Nigel G. Adams

The selected ion flow tube (SIFT); A technique for studying ion-neutral reactions

Abstract A technique is described by which a mass-selected, low-energy, positive ion beam, derived from a gas-discharge source in conjunction with a quadrupole mass filter, is injected into a flowing gas. The subsequent reaction of these ions with neutral molecules introduced into the carrier gas downstream of the injection point is studied using downstream mass spectrometer sampling in a manner identical to the well established flowing afterglow technique. We have termed our apparatus SIFT (Selected Ion Flow Tube). The rate coefficients for some sixteen ion-molecule reactions at room temperature measured using SIFT are presented, many of which have previously been reported. The branching ratio of the (He + + N 2 )-reaction is also reported and the value compared with those previously obtained. Some particular points of merit of the SIFT technique are discussed and ways in which it may be utilized to investigate some outstanding problems in ionic reaction studies are indicated.

A laboratory study of the reactions of N+, N2+, N3+, N4+, O+, O2+, and NO+ ions with several molecules at 300 K

A study has been made of the rate coefficients and product ion distributions for the reactions at 300 K of the ions N+, N2+, N3+, N4+, O+, O2+, and NO+ with CH3NH2, NH3, H2S, CH3OH, H2CO, COS, O2, H2O, CH4, CO2, CO, H2, and N2 molecules listed in increasing order of their ionization energies. These measurements are intended as a contribution to stratospheric chemistry. In the binary reactions of the ions of large recombination energy with molecules of low ionization energy, multiple ion products generally result and the rate coefficients are close to gas kinetic. Conversely, the low recombination energy ions NO+ and O2+ generally undergo ternary association reactions with the large ionization energy molecules. The reactions of N2+ and N4+ are very similar, the most common mechanism apparently being direct charge transfer usually followed by fragmentation, the nitrogen–nitrogen bonds in the reacting ions remaining intact. The N+ and N3+ reactions differ from the N2+ and N4+ reactions in that they show a gr...

Product-ion distributions for some ion-molecule reactions

The reactions of He+ ions with N2, O2, CO2, and CH4, and of C+ and N+ ions with O2 are presented. The measurements were carried out in the SIFT apparatus (selected ion flow tube), which involves the injection of a mass-selected positive-ion beam into a flowing neutral gas into which a reactant gas is introduced at a position downstream in the flow. Data acquisition and analysis are made by the flowing afterglow technique. The results for the He++N2 reaction agree with the previously established branching ratio. For the other reactions, considerable differences are apparent between the product distributions obtained and those previously reported and an explanation is attempted. A brief discussion of the reaction mechanism is presented for each reaction in the light of the determined product distributions, and the anticipated development of the SIFT technique for ion-molecule reaction mechanisms is outlined.

An experimental survey of the reactions of NHn+ ions (n = 0 to 4) with several diatomic and polyatomic molecules at 300 K

A study has been made of the binary reactions at 300 K in a SIFT apparatus of the ions N+, NH+, NH2+, NH3+, and NH4+ with CH3NH2, NO, NH3, H2S, CH3OH, H2CO, COS, O2, H2O, CH4, CO2, CO,H2, and N2. The N+, NH+, and NH2+ reactions are generally fast, their rate coefficients being close to the collisional limit, and multiple product channels result, especially for reactions with the polyatomic molecules. Several reaction mechanisms are apparent, including charge transfer, proton transfer, and H‐atom abstraction, although it is argued that many of the reactions proceed via a short‐lived, ion–molecule complex which separates along the observed reaction channels. The NH3+ ions are seen to be generally less reactive than the lesser hydrogenated ions, the product of most of the reactions being HN4+ ions which do not undergo binary reactions with any of the molecules (except CH3NH2) but rather undergo ternary association reactions with a few of them. A few measurements were made using the deuterated analogs ND+, ND...

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 .

Temperature dependence of rate coefficients for reactions of ions with dipolar molecules

Abstract We present a comparison of theoretical and experimental rate coefficients for the reactions of H 3 + , HCO + , H − , C + and O + with HCN, and H 3 + with HCl, over the temperature range 205–540 K. The theoretical results are obtained using an adiabatic capture theory which is selective in the initial rotational states of the reacting dipolar molecules. The experiments were performed using a variable-temperature SIFT apparatus. For the H 3 + and HCO + proton transfer reactions in particular, there is excellent agreement between theory and experiment, with the rate coefficients increasing sharply with decreasing temperature.

Reaction of simple hydrocarbon ions with molecules at thermal energies

Abstract The results of an experimental study of the reactions of C+, CH+, CH2+, CH3+, and CH4+ with H2, N2, O2, CO, CO2, H2O and CH4 at a temperature of ∼300 K are presented. The measurements were carried out in a SIFT apparatus (Selected Ion Flow Tube). The rate coefficients for some thirty-one 2-body and 3-body reactions are presented together with the ionized products of each reaction. Several types are apparent, including proton transfer, H-atom abstraction, charge transfer and 3-body association, the last describing the reactions of the CH3+ ion except for its reaction with CH4 in which another saturated hydrocarbon ion, C2H5+, is the only product. General conclusions are drawn concerning the reactivities of the various ionized and neutral reactants, for example all the ions react extremely rapidly with H2O molecules, a result consistent with theoretical predictions which take account of the large permanent dipole moment of H2O. In some cases attempts are made to describe the nature of individual reactions. The magnitudes of the rate coefficients and the observed ionized products are in reasonable agreement with published data for the limited number of the reactions which have been studied previously.

Laser induced fluorescence and vacuum ultraviolet spectroscopic studies of H‐atom production in the dissociative recombination of some protonated ions

The flowing afterglow technique, coupled with laser induced fluorescence (LIF) and vacuum ultraviolet (vuv) absorption spectroscopy, has been used to determine the fractional H‐atom contributions, fH, to the product distributions for the dissociative recombination of a series of protonated ions (N2H+, HCO+, HCO+2, N2OH+, OCSH+, H2CN+, H3O+, H3S+, NH+4, and CH+5 ) with electrons. The measurements were made at 300 K in two separate ways in two laboratories by (i) directly determining the H‐atom number density using vuv absorption spectroscopy at the Lα (121.6 nm) wavelength and (ii) converting the H atoms to OH radicals using the reaction H+NO2→OH+NO followed by LIF to determine the OH number density. The agreement between the two techniques is excellent and values of fH varying from ∼0.2 (for OCSH+ ) to 1.2 (for CH+5 ) have been obtained showing that in some of the cases recombination can lead to the ejection of two separate H atoms. Comparison of the oxygen/sulphur analogs, HCO+2/OCSH+ and H3O+/H3S+ showe...

A study of the reactions of the ground and metastable states of C+, N+, S+ and N2+ at 300 K

Abstract The reactions of the ground and metastable states of C + , N + , S + and N 2 + with H 2 , CO, NO, O 2 , CO 2 , H 2 O, NH 3 and CH 4 at 300 K have been studied using a selected ion flow tube (SIFT) apparatus. Reaction rate coefficients have been determined for the ground and metastable states of N + , S + and N 2 + , and for the last two ions it has been possible in some reactions to distinguish two metastable states through their different reaction rate coefficients. The measured ground state rate coefficients generally agree with previous measurements and the metastable rate coefficients are usually higher, approaching gas kinetic values. In the case of S + and N 2 + , collisional quenching of the metastable to the ground state ions by the reactant gas has been observed and this process can account for up to half of the loss of the metastable ions. Product ion distributions for the mixture of ground and metastable states have been determined for the reactions of all four ions and from these it has been possible to unfold the product ion distributions for the metastable state using the values previously reported for the ground state ions. The similarities and differences in the reaction mechanisms of the ground and metastable state ions are briefly discussed.

The rate coefficients for several ternary association reactions at thermal energies

Abstract The rate coefficients k for the ternary association reactions of CH 3 + and CD 3 + with H 2 , N 2 , O 2 CO and CO 2 N 2 + with N 2 , and C + with H 2 and D 2 have been measured within the temperature range 80–520 K in helium buffer gas. In every case, k ∝ AT − n and the magnitudes of k are greater when the deuterated species are involved.