Lucia M. Babcock

Selected ion flow tube studies of H3O+(H2O)0.1 reactions with sulfides and thiols

Abstract Rate coefficients and product distributions have been determined in a selected ion flow tube at 300 K for the reactions of H 3 O + and the water cluster ion, H 3 O + ·H 2 O, with the sulfur-containing compounds H 2 S, CH 3 SH C 2 H 5 SH, n-C 3 H 7 SH, iso-C 3 H 7 SH, (CH 3 ) 2 S, CH 3 SC 2 H 5 , and C 4 H 4 S. The reactions of H 3 O + have high efficiency, following the trend established in reactions with other molecules, and are non-dissociative proton transfer reactions. For H 3 O + ·H 2 O, almost all of the reactions also proceed approximately at the collision rate, with rate coefficients slightly smaller than for the analogous H 3 O + reactions, consistent with the slightly larger reduced mass. The products are more varied, however. Proton transfer and/or ligand switching (presumably with internal H + transfer as, for example, in the production of CH 3 SH + 2 ·H 2 O) are observed to occur. Indeed, in some cases, proton transfer is observed where it should not be energetically possible. For example, in the cases of n- and iso-C 3 H 7 SH, both product channels are observed. The possibilities of neutral water dimer production, reactions being entropy driven, isomerization of the proton acceptor in the reaction, thermal dissociation of cluster ion products and uncertainties in the thermochemical data are all considered. Applications to interstellar gas clouds and the terrestrial atmosphere are discussed briefly.

GAS-PHASE REACTIVITY OF HS2H+. AND S2+.: AN INVESTIGATION OF THE GAS BASICITY AND PROTON AFFINITY OF HS2.

Abstract The reactions of HS2H+· with the series of reference bases: H2S, CH2O, C3H3F3O (1,1,1-trifluoropropanone), C2H5I, C6H4F2 (o-difluorobenzene), HCO2H, C4H8 (trans-2-butene), c-C3H6, n-C3H6, CH3OH, CH3SH, and C2H5OH, with proton affinities ranging from 168.5–185.6 kcal/mol, have been investigated with a selected ion flow tube (SIFT) to bracket the gas basicity (GB) and proton affinity (PA) of the hydrothiosulfeno radical (HS2·). The recently developed thermokinetic method of Bouchoux et al. [Int. J. Mass Spectrom. Ion Processes 153 (1996) 37] applied to the data gives GB(HS2·) = 169.8 ± 2.2 kcal/mol and PA(HS2·) = 178.0 ± 2.4 kcal/mol, consistent with but more accurate than the simple bracketing procedure. The proton affinity is used to calculate the enthalpy of formation of HS2·, giving ΔHf○298(HS2·) = 25.0 ± 2.5 kcal/mol; this result is compared with the relatively few other reported determinations of this quantity. The HS2· radical is of potential importance to chemical processes in interstellar clouds (ISC), as well as to fuel refinery and atmospheric chemistry. The HS2H+· reactions and a parallel study of the reactions of S2+· are discussed.

Dissociative charge transfer in reactions of CCl4 and SF6 with ions having recombination energies between 6.4 eV and 24.5 eV

Abstract Gas mixtures of CCl 4 and SF 6 with rare gases and simple diatomic gases in reactive plasmas are often used to etch insulating and semiconductor layers. However, much of the kinetic and product ion information for ion-molecule processes that occur in such plasmas is not known. To improve this situation, a selected ion flow tube (SIFT) study has been made of reactions of CCl 4 and SF 6 variously with D 3 + , H 3 + , D + , N + , D 2 + , N 2 + , Ar + , Ne + , He 2 + , and He + at 298 K. Because water is a common plasma impurity, the reactions of H 2 O + and H 3 O + have also been included. With this information, better models can be developed to predict plasma conditions that are optimal for etching. Reactions generally proceed by dissociative charge transfer with rate coefficients close to the collisional values. The degree of fragmentation (into Cl + , CCl + , CCl 2 + , CCl 3 + and SF 3 + , SF 4 + , SF 5 + ) and the energy thresholds at which products are observed are frequently consistent with a long-range mechanism in which the available energy goes into fragmentation. Notable exceptions to this are reactions of D 3 + and H 3 O + with CCl 4 and D + , H 3 + , H 2 O + , and H 3 O + with SF 6 . Rate coefficients and product ion information are discussed in terms of photoelectron spectroscopy (PES) and photoionization (coincidence) data (e.g. threshold photoelectron-photoion coincidence (TPEPICO) and PEPICO techniques) available in the literature. From this comparison, a better fundamental understanding of the dynamics of charge transfer is obtained. In addition to the reactions of neutral CCl 4 and SF 6 gases, Cl 2 + , CCl 2 + , CCl 3 + and SF + , SF 2 + , SF 3 + , SF 4 + , SF 5 + studies with H 2 have also been conducted.

Application of a sputtering glow discharge ion source to a flowing afterglow study of transition metal ion chemistry

The use of a sputtering discharge ion source in the study of gas phase transition metal ion chemistry under conditions where three-body processes predominate has been explored. Experiments were carried out in which the glow discharge was interfaced with a flowing afterglow reactor in order to determine its effectiveness in the production of metal ions with signal characteristics suitable for use in this environment. These initial experiments have indicated that a variety of metal ions can be formed with adequate signal-to-noise ratios. Because of the specific instrument configuration, a mixture of helium and argon was used as the discharge gas in order to minimize noise resulting from argon metastables. Helium metastables produced in the discharge were quenched by injecting argon into the flowtube immediately downstream from the discharge. Metal ion signals within the flowtube were observed to stabilize approximately 10 min after initiation of the discharge, and remained stable for the duration of subsequent analyses. Experiments in which Co+, Ni+, Cu+, and Zn+ were allowed to react with ethane were carried out. Overall rate constants and observed products in the reactions involving Ni+, Cu+ and Zn+ were consistent with those reported for ground state ions. Reaction of CO+ with CH3I indicated that a portion of these ions was in an excited state.

Molecular ion recombination in trapped and flowing plasmas: methods, recent results, new goals, open questions

Two major techniques have been used to study electron-ion recombination, afterglows and storage rings. These have differing strengths and limitations for the determination of recombination rate constants/cross-sections, including those as a function of temperature/energy, and for the identification and quantification of the neutral products, both rovibronically excited and ground state. Here, the afterglow techniques will be reviewed (the storage ring is considered by Larsson) with emphasis on new developments. In this vein, there will be discussion of (i) rate constants for larger species, (ii) temperature dependencies of rate constants including those for some isomers, and (iii) electronically excited states; all these connect with other papers in these proceedings. New developments are discussed; in particular a means of quantifying the neutral product distributions, and possible new directions are suggested.

Gas-phase reactivity of SO+·: a selected ion flow tube study

Abstract We present a systematic study of the reactions of SO+·(X 2Πr), an important ion in space plasmas, with organic molecules of interstellar interest. A selected ion flow tube has been used to investigate the reactions of SO+· with CH4, C2H6, C3H8, C2H2, C2H4, C3H4 (allene), n-C3H6, CH3OH, C2H5OH, CH3OCH3, OCS, CH2O, CH3CHO, CH3C(O)CH3, HCO2H, and HCO2CH3, and additionally the reactions of S2+· with C2H2 and O2+· with CH4, C2H2, C3H4 (allene), n-C3H6, CH3OCH3, and HCO2H at 294.5 ± 2.5 K. With just a few exceptions the reactions proceed at or near their theoretical collisional capture rates. Apart from the smaller and more saturated hydrocarbons and OCS, the reactions of SO+· are dominated by heterogenic abstractions of R− (R = H, OH, CH3, OCH3). Charge transfer, where it is exothermic, occurs in competition with the abstraction channels. Hydride abstraction is particularly prevalent, forming the thioperoxy radical, HSO·, or its structural isomer, SOH·. Hydroxide abstraction to form the hydroxysulfinyl radical, HOSO·, occurs in some of the reactions with oxygen-bearing molecules. Where neutral, the abstraction products are inferred from the calculated reaction energetics; however, they are frequently detected directly in their protonated forms. This suggests a two-step reaction mechanism whereby competition for a proton occurs between leaving partners in the exit channel of the activated complex. In the reaction of SO+· with HCO2CH3, the protonated methoxysulfinyl radical, CH3OSOH+·, is observed for the first time. The reactions of SO+· with the smaller unsaturated hydrocarbons are more complex, and largely involve rupture of the S–O bond and a C–C bond to form products containing C–S and C–O bonds. The SO+· reactions are discussed in terms of their mechanisms, product formation, thermodynamics, and interstellar significance, and are compared with the related reactions of S2+· and O2+·.

Technique for distinguishing and determining the origin of photon emissions from complex plasmas

Experimental studies of electron-ion recombination have traditionally been made in three ways: (1) the determination of recombination rate coefficients, some as a function of temperature, (2) the identification of ground state neutral products and (3) the detection of excited state products. Initial studies of (1) were made in stationary afterglows, predominantly by Biondi and co-workers1 and Smith and co-workers2 and were followed by investigations using the more versatile flowing afterglow.3 In parallel with the latter studies, emission spectroscopy was used primarily for detecting excited state products in ion- and metastable-neutral reactions,4, 5, 6 but with some studies in which excited products of recombination were detected. Studies were later made by Adams and co-workers7,8 and by Johnsen and co-workers9 to detect the ground state products of recombination, a more difficult problem but where substantial progress has been made. At the same time, determination of product distributions was also starting in storage rings.10,11 In this area, storage ring studies have been more numerous, due predominantly to the degree of effort put into the technique by a number of workers. Also, with this method, it is possible to inject a single ion species into the ring, and for heteronuclear ions to radiationally cool the vibrations.12 In the flowing afterglow, it is more complicated to create a single ion plasma, ensure that the ions are vibrationaly relaxed and to detect all of the products simultaneously.

Product channeling in the reactions of CS+(X 2Σ+) with simple carboxylic acids and esters

Abstract In the present work, a selected ion flow tube was used to study the reactions of CS + ( X 2 Σ + ) with H 2 , CO, and a series of carboxylic acids and esters, RCO 2 R′ (R, R′ = H, CH 3 , C 2 H 5 ), at 296 ± 4 K. The CS + ion is expected to be a reasonable ionic analogue of the neutral pseudohalogen, CN, because of its strong chemical bond and isovalency. The reactions of CS + with the RCO 2 R′ series were all fast at greater than 80% of the theoretical upper-limit ion/dipole capture rate coefficient. Numerous binary ion products were observed, the most prevalent being the acylium ion (RCO + ), the radical ion of the neutral reactant, and HCS + ; no ternary association products were observed. Despite the abundance of product channels, these reactions were simple mechanistically, proceeding either by the induction of lone pair electrons on the carboxyl O atoms, or by H-atom transfer, which was chiefly homolytic rather than heterolytic. Induction of an O-atom lone pair by the C-terminus of CS + into a molecular orbital of the activated complex is believed to lead to product formation through an oxonium ylide intermediate. The CS + was converted, with few exceptions, into the small product molecules OCS, CS, and HCS + . Except in one instance, rupture of the strong C S bond did not occur. The reactions of CS + with the RCO 2 R′ series are compared with the analogous reactions of S 2 + , SO + , S + , H 3 O + , NO + , and O 2 + which have been studied previously. The reactive behavior of CS + is discussed with regard to the pseudohalogen character of this ion and its potential role in studies of chemical reaction dynamics. Additionally, the rate coefficients and product distributions for the reactions of S + with NO 2 and C 3 H 4 (allene) at 296 ± 4 K are presented.

Pulsed technique for observing infrared emissions from ionic gas phase reactions at low reactant ion concentrations

A technique has been developed to detect infrared emissions from the products of ionic reactions in plasmas. The technique employs dual-phase digital lock-in amplification and cold filtering to permit the detection of the weak infrared chemiluminescence (IRCL) with a solid-state detector. A novel method of cleanly modulating plasma chemiluminescence by the pulsed introduction of reagent gases has been developed and implemented. This new technique has been tested by studying the well-characterized H-atom reactions, H+Cl2→HCl(v=0–4)+Cl and H+NO2→OH(v=0–3)+NO. Rotational and vibrational distributions have been measured for these two reactions and are presented and compared with previous determinations. Additionally, the associative electron detachment reaction, H+Cl−→HCl(v=0–2)+e, has been studied, demonstrating that IRCL can be collected from reactions occurring at a low number density approaching that of the plasma ionization (∼4×1010 cm−3). The resolution, and hence, the information content of the collect...

Ionic Processes in Low Temperature Interstellar Molecular Plasmas

Astrophysical plasmas vary widely in their temperatures and compositions. Some of the most tenuous, coldest, and spatially extensive are the diffuse and dense interstellar clouds, in the latter of which stars are forming. In these regions, including circumstellar shells, ∼130 molecules have been detected with ∼10% being ions. The physical properties and compositions of these regions and the predominant gas phase ionic routes to the molecules are discussed. Examples of recent successes are given and some current problems highlighted.