory combined with a family of basis sets that systematically converge to the complete basis set limit. We will review recent state-of-the-art calculations of electron affinities for selected atoms and molecules, clearly distinguishing between the basis set convergence error and the error intrinsic to the theoretical method being used.

* Pacific Northwest Laboratory is operated for the U.S. Department of Energy by Battelle Memorial Institute under contract DE-AC06-76RLO 1830.

458407: Technique for distinguishing and determining the origin of photon emissions from He⁺/Ar⁺ plasmas: Recombination emission with addition of OCS, CS₂ and H₂S

ACCEPTED
Topic Selection: Dissociative Recombination of Molecules with Electrons: Afterglow Experiments
Invited: Y
Preferred Presentation Format: OralOnly
Consider for Sci-Mix: N
Conforms to Bylaw 6: Y
Last Modified: 2001-04-18

Abstract
In plasmas containing reactive gases, photon emissions can occur from a number of reactions. For many situations, it is not possible to ensure that the overall emission results only from a reaction of interest, for example electron-ion recombination. Other emissions from reactions such as the ion-molecule reactions creating the recombining ions often interfere. A flow tube technique has been developed for distinguishing the emissions from specific reactions in the presence of other, often more intense emissions. This technique will be described and illustrated through its application to emissions (in the wavelength range 180-800 nm), from dissociative electron-ion recombination, emanating from He⁺/Ar⁺ plasmas containing OCS, CS₂ and H₂S. In such plasmas, recombination emissions (variously from electronically excited states of S, CS and SH) have been detected from OCS⁺, HOCS⁺, CS₂⁺, HCS₂⁺ and H₃S⁺ recombinations with electrons and these studies will be discussed. Financial support from ACS/PRF for ion-molecule reaction studies (33892-AC6) and NASA for the recombination studies (NAG5-8951) is gratefully acknowledged.

458667: Dissociative recombination and deuterium fractionation in interstellar clouds

Abstract
The recent detection of doubly deuterated ammonia in the dense quiescent interstellar cloud L134N by Roueff et al. 2000, suggests that gas phase mechanisms are at work. The general features of deuterium enrichment in interstellar clouds via gas phase chemical processes will be reviewed. Particular emphasis will be put on the branching ratios in dissociative recombination of deuterated molecular ions.

458815: Great enhancements in dissociative electron attachment to chlorine-containing molecules adsorbed on H₂O / NH₃ ice

ACCEPTED
Topic Selection: Dissociative Recombination of Molecules with Electrons: Electron Attachment
Invited: N
Comments to Organizer: We strongly request for an oral presentation
Preferred Presentation Format: OralOnly
Consider for Sci-Mix: N
Special Equipment Needs: standard
Conforms to Bylaw 6: Y
Last Modified: 2001-04-19

Abstract
Dissociative electron attachment (DEA) to molecules in polar media may be an important process in the Earth’s atmosphere and environment. We report that the presence of H₂O / NH₃ greatly enhances DEA of ~0 eV electrons to CF₂Cl₂, CFCl₃ and HCl molecules, respectively. The absolute DEA cross sections for these molecules adsorbed on H₂O / NH₃ ice are measured to be two to three orders of magnitude larger than those in the gas phase. This enhancement is due to the transfer of electrons trapped in the precursors of the fully solvated state in water or ammonia ice to chlorine-containing molecules that then dissociate. The results indicate that DEA to these ozone-depleting molecules adsorbed on polar stratospheric clouds under cosmic ray radiation is a very efficient process. The implication of this observation to atmospheric ozone depletion will be discussed.

459044: Chiral recognition via excess electron attachment to the 1,3-butandiol/2-butanol complex: Ab initio study

ACCEPTED
Topic Selection: Dissociative Recombination of Molecules with Electrons: Electron Attachment
Invited: N
Preferred Presentation Format: OralOnly
Consider for Sci-Mix: N
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Last Modified: 2001-04-19

Abstract
Investigations into the different dipole bound isomers of 1,3-Butandiol/2-Butanol were performed by theoretical ab initio calculations. Four sterochemical isomers of this complex were optimized, including the RR, RS, SR, and SS chiral species for the complex between 1,3-butandiol and 2-butanol, respectively . Interestingly enough, the RS complex seems to be the most stable acceptor of the excess electron, which is in agreement with the concept that complexes between two molecules of different chiralities leads to molecular stabilization. In order for a chemical system to form a dipole bound anionic state it must posses a dipole moment that exceeds 3.5 debye. Thus we prove that in order for chiral recognition by excess electron attachment to occur, the complexes formed must differ, to some degree, in their dipole moments. If the chirally recognized compounds do in fact have substantial dipoles and the adiabatic electron affinity is high enough then an experimental technique such as Rydberg electron-transfer spectroscopy can be used to verify the calculated results.

459203: Photodissociation and unimolecular dissociation of molecules

ACCEPTED
Topic Selection: Dissociative Recombination of Molecules with Electrons: Molecular dissociation dynamics
Invited: Y
Preferred Presentation Format: Oral
Consider for Sci-Mix: N
Conforms to Bylaw 6: Y
Last Modified: 2001-04-19

Abstract
The theory of photodissociation and unimolecular dissociation of polyatomic molecules will be discussed. Both the construction of reliable multi-dimensional potential energy surfaces and the numerical methods for performing dynamics calculations will be addressed. As particular examples we will elucidate the Renner-Teller induced predissociation of HNO in the first excited singlet state and the dissociation of ozone in the ground state. In the case of HNO, we will focus on mixing effects between resonances belonging to different electronic states and the comparison with spectroscopic data. In the case of ozone, we will discuss the spectroscopy at the dissociation threshold and possible implications for recombination processes.

459414: Electronic dissociative recombination of an ion of astrophysical interest : Quantum chemical calculations

ACCEPTED
Topic Selection: Dissociative Recombination of Molecules with Electrons: Astrophysics and Ionospheres
Invited: Y
Preferred Presentation Format: Oral
Consider for Sci-Mix: N
Conforms to Bylaw 6: Y
Last Modified: 2001-04-20

Abstract
Quantum chemical calculations have proven to be extremely useful for the understanding of electronic dissociative recombination processes. Such processes may occur by direct and/or indirect mechanisms; while the former involves a direct transition from the ionic state to the final neutral dissociative state, the second one is a two step process involving the formation of a vibrationally (rotationally) excited state of the neutral molecule. For this last case, either autoionization takes place or the state obtained after binding of the electron relaxes to lower energy states, leading if they have a dissociative character, to fragmentation of the molecule. Understanding such processes requires the knowledge of the potential energy surfaces governing the corresponding mechanism i.e the potential surfaces of the ionic, excited and dissociative states of the parent neutral molecule. The difficulty in treating such problems resides in, first of all, the description of excited and dissociative surfaces. This is not a trivial task because, very often, the nature of the states changes along such channels; carefully designed wavefunctions are thus necessary to follow those transformations. But the difficulty is also in the even handed treatment required for all the states involved in the mechanism. State of the art ab-initio quantum chemical methods are needed for a quantitative study of this kind of process. To illustrate this matter, a recent work undertaken on the formation of cyclopropenylidene will be presented. Why cyclopropenylidene ? Cyclopropenylidene (c-C3H2) is a molecule widely spread in the galaxy and is present in environments as different as dense or diffuse interstellar clouds. In space, its formation is attributed to the electronic dissociative recombination of cyclopropenyl ion : c-C3H3+ + e- -> c-C3H2 + H. Although numerous theoretical and experimental investigations are available for the cyclic neutral species, its way of formation is still not understood. Therefore an extensive quantum chemical study has been undertaken on the electronic dissociative recombination of the cyclopropenyl ion using selected wavefunctions and large scale CI calculations. The results will be presented and discussed in relation with the few available experimental data.

459686: Stark mixing in dissociative recombination

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Theory
Invited: Y
Preferred Presentation Format: OralOnly
Consider for Sci-Mix: N
Conforms to Bylaw 6: Y
Last Modified: 2001-04-20

Abstract
The indirect mechanism of dissociative recombination involves the vibrational excitation of the ion with the subsequent capture of the colliding electron into a Rydberg state nl. Rydberg molecules are formed in high n and low l. The angular momentum l is redistributed¹ to intermediate and large l by the very efficient process of angular momentum (Stark) Mixing. The much larger lifetimes of these high l-states towards autoionization/radiative relaxation/predissociation can therefore increase the probability that collisional deactivation of the Rydbergs by electrons or neutrals and rearrangement collisions can occur. In this paper, probabilities² are presented for nl->nl' transitions in a Rydberg atom induced by the time dependent (dipole) electric field generated by adiabatic collision with charged particles.

459806: Electronic accessibility of product channels in photodissociation: Implications for dissociative recombination processes

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Molecular dissociation dynamics
Invited: Y
Preferred Presentation Format: OralOnly
Consider for Sci-Mix: N
Conforms to Bylaw 6: Y
Last Modified: 2001-04-19

Abstract
The electronic and nuclear dynamics during the photodissociation of polyatomic molecules and the latter portion of the dissociative recombination of electrons with polyatomic ions potentially share a common theoretical description. A central question is how to predict which polyatomic neutral fragments are formed, and in what branching ratios, when the ABC* complex, formed by either electron ion recombination or by photoexcitation of a neutral molecule to an excited electronic state, breaks up into polyatomic neutral fragments. The talk reviews results on product branching in several recent photodissociation experiments in my laboratory, including experiments on methyl- and trimethyl amine, nitric acid, allyl chloride and methyl vinyl ether, to come to an understanding of how the change in electronic wavefunction required for the reactant complex ABC* to evolve into each product channel can influence the branching between product channels.

459888: Studies of electron-molecular ion dissociative recombination using ion storage rings

Abstract
Heavy-ion storage rings have been used for almost a decade to study dissociative recombination. In the first experiments, relative and absolute recombination cross sections were measured. These experiments were developed to include measurements of product branching ratios, electronic excitation in recombination products, dynamics of three-body breakups in recombination of triatomic molecular ions, and the study of recombination of selected vibrational levels. A brief review of this development will be given, and an attempt of an outlook will be given. Dissociative recombination of H3+ continues to be a controversial topic, as witnessed not least from several of the talks at this meeting. We hope to have new results on H3+ to present at this talk.

459898: Dissociative recombination of slow electrons and molecular oxygen ions in the strong laser field

Abstract
The dissociative recombination of electron with molecular oxygen ion in the ground state in the external monochromatic laser field is considered utilizing stationary formalism of radiative collision T-matrix in the Multichannel Quantum Defect Theory. It is possible because radiation efficiently interacts with intermediate Rydberg states only. Laser induced transitions to the Schumann-Runge continuum are investigated for both direct and dipole-bounded transitions. The channel interference picture is analyzed. Total cross section is represented by sum of two terms. The first one describes a transition not accompained by an exchange of energy with the field while the second one is responsible for a transition accompanied by the emission of a field quantum. The dependence of cross section on the field strength and the angle between radiation polarization vector and electron beam direction is studied. The result is compared with the experiment. This work was supported by INTAS project #99-00039.

459935: Molecular autoionization and predissociation as "Inverse" dissociative recombination

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Ionization
Invited: Y
Preferred Presentation Format: OralOnly
Consider for Sci-Mix: N
Special Equipment Needs: PowerPoint Presentation
Conforms to Bylaw 6: Y
Last Modified: 2001-04-20

Abstract
Autoionization might be considered to be reverse dissociative recombination of a molecular ion with an electron leading to an excited state of a neutral molecule. In that regard, autoionization and predissoication are of special interest. We present recent studies of the vibrational autoionization of NO excited to various Rydberg states. Using time-of-flight photoelectron spectrometry, we measure the rotational distributions of the final-state ion and the angular distribution of the associated photoelectron.

For the ns Rydberg series with n=13 - 16 we find that the final-state rotational distribution that results from excitation of a specific NO A state v=1 J level is asymmetric about DELTA J and varies markedly with J within the same ns Rydberg state. Moreover, the photoelectron angular distributions show a strong propensity for perpendicular ejection to the polarization vector of the light beam that prepares the aligned Rydberg state.

460322: Dissociative recombination of Xe₂⁺ and XeH⁺

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Topic Selection: Dissociative Recombination of Molecules with Electrons: Afterglow Experiments
Invited: Y
Preferred Presentation Format: OralOnly
Consider for Sci-Mix: N
Conforms to Bylaw 6: Y
Last Modified: 2001-04-18

Abstract
The associative ionization and dissociative recombination of Xe2+ has been studied using a Time-of-Flight mass spectrometry, Photoelectron Spectroscopy method. It has been found that xenon atoms excited to nf states with n=4-8 rapidly undergo associative ionization while those in np states with n=8-11 do not. The resulting Xe2+ ions recombine with electrons to form Xe atoms predominantly in the 6p and 5d excited states. (This technique is not sensitive to ground state atoms). Comparison with results from other studies will be presented. In an afterglow experiment, it was found that unlike lighter rare gas hydrides, XeH+ displays a large dissociative recombination rate. Calculations of potential energy curves have been performed for XeH+ and for KrH+. While the latter exhibits a crossing away from the ground vibrational state, XeH+ has a favorable crossing through which recombination can proceed. The results of these calculations will be presented.

460601: Channel mixing effects in dissociative recombination and related processes

ACCEPTED
Topic Selection: Dissociative Recombination of Molecules with Electrons: Theory
Invited: Y
Preferred Presentation Format: OralOnly
Consider for Sci-Mix: Y
Special Equipment Needs: Standard equipment
Conforms to Bylaw 6: Y
Last Modified: 2001-04-20

Abstract
Being in Chicago, we will review the impact of U. Fano's seminal paper on configuration interaction (Phys. Rev. 124, 1866, 1961) on molecular and in particular dissociative processes. We will show from several examples that besides being responsible of sharp resonance structures due to discrete levels embedded in ionization or dissociation continua, configuration interaction, generalized in terms of channel mixing, may affect substantially the dynamics of excited molecules, at short range (vibrational excitation and deexcitation) as well as at long range (branching ratios in dissociative recombination). Various kind of channels, open or closed, associated to electronic or nuclear continua, and of channel interactions (electronic as well as non-adiabatic) must be treated on equal footing in order to get a complete picture of the molecular dynamics.

460703: Associative ionization with cold atoms

Abstract
We will discuss the possibility offered by photoassociation of cold atoms, to perform the spectroscopy of vibrational levels of molecular autoionizing states. This may provide a powerful tool to identify the channels responsible for associative ionization or dissociative recombination in alkali systems.

461038: The dissociative recombination of the helium molecular ion

The dissociative Recombination reaction has been studied theoretically and experimentally for about four decades. Because of the complexity of this process there is not yet a definite agreement between theory and experiment. In our laboratory, the DR of helium molecular ion in thermal plasma was studied using Time of Flight (TOF) spectroscopy. The observed final states for the DR atomic products were one helium atom in the ground state He (1s2; 1S) and the other atom in one of the following excited states: He(1s3s ; 3S), He(1s3s; 1S), He(1s3p; 3P), He(1s3d ; 3D), He(1s3d; 1D), He(1s3p;1P), He(1s2s; 3S), or He(1s2s;1S).

Symmetry requirements restrict the rotational states of homonuclear diatomic molecules with nuclear spin zero to odd J rotational states. However, in our experiment we observed both even and odd J rotational states. The observation of the symmetry-forbidden even J rotational states[1] of the He2+ can be explained by examination of the indirect DR process. It proceeds by the formation of a barely bound Rydberg state, with gerade and ungerade symmetry, which can have even and odd angular momentum states. For these transient Rydberg states the potential energy becomes, in the limit of large principal quantum number, identical with the potential energy of the molecular ion.

Our experimental results shows final atomic products in n=2 and predominantly n=3 manifold states, and possibly totally ground state final products (with both atomic products of DR in the ground state). Results similar to ours were obtained in experiments done using optical methods[2] which clearly shows the final product atoms in excited states 3S and 3D. Further theoretical modeling for this complex process is required. Thanks for partial support to NASA and LLNL Research Collaborations Program for HBCU/MI.

[1] L. Coman, M. Guna, L. Simons and K. Hardy, Phys. Rev. Lett., 83, 14, 2715 (1999) [2] V. A Ivanov, N. P. Penkin, Yu. E. Skoblo Opt. Spectrosc. (URSS) 54 (5) 552 (1983)

468078: Branching ratio in the dissociative recombination of polyatomic ions

ACCEPTED
Topic Selection: Dissociative Recombination of Molecules with Electrons: Dissociative Recombination of H₃⁺
Invited: N
Preferred Presentation Format: OralOnly
Consider for Sci-Mix: N
Conforms to Bylaw 6: Y
Last Modified: 2001-04-17

Abstract
A new "electronic phase space" theoretical model will be presented. The model allows for the estimation of the branching ratio in the dissociative recombination of polyatomic molecular ions. The theory requires the knowledge of the multiplicity of the states involved, the kinetic energy releases and the binding energies of the molecular fragments. The vibrational states population of the molecular fragments can be estimated as well. The model, which is parameter free, is compared with the existing experimental results for H₃⁺.

468878: Dissociative recombination of atmospherically relevant ions

Abstract
Molecular atmospheric physics aims at understanding the properties and composition of our atmosphere. This aim requires knowledge of the underlying microscopic physico-chemical processes. The same microscopic properties form the basis of many of the diagnostic tools, which are needed to determine the composition of our atmosphere in the absence of sufficiently accurate models or to corroborate these models. Light-driven reactions are obviously relevant, both by solar light and by thermal light from the earth.

I will present data on NO⁺ and O₂⁺ using the heavy ion storage ring, CRYRING at Stockholm. The data are needed to quantify the efficiency of this mutual neutralization process, which dominates the electron sink in the thermosphere. Both the underlying physical mechanisms and the implications for the study of the glows in our atmosphere will be discussed. Atomic products can be formed in different final states, some of which give rise to emission of radiation. The relative efficiency of the atomic states is determined and confronted with in situ data obtained in a rocket experiment. In the case of NO⁺ the products and lifetime of the metastable excited state has been determined.

476301: Dissociative recombination of vibrationally excited levels of O₂⁺

Abstract
Dissociative recombination (DR) is the primary mechanism for electron loss in the atmospheres of Mars and Venus. Within these atmospheres, oxygen is the most abundant molecular ion and is produced in a wide range of vibrationally excited levels by the reaction of atomic oxygen ions with carbon dioxide. Present knowledge of oxygen DR is limited to the ground vibrational level and to an uncharacterized distribution of excited vibrational levels. Measurements of the DR cross section and product distribution of individual vibrational levels are planned at CRYRING using a high pressure electron-impact ion source to produce controlled vibrational distributions in the molecular ion beam that are fully characterized by observing the products of dissociative charge transfer of the ions in cesium vapor. This work is in collaboration with W. van der Zande (FOM), M. Larsson (MSL), and their colleagues and is partially funded by NASA Planetary Atmospheres.