M. McFarland

Flow‐drift technique for ion mobility and ion‐molecule reaction rate constant measurements. II. Positive ion reactions of N+, O+, and H2+ with O2 and O+ with N2 from thermal to [inverted lazy s]2 eV

The positive ion‐molecule reactions N++O2→ NO++O→ O2++N, N2++O2→ O2++N2, O++O2→ O2++O, O++N2→ NO++N, were measured in a newly constructed flow‐drift tube apparatus. Reactions (a), (b), and (c) were measured from thermal energy to approximately 2 eV ion kinetic energy in the center of mass. Reaction (d) was measured from 0.3–3 eV ion kinetic energy. The data agree well with previous thermal values at low energies and agree well with beam data at the highest energy. In the case of all the reaction except (a), there is an initial decrease of the rate constant with increasing energy followed by an increase.

Flow‐drift technique for ion mobility and ion‐molecule reaction rate constant measurements. I. Apparatus and mobility measurements

The present paper describes the construction and operation of a new experimental device that combines the chemical versatility of a conventional flowing afterglow system with the energy variability of a drift tube. This allows the measurement of both positive and negative ion mobilities not previously measured. Ion mobility measurements offer a significant constraint upon the ion‐neutral intermolecular potential and are therefore of value in testing either empirical or quantum mechanical theory. The mobilities of He+, He2+, H+, D+, O+, N+, Ar+, H2+, H3+, H2+, H−, O−, and OH− in helium and H3+ in H2 are presented in the present paper. The following papers describe positive ion‐neutral and negative ion‐neutral reaction rate constant measurements in the same device.

Effects of ion speed distributions in flow‐drift tube studies of ion–neutral reactions

The effects of non‐Maxwellian ion speed distributions on ion–neutral reaction rate constants measured in drift tubes are examined experimentally and are compared to the predictions of recent theories. The rate constants of strongly kinetic‐energy‐dependent ion–molecule reactions of O+ with O2, N2, and NO are measured separately in helium and argon buffer gases, in which the O+ speed distributions are expected to be very different. The differences between the helium‐buffered and argon‐buffered rate constants are often substantial. When different, the argon‐buffered values are generally larger than the helium‐buffered values at the same mean energy, indicating that the O+‐in‐argon distribution has a larger high‐energy ’’tail’’ than the O+‐in‐helium distribution. The differences between the two sets of data are compared to predictions from (a) the Monte Carlo trajectory calculations of Lin and Bardsley, and (b) the moment solution of the Boltzmann equation of Viehland and Mason, both described in accompanyin...

Flow‐drift technique for ion mobility and ion‐molecule reaction rate constant measurements. III. Negative ion reactions of O− with CO, NO, H2, and D2

A new combined flowing afterglow‐drift tube experimental appartus has been used to measure the reaction rate constants as a function of ion kinetic energy for the following negative ion reactions: O−+CO→ CO2+e, O−+NO→ NO2+e, O−+H2→ H2O+e → OH−H, O−+D2→ D2O+e → OD−+D. Reaction (1) has been measured from thermal energy to ∼ 3 eV, (2) from thermal to ∼ 1.0 eV, (3) from thermal to ∼ 0.5 eV, and (4) from thermal to ∼ 0.8 eV. Reactions (1) and (2) are found to have rate constants that decrease by an order of magnitude with increasing ion kinetic energy, while Reactions (3) and (4) have total rate constants independent of energy. The channels (3b) and (4b) increase markedly with energy at the expense of the associative‐detachment channels (3a) and (4a). The present results are compared with existing available data on these reactions.

Rate constants for the reactions of O2+(a 4Πu) ions with N2, Ar, CO, CO2,H2, and O2 at relative kinetic energies 0.04–2 eV

Rate constants for the reaction of O+2(a 4Πu) metastable ions with N2, Ar, CO, CO2, H2, and O2, which do not react with O+2(X 2Πg, v=0) ground‐state ions, have been measured for the relative kinetic energies 0.04–2 eV in a flow‐drift tube. With the exception of O2 and CO, most of the rate constants are near the respective Langevin limits. The rate constant for the reaction with CO exhibits a pronounced minimum at 0.3 eV; the other reactions show much less variation with energy. Up to 2 eV, quenching of the a 4Πu metastable state to the X 2Πg ground state of O+2 appears to be small compared to the reactive losses of O+2(a 4Πu) ions. The reactive process is charge transfer, except for CO and H2. For CO, CO+ and CO+2 are formed roughly in the ratio 5:1 and for H2, only O2H+ is formed.

Negative ion‐molecule reactions with atomic hydrogen in the gas phase at 296 °K

Gas phase reaction rates have been measured for the reactions of Cl−, I−, OH−, O2−, SF6, and some hydrates of Cl−, OH−, and O2− with atomic hydrogen in a flowing afterglow system at 296°K. In general, the reaction mechanism is associative detachment, X−+H→HX+e, and the rate constants are very large (∼ 10−9 cm3/sec). The reactivities of Cl−, OH−, and O2− are reduced by clustering with H2O. The reaction of I− with H is immeasurably slow, and SF6− reacts with H to form SF5− which does not react further.

Partial Charge‐Transfer Reactions at Thermal Energies

Thermal energy reaction rate constants are reported for the partial charge‐transfer reactions Ca2+ + NO → Ca+ + NO+ and Mg2+ + X → Mg+ + X+ with X as Xe, NO, O2, N2O, CO2, CO, SO2, NH3, and NO2. The rate constants for single electron transfer of polyatomic neutrals are often three orders of magnitude larger than the rate of electron transfer from Xe. Within a curve crossing model the molecular data yield approximate efficiencies of electron transfer from 2.5 to 14.4 A. The difficulties of using a curve crossing model for polyatomic molecules and a simple Landau—Zener calculation at thermal energies are briefly discussed.

Translational and internal energy dependences of some ion–neutral reactions

The forward and reverse rate constants for the proton‐transfer reaction, N2OH++CO?COH++N2O, have been measured as a function of temperature in a variable‐temperature flowing afterglow and as a function of relative kinetic energy in a flow‐drift tube using both helium and argon buffer gases. The temperature variation of the rate constant was used to obtain ΔH0 =−4.4±0.4 kcal/mole and ΔS0=−4.0±1.0 entropy units. Furthermore, the comparison of the data taken as a function of temperature with those taken as a function of relative kinetic energy in different buffer gases shows that there are marked effects of ion vibrational energy on these rate constants. Similar data on the charge–transfer reaction, CO2++O2→O2++CO2, show the same effects. In addition, relative kinetic energy data on the proton transfer, ArH++O2→O2H++Ar, also imply strong effects of vibrational excitation on the rate constant.

Collisional Detachment Studies of NO

Rate constants for collisional detachment of NO−, NO− + X → NO + X + e, have been measured from 193 to 506°K for X as He, Ne, H2, NO, CO, CO2, N2O, and NH3. The data fit Arrhenius plots kd=Ae−E/kT with both A and E showing large variations with X. From equilibrium constant considerations, electron attachment rates to NO have been calculated. An estimate for the electron affinity of NO3 is obtained from some auxiliary considerations. Rate constants for the charge‐transfer reactions of NO− with O2 and NO2 were found to be 5 and 7.4 × 10−10  cm3sec−1, respectively.

Rate constants for the thermal energy reactions of He+, He(21S), and He(23S) with HCl and HBr

The rate constants for the reactions of He+, He(21S), and He(23S) with HCl and HBr are reported. The charge‐transfer reactions are dissociative and the rate constants lie between the Langevin and locked dipole rates. The Penning ionization rate constants are also large. The ratio of singlet‐to‐triplet cross sections is 2.3.