E. Alge

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.

Ion-ion mutual neutralization and ion-neutral switching reactions of some stratospheric ions

Abstract Following the recent mass spectrometric observations of the ambient stratospheric positive and negative ions we have carried out co-ordinated laboratory experiments using a selected ion flow tube apparatus and a flowing afterglow apparatus for the following purposes: (i) to consider whether CH 3 CN is a viable candidate molecule for the species X in the observed stratospheric ion series H + (H 2 O n (X) m and (ii) to determine the binary mutual neutralization rate coefficients α i for the reactions ofH + (H 2 O 4 and H + (H 2 O)(CH 3 CN) 3 with several of the negative ion species observed in the stratosphere. We conclude from (i) that CH 3 CN is indeed a viable candidate for X and from (ii) that the α i for stratospheric ions are within the limited range (5–6) × 10 −8 cm 3 s −1 .

Isotope exchange and collisional association in the reactions of CH3+ and its deuterated analogs with H2, HD, and D2

The rate coefficients and product ion distributions have been determined at 80, 205, and 295 K for the reactions of CH3+, CH2D+, CHD2+, and CD3+ with H2, HD, and D2 using a SIFT apparatus. Two types of reactions were observed, those involving isotope (H/D) exchange and those in which ion–molecule collisionally stabilized association occurs. Both exoergic and endoergic isotope exchange occurred and often more than one product was observed. Example reactions are (1) CH3++HD⇄CH2D++H2, (2) CD3++H2→CHD2++HD, CH2D++D2,CH2D3+. The exchange reactions are exoergic when H atoms in the reactant ion are exchanged for D atoms from the reactant neutral. As expected, the reverse (endoergic) rate coefficients kr decrease with decreasing temperature, whereas the forward (exoergic) rate coefficients kf increase with decreasing temperature. From the kf and kr for some of these reactions van’t Hoff plots have been constructed and thus ΔH° and ΔS° have been obtained. Using the experimentally determined values for ΔH°, the dif...

Anomalous temperature dependence of the coefficient of electron attachment to hexafluorobenzene

Abstract Electron attachment to C 6 F 6 has been studied over the temperature range 200–450 K using a FALP apparatus. The attachment coefficient, β, has been determined to be 1.4×10 −7 cm 3 s −1 at 200 K and 1.1×10 −7 cm 3 s −1 at 300 K. However a dramatic reduction of β occurs as the temperature is increased above 300 K, β decreasing to about 10 −9 cm 3 s −1 at 450 K. A β which reduces with increasing temperature is rare and the rate of reduction observed for C 6 F 6 is unprecedented. During this study, the appearance potential of C 5 F 3 + ions from C 6 F 6 was determined (much more accurately than previously) to be 15.85±0.01 eV.

Further studies of the N2+ + N2 → N4+ association reaction

Abstract Data are presented which strongly suggest that stabilisation of the excited intermediate (N 4 + )* complex in the reaction (1) N 2 + + 2N 2 (rate coefficient k 1 ) occurs via N 2 switching whereas for (2) N 2 + + N 2 + He (rate coefficient k 2 ) it occurs via superelastic He collisions. This explains the differing temperature variations of k 1 and k 2 previously obtained for these reactions. Drift tube data are also presented which show how k 1 varies with N 2 + /N 2 centre-of-mass energy as compared with thermal energy.

Rate coefficients at 300 K for the vibrational energy transfer reactions from N2(v=1) to O+2(v=0) and NO+(v=0)

Rate coefficients for the reactions N2(v=1)+O+2 (v=0)→N2(v=0)+O+2 (v=1) and N2(v=1)+NO+(v=0)→N2(v=0)+NO+(v=1) have been measured in a SIFT/flowing afterglow system at 300 K, the first measurements of thermal energy vibrational transfer from neutrals to ions. The values are k=1×10−12 and <10−13 cm3 s−1, respectively. These values are consistent with an intermediate complex vibrational predissociation model which has been successful in rationalizing earlier ion vibrational relaxations by neutral collisions. The NO+ result establishes that the efficient quenching of NO+(v=1) by N2 is not a resonant V→V process as was previously assumed.