N. G. Adams

The selected ion flow tube (SIFT): studies of ion-neutral reactions

Publisher Summary This chapter focuses on the role of Selected Ion Flow Tube (SIFT) in examining ion-neutral reactions. The SIFT technique is a natural extension of the Flowing Afterglow (FA) technique. It builds on and extends the versatility of the FA for the study of ion-neutral reactions under truly thermal conditions. Under this technique, the ions enter a quadrupole mass filter that can be set to pass ions of a given mass-to-charge ratio. These mass selected ions are injected at low energy via a small aperture into a flow tube along which they are convected by a fast-flowing carrier gas at a pressure of typically 0.5 Torr. In the SIFT, the ions are created in a remote ion source and not in the carrier gas-reactant-gas mixture. Thus, crucially, the ion source gas is excluded from the carrier gas and therefore from the reaction zone. Reactions, therefore, between the primary ions and their source gas cannot occur. Various types of ion source have been used in SIFT experiments, the choice depending on the ion types required. The simplest to use are low-pressure sources because they do not offer excessive gas loading to the diffusion pump. They can be built directly into the SIFT chamber.

Attachment coefficients for the reactions of electrons with CCl4, CCl3F, CCl2F2, CHCl3, Cl2 and SF6 determined between 200 and 600 K using the FALP technique

The rate coefficients, beta , for the attachment reactions of electrons with CCl4, CCl3F, CCl2F2, CHCl3, Cl2 and SF6 have been measured under truly thermal conditions over the approximate temperature range 200-600K using a flowing-afterglow/Langmuir probe apparatus. The beta values obtained at 300K are 3.9*10-7, 2.6*10-7, 3.2*10-9, 4.4*10-9, 2.0*10-9 and 3.1*10-7 cm3 s-1 respectively. From the variation with temperature of beta for the CCl2F2, CHCl3 and Cl2 dissociative attachment reactions, activation energies, Ea, of 0.15, 0.12 and 0.05 eV respectively were derived. The beta for CCl4, CCl3F and SF6 are close to their theoretical limiting values within the temperature range investigated. While Cl- was the only product ion observed for the reactions involving chlorine-containing molecules, both SF6- and SF5- were observed for the SF6 reaction. The data obtained are compared with previous data and the separate influences of electron temperature and gas temperature are noted.

Measurements of the dissociative recombination coefficients of O2+, NO+ and NH4+ in the temperature range 200-600K

Measurements are presented for alpha t, the dissociative recombination coefficients with electrons of O2+, NH4+ and NO+ under truly thermalised conditions within the temperature range 200-600K using a flowing afterglow/Langmuir probe apparatus, alpha t(O2+) is found to vary as approximately T-0.7 in close accord with previous pulsed afterglow data for alpha t(O2+) and alpha e(O2+) and with values for alpha e(O2+) inferred from ion trap data. alpha t(NH4+) is found to vary as approximately T-.06. alpha t(NO+) is found to vary as approximately T-0.9 which is reasonably consistent with previous pulsed afterglow data for alpha t(NO+). These data are also compared with values of alpha 3(NO+) measured in a pulsed afterglow experiment and those derived from ion trap and merged beam cross section data and from atmospheric observations.

Measurements of dissociative recombination coefficients of H+3, HCO+, N2H+, and CH+5 at 95 and 300 K using the FALP apparatus

Meaurements are presented of the dissociative recombination coefficients αt for reactions of electrons with H+3 , D+3 , HCO+, DCO+, N2H+, N2D+, and CH+5 ions at 95 and 300 K. The measurements were made under truly thermalized conditions using a flowing afterglow (FALP) apparatus. Contrary to previous stationary afterglow (SA) studies, αt(H+3) was found to be immeasurably small [≲2 (−8) cm3 s−1] at both temperatures which is consistent with recent theoretical predictions. However some evidence was obtained indicating that vibrationally excited H+3 recombined efficiently, which is also in accordance with the recent theory. At 300 K, αt(HCO+)=1.1 (−7) cm3 s−1 and αt(N2H+)=1.7 (−7) cm3 s−1 and both were larger by a factor of three at 95 K. αt(CH+5) =1.1 (−6) cm3 s−1 at 300 K and 1.5 (−6) cm3 s−1 at 95 K. The αt(HCO+) data are compared with previous SA data and the αt(N2H+) and αt(CH+5) data are discussed in relation to the recombination coefficients αe(N2H+) and αe(CH+5) derived from merged beam (MB) cross se...

Rate coefficients for the attachment reactions of electrons with c-C7F14, CH3Br, CF3Br, CH2Br2 and CH3I determined between 200 and 600K using the FALP technique

The rate coefficients, beta , for the attachment reactions of electrons with c-C7F14, CH3Br, CF3Br, CH2Br2 and CH3I have been determined under truly thermal conditions at temperatures in the range 200-600K using the flowing afterglow/Langmuir probe technique. The beta values (in units of CM3 s-1) at 300K are 6.8*10-8, 6*10-12, 1.6*10-8, 9.3*10-8 and 1.2*10-7 respectively. The beta for all of the reactions increase with temperature and approach the theoretical maximum value of beta at high temperatures. From Arrhenius plots, activation energies for the reactions have been determined to be 40, 300, 80, 50 and 25 meV respectively. All of the reactions, except for that with c-C7F14 (for which electron attachment is apparently non-dissociative), proceed by exoergic dissociative attachment producing halogen atomic negative ions. The data are compared with previous data where these are available.

An absolute proton affinity scale in the ∼130-140 kcal mol−1 range

The dependences on temperature of the rate coefficients for the endothermic proton transfer reactions of HBr+ with CO2 and CH4 have been obtained in a variable‐temperature selected ion flow tube. The measurements have been used to determine the 300 K proton affinity of CO2, P.A.(CO2), =128.5±1.0 kcal mol−1, utilizing the literature value of P.A.(Br)=131.8 kcal mol−1, obtained from the dissociation energy of HBr+, as a primary standard. The proton affinity difference between CO2 and CH4 has been substantiated by equilibrium constant measurements as a function of temperature for proton transfer between CO2 and CH4. Similar equilibrium constant measurements have been used to determine the proton affinities of HCl, N2 O, HBr, and CO, giving a proton affinity ladder ordered (in kcal mol−1) as CO(141.4), HBr(138.8), N2 O(137.3), HCl(133.0), Br(131.8), CH4 (130.0), and CO2(128.5). Proton affinities have also been determined for Br2(140.0), NO(127.0), and CF4(126.5), the last two values being obtained from select...

Laboratory measurements of ion-molecule reactions pertaining to interstellar hydrocarbon synthesis

Measurements on a variety of three-body association reactions between hydrocarbon ions and molecular hydrogen in the presence of helium have been undertaken. The rate coefficients of most of the systems studied are vanishingly small (<1 x 10/sup -30/ cm/sup 6/ s/sup -1/) even at 80 K. The results cast severe doubt on a recent gas-phase model of complex hydrocarbon formation in dense interstellar clouds in which radiative association reactions between hydrocarbon ions and molecular hydrogen play a key role. In addition to the three-body reactions, some two-body reactions between hydrocarbon ions and molecular hydrogen and some reactions between carbon ions (C/sup +/) and hydrocarbon neutrals have been studied.

The application of Langmuir probes to the study of flowing afterglow plasmas

The application of the single Langmuir probe techniques to the measurement of electron number densities and temperatures in helium flowing afterglow plasmas is described. Data are presented relating to electron loss by both ambipolar diffusion and recombination, which illustrates the value and quality of the diagnostic technique. Results are also presented which illustrate how metastable atom densities can be readily determined by exploiting the Penning effect, and also probe determinations of electron temperature which demonstrate the role of metastable atoms in elevating the plasma electron temperature. In conclusion, the potential value of the probe-flowing afterglow combination is discussed with reference to the determination of positive and negative ion densities and ionic mass, of ion-molecule reaction rates and electron temperature relaxation rates.

Molecular synthesis in interstellar clouds: some relevant laboratory measurements

Data are presented which have resulted from a systematic program of measurements of the reaction rate coefficients and product distributions of positive ion/molecule reactions which are probably important reaction channels for the synthesis of molecular species observed in interstellar gas clouds. In particular, the reactions of the CH/sub n//sup +/ and C/sub 2/H/sub n//sup +/ ions (n=0 to 4) with several molecules have received special attention, and several likely channels for the synthesis of observed interstellar species have been identified. Special consideration is given to the few reactions which proceed via rapid three-body association in the laboratory experiments, since it is suggested that these association reactions may proceed at significant two-body rates which involve radiative stabilization in low-pressure interstellar clouds. This mechanism can thus result in the formation of a relatively large molecule in a single interaction. These ideas are utilized to explain tentatively the relative abundances of some hydrogen-carbon-nitrogen molecules in Ori A and Sgr B2.

The reaction O+2+CH4 → CH3O+2+H studied from 20 to 560 K in a supersonic jet and in a SIFT

Rate coefficients k have been determined for the reaction O+2+CH4 → CH3O+2+H at several temperatures in the uniquely wide temperature range 20–560 K using a new expanding jet (CRESU) apparatus and a selected ion flow tube (SIFT) apparatus. In the overlapping temperature range of the experiments the rate coefficients are in excellent agreement. A minimum is observed in the rate constant near 300 K but the outstanding feature is the rapid increase at low temperatures from the minimum value k (290 K)=5.4×10−12 cm3 s−1 to k (20 K)=4.7×10−10 cm3 s−1, the latter being about half of the collisional limiting value kc for the reaction (kc=1.16×10−9 cm3 s−1). Indeed the data show that k → kc as T → 0 K and the low temperature values can be fitted to a power law of the form k=1.1×10−7 T−1.8. The results strongly indicate that the CH3O+2 product ion is formed via rearrangement in a long‐lived intermediate (CH4O2)+ complex, the lifetime against unimolecular decomposition of which largely controls the rate of the react...