I. Dotan

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...

Rate constants for the reactions of O+ with N2 and O2 as a function of temperature (300–1800 K)

We have measured the rate constants for the reactions of Ar+ with CO2 and SO2 from 300 to 1500 K in a high temperature flowing afterglow. For the reaction with CO2, we have found that all modes of energy, i.e., translation, rotation, and vibration, affect the rate constant to the same degree up to a total energy of 0.4 eV. Above 0.4 eV total energy, internal energy decreases the rate constants more effectively than does translational energy. For the reaction of Ar+ with SO2, the rate constants go through a minimum at about 900 K. By comparing our results to drift tube data, we derive rate constants for reaction from the υ=0 and υ>0 vibrational levels. At low energy, the vibrationally excited SO2 molecules react with Ar+ approximately twice as fast as the ground state molecules. Both vibrational modes have similar temperature dependences.

Rate constants for the reactions of O2+, NO2+, NO+, H3O+, CO3−, NO2−, and halide ions with N2O5 at 300 K

The reactions of N2O5 with CO3−, NO2−, and the halide ions, F−, Cl−, Br−, and I−, have been found to be very fast (k∼10−9 cm3 s−1) at 300 K and to produce the NO3− ion. It is inferred from the thermochemistry of the halide reactions that the neutral products must be XNO2. The positive ions O2+ and NO+ react with N2O5 to produce NO2+, while H3O+ reacts with N2O5 to form NO2+ and H2NO3+. NO2+ was observed not to react readily with N2O5.

Energy dependencies of the reactions of Ar+ with H2, N2, CO, O2, CO2, N2O, and COS

Reactions of Ar+ ions with H2, N2, O2, CO, CO2, N2O, and COS have been meausred in a flow‐drift tube at center‐of‐mass energies of 0.04–2.5 eV. The reactions with H2 and COS are fast and show only small energy dependences. The reaction with N2 is very slow at thermal energies and the rate coefficient increases very fast with increasing energy. The rate coefficients for the reactions with CO2 and N2O decrease with increasing energy over the whole energy range studied. For the reactions with O2 and CO, we find minima of the rate coefficient at 0.4 and 0.8 eV, respectively, followed by a dramatic increase at higher energies.

A study of the reaction O−3+CO2?CO−3+O2 and its implication on the thermochemistry of CO3 and O3 and their negative ions

The reaction O−3+CO2?CO−3+O2 has been examined in the forward and reverse directions in a variable‐temperature flowing afterglow from 200 to 600 K and in a flow‐drift tube at mean relative kinetic energies from 0.04 to 1 eV. The forward direction is clearly established as the exothermic direction. Furthermore, collisional dissociation of CO−3 and O−3 ions in the flow‐drift tube at high E/N to form O− shows that CO−3 is the more stable ion. All of this implies that D (CO2+O−) ≳D (O2+O−). Kinetic‐equilibrium studies at the higher temperatures show that the reverse rate constant is less than 6×10−15 cm3 s−1 below 600 K. When this is combined with the estimated entropy change of the reaction one obtains the quantitative lower limit D (CO2+O−) −D (O2+O−) ?0.58 eV. The reaction OH−+O3→O−3+OH is found to be fast, thereby establishing lower limits for the electron affinity of O3 and the O− bond dissociation energy of O−3. When taken with the above limit for the relative CO−3 and O−3 bond dissociation energy one o...

Mobilities of various mass‐identified positive and negative ions in helium and argon

The mobilities of the ions CH3O2+, NO+⋅H2O, C2H2−, Cl−, O2−, O3−, NO2−, CO3−, SO3−, SO2F−, SF5−, and SF6− in helium and 0+, O2+, O3−, and CO3− in argon have been measured at room temperature with a flow–drift tube. The measurements were obtained over the range 5⩽E/N⩽120 Td for helium and 5⩽E/N ⩽270 Td for argon (1 Td=10−17 V⋅cm2). The E/N dependence of these data fits the general pattern observed previously for a variety of ions in helium and argon, with the exception that the O+ mobilities in argon exhibit a rare minimum. Several of the polyatomic ions, notably CH3O2+, NO+⋅H2O, and O3−, were observed to break up at elevated E/N.

Studies of the reaction of O+2 with deuterated methanes

In the gas phase O+2 reacts with methane at 300 K to produce a hydrogen atom and the CH3O+2 ion. The structure of this ion has recently been determined to be H2COOH+, methylene hydroperoxide ion. The reaction rate coefficients and product distributions have now been measured at 300 K for the CHnD4−n isotopes. The reaction shows both inter‐ and intramolecular isotope effects, e.g., CH2D2 reacts more slowly than methane and more rapidly than CD4, but loses hydrogen or deuterium with equal probability. The ion readily transfers HO+ to alkenes, CS2, and many other neutral molecules. The reaction with CS2 has been used to investigate the isotopic distribution within mixed isotope product ions. In addition, the reaction rate coefficients for both CH4 and CD4 have been measured as functions of temperature between 20 and 500 K; in both cases a clear minimum is observed in the reaction rate coefficient near room temperature. A mechanism for the reaction is proposed which allows us to model the temperature dependen...

Kinetic energy dependence of the branching ratios of the reaction of N+ ions with O2

The kinetic energy dependence of the branching ratios of the reaction of N+ with O2 has been determined in the collision energy range from 0.06 to 1.8 eV using a selected‐ion flow‐drift tube. At energies near thermal, the branching ratios were found to be the following percentages: O2+ 51±4, NO+ 43±4, and O+ 6±4, in good agreement with the results of some of the earlier thermal‐energy studies. At relative kinetic energies above about 0.1 eV, the O+2 channel increases and eventually becomes dominant, at the expense of the NO+ channel. At the highest energy investigated in the present study, the branching ratios obtained are 79%, 19%, and 2%, respectively, and join rather smoothly with crossed‐beam data extending to higher energies. The O+ channel remains a minor channel over the energy range studied here. These energy‐dependent branching ratios are in striking contrast to the total rate constant for this reaction, which is virtually energy‐independent for energies below 1 eV.

Flowing afterglow apparatus for the study of ion-molecule reactions at high temperatures

We describe two versions of a high temperature flowing afterglow apparatus. With a stainless steel flow tube wrapped with heating tape we have obtained data over the range 300–1300 K. In a version with a ceramic flow tube in a commercial furnace we have obtained data over the range 300–1600 K. The ceramic version is designed to take data up to 1800 K, but we have encountered experimental problems at the upper temperature range. The design modifications to a standard flowing afterglow needed to make measurements at elevated temperatures are described in detail, as are problems associated with operating at elevated temperatures. Samples of data are given.

Temperature Dependences for the Reactions of O- and O2- with O2(a1Δg) from 200 to 700 K

Rate constants and product ion distributions for the O- and O2- reactions with O2(a 1Deltag) were measured as a function of temperature from 200 to 700 K. The measurements were made in a selected ion flow tube (SIFT) using a newly calibrated O2(a 1Deltag) emission detection scheme with a chemical singlet oxygen generator. The rate constant for the O2- reaction is approximately 7 x 10(-10) cm3 s-1 at all temperatures, approaching the Langevin collision rate constant. Electron detachment was the only product observed with O2-. The O- reaction shows a positive temperature dependence in the rate constant from 200 to 700 K. The product branching ratios show that almost all of the products at 200 K are electron detachment, with an increasing contribution from the slightly endothermic charge-transfer channel up to 700 K, accounting for 75% of the products at that temperature. The increase in the overall rate constant can be attributed to this increase in the contribution the endothermic channel. The charge-transfer product channel rate constant follows the Arrhenius form, and the detachment product channel rate constant is essentially independent of temperature with a value of approximately 6.1 x 10(-11) cm3 s-1.