R. Passarella

Diagnostics of flow tube techniques for ion/molecule reactions

An in-depth study of the dependence of reaction rate coefficients on system parameters in a selected ion flow tube (SIFT) has been performed. The effects due to differing flow tube diameters, lengths, and reactant gas inlet designs, have been quantified for the thermal reaction Ar+ + O2 → O2+ + Ar. Four different reactant gas inlets in three flow tubes of different size have been compared. The trend is toward a consistent rate constant for all inlets at longer residence times and larger flow tube diameters. Results from the premixing of the O2 in the helium buffer flow substantiate these observations.

Thermal energy reactions involving Ar+ monomer and dimer with N2, H2, Xe, and Kr

Rates and products of reaction are reported for the thermal energy charge transfer of Ar+ and Ar+2 with N2, H2, Xe, and Kr using a selected ion flow tube (SIFT). New rate coefficients were measured for the reactions Ar++Xe→Xe++Ar, Ar+2+N2 →Ar+⋅N2+Ar, Ar+2+H2 →ArH++H+Ar, and Ar+2+H2 →ArH+2+Ar; the rates are 4.3×10−13, 2.2×10−10, 3.6×10−10, and 1.1×10−10 cm3/s, respectively. The finding of (Ar⋅N2)+ as the product of the reaction of Ar+2 with N2 shows the bond energy of Ar+⋅N2≥1.27 eV in agreement with other techniques. In the case of H2, two reaction products are observed for reaction with Ar+2: ArH+ (85±10%) and ArH+2, but not Ar2H+ as suggested by earlier studies. Lower limits for the proton affinity of Ar (3.69 eV) and the dissociation energy for H+2⋅Ar (0.97 eV) are determined from the present work. Considering all the reaction rates, the proximity of the recombination energy of Ar+ and Ar+2 to a band in the photoelectron spectrum of a specific neutral reactant offers an explanation for the observed ord...

Studies of the energy dependence of reactions of Ar+ and Ar+2 with CH4 and CS2

The rate coefficients for the reactions of Ar+ and Ar+2 with CH4 and CS2 are obtained as a function of energy by using a selected ion flow tube with an electric drift field (SIFT‐DRIFT). A diagnostic analysis of the system and technique has been completed and allows rate coefficients to be reported with a high level of confidence. Rate coefficients and product branching ratios are determined for the methane reactions from thermal (room temperature) to center‐of‐mass kinetic energies of about 0.15 eV and for the carbon disulfide reactions from thermal to about 0.6 eV. The overall rate coefficients for these reactions do not vary a great deal over the investigated energy range. The greatest dependence is found for the slowest reaction Ar+ with CS2, where a minimum in the rate coefficient is observed near 0.15 eV. The behavior of the branching ratios for S+ and CS+2 suggests curve crossings between a repulsive state and the A, B, and C electronic states of CS+2.

Gas-phase reactions of sulfides, mercaptans, and dimethyl methylphosphonate with ionic species derived from argon and water

Abstract Rate coefficients for gas-phase ion/molecule reactions involving sulfides, mercaptans of the form RSR′ (where R  H, CH 3 , CH 3 CH 2 ), and dimethyl methylphosphonate (DMMP) are measured using a selected ion flow tube (SIFT) technique. The ionic reactants include Ar + , Ar + 2 , OH + , H 2 O + , H 3 O + (H 2 O) n with n  0–3. All the rate coefficients indicate relatively fast, nearly collisional reactions. Ar + and Ar + 2 react with DMMP to produce the parent ion (charge transfer channel) as well as ionic fragments of DMMP. Reactions of DMMP and RSR′ with H 3 O + result in proton transfer. Both proton and charge transfer products are observed when H 2 O + reacts with DMMP. Hydration of H 3 O + by up to three water molecules reduces the proton transfer reaction rate with DMMP by a factor of two.

Curvature in the plots for the determination of rate constants derived from fast-flow techniques

Abstract The determination of rate constants by using flow techniques typically is made under pseudo-first-order reaction conditions, whereby rates are derived from semilogarithmic plots of ion intensity versus reactant gas concentration. Reproducible curvature in such plots is often taken as an indication of measurable differences in the reactivity of various ion states. In the present study, curvature is traced to effects arising from the mass flow of the buffer gas. These results, in conjunction with previous investigations, suggest that accurate determination of rate constants using flow techniques requires a judicious selection of flow conditions and reactant gas inlet design with due consideration to the flow tube dimensions.

The energy dependence of the reaction of N+2 with CS2

The reaction of N+2 with CS2 is studied as a function of center-of-mass kinetic energy over the range from thermal to 0.5 eV. The overall reaction displays little dependence on energy, with an average of all determinations given (1.2 ± 0.1) × 10−9 cm3 s−1, about 80% of the calculated collisional value. Two products are observed, namely S+ and CS+2; the ratio S+:CS+2 of the primary products changes from about 60:40 to about 20:80 as the center-of-mass kinetic energy increases from 0.15 to 0.65 eV. The reduced zero-field mobility of N+2 in He was determined to be 20 ± 1 cm2V−1s−1, in good agreement with previously reported values.