J. B. Marquette

Study of the reactions N+2+2N2→N+4+N2 and O+2+2O2→O+4+O2 from 20 to 160 K by the CRESU technique

Rate coefficients k1 and k2 have been determined for the association reaction N+2+2 N2→k1N+4+N2 and O+2+2 O2→k2O+4+O2 in the temperature range 20–160 K, using a supersonic jet apparatus (CRESU), which is described in detail. For the highest temperatures where other measurements exist, the measured values of the rate coefficients are in excellent agreement with previous measurements. The temperature dependence of k1 can be represented by a power law of the form k1=6.0×10−29 (300/T)1.85 from 20 K to higher temperatures. For k2, only lower limits for 20 and 45 K are reported but the present data do not confirm the existence of a maximum in k2 as suggested by other authors. These results are examined in the light of recent theoretical arguments.

Design and testing of axisymmetric nozzles for ion‐molecule reaction studies between 20 °K and 160 °K

Axisymmetric contoured Laval nozzles have been calculated in order to obtain uniform supersonic flow. For nitrogen and oxygen gases the calculation uses only similarity techniques, but for helium, complete calculations of the isentropic core and the boundary layer were performed for two sets of boundary conditions. Experimental testing has shown that the nitrogen or oxygen flows exhibit a good uniformity but that the helium flow was uniform only for one set of boundary conditions. The purpose of this work is to provide flow reactors for studies of ion‐molecule reactions in the real temperature range of interstellar clouds (T<80 °K).

Reactions of N+(3P) ions with normal, para, and deuterated hydrogens at low temperatures

The reactions of N+(3P) ions with normal, para, and deuterated hydrogens have been examined at temperatures below 163 K by using the CRESU technique with both helium and nitrogen buffer gases. All these reactions have decreasing rate coefficients with decreasing temperature. Comparison of the data for normal and para‐H2 reveals the drastic importance of rotational energy in promoting these processes. Analysis of these results shows a better agreement between the n‐H2, p‐H2, and HD data if the spin–orbit energy of N+(3P) is also considered as efficient as kinetic and rotational energies in driving the reactions. It follows that the endothermicity of the reaction N+(3P0) +H2(X 1Σ+g,J=0) →NH+(X 2Π,J=0) +H(2S) is (18±2) meV. This yields a proton affinity of N equal to (3.524±0.003) eV and a dissociation energy of NH of (3.42±0.01) eV.

Reactions of He+ and N+ ions with several molecules at 8 K

Abstract The rate constants for the reactions of He+ ions with N2, O2 and CO and N+ ions with O2, CO and CH4 have been measured at 8 K in a supersonic jet apparatus (CRESU). The reactions are all fast and the rate constants do not change in going from 300 K (88 K in the case of N+ + CO) to 8 K. This indicates that the existence of quadrupole moments and anisotropic polarizabilities of the linear neutrals do not contribute to a significant change in collision rate constant or reaction efficiency over this extreme temperature range.

Ion—polar-molecule reactions: A CRESU study of He+, C+, N+ + H2O, NH3 at 27, 68 and 163 K

The first measurements of ion—polar-molecule reaction rate constants at very low temperatures are presented. They have been obtained using the CRESU (cine_.tique de reactions en ecoulement supersonique uniforme) technique for H+.C+ and N+ ions reacting with H2O and NH3 at 27 and 68 K in helium buffer. Additional data have been obtained for N+ reactions at 163 K in nitrogen buffer. In the 27–300 K (27–163 K for N+ + NH3) temperature range, all the results yield a power law, k = k0T−n (0 <n < 1), for the rate coefficient of each reaction, which should be applied in interstellar cloud model in place of the room-temperature values. The results are compared with various theoretical calculations. Rather good agreement is found although no general behavior can be simply drawn from these experiments.

The reaction O+2+CH4 → CH3O+2+H studied from 20 to 560 K in a supersonic jet and in a SIFT

Rate coefficients k have been determined for the reaction O+2+CH4 → CH3O+2+H at several temperatures in the uniquely wide temperature range 20–560 K using a new expanding jet (CRESU) apparatus and a selected ion flow tube (SIFT) apparatus. In the overlapping temperature range of the experiments the rate coefficients are in excellent agreement. A minimum is observed in the rate constant near 300 K but the outstanding feature is the rapid increase at low temperatures from the minimum value k (290 K)=5.4×10−12 cm3 s−1 to k (20 K)=4.7×10−10 cm3 s−1, the latter being about half of the collisional limiting value kc for the reaction (kc=1.16×10−9 cm3 s−1). Indeed the data show that k → kc as T → 0 K and the low temperature values can be fitted to a power law of the form k=1.1×10−7 T−1.8. The results strongly indicate that the CH3O+2 product ion is formed via rearrangement in a long‐lived intermediate (CH4O2)+ complex, the lifetime against unimolecular decomposition of which largely controls the rate of the react...

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...

Reactions of Ar+ with H2, N2, O2, and CO at 20, 30, and 70 K

Reactions of Ar+(2P3/2) ions with H2, N2, O2, and CO have been studied with the CRESU apparatus at 30 K in argon buffer gas and with the newly developed mass‐selected ion injector (CRESUS configuration) at 20 and 70 K in helium buffer gas. The atom exchange reaction with H2 is rather fast, with a rate coefficient k showing a small energy dependence well represented by k=1.5×10−9E0.16 cm3 s−1 with the collision energy in the center‐of‐mass frame, E, in the range 2.5×10−3−0.65 eV. This result is more compatible with a frozen‐rotor capture model rather than with a threshold model for endoergic reactions. Concerning the reaction with N2, a competition between the two exit channels involving the two first vibrational levels of the N+2 product ion explains the minimum of the rate constant suggested at about 140 K by previous SIFT experiments [D. Smith and N. G. Adams, Phys. Rev. A 23, 2327 (1981)] and presently confirmed. The rate coefficients for the charge transfer reactions with O2 and CO increase with decre...

Low-temperature reactions of He+ and C+ with HCl, SO2 and H2S

Abstract CRESU measurements of rate coefficients for the reactions of He + and C + with HCl, SO 2 and H 2 S are reported at 27 and 68 K. The results are compared with those obtained using three different ion-dipole theories and good agreement is obtained.

Low-temperature reactions of some atomic ions with molecules of large quadrupole moment: C6F6 and c-C6H12

Abstract Reactions of He + ,C + and N + ions with C 6 F 6 , and c -C 6 H 6 were examined at 27, 68 and 297 K using both CRESU and SIFT techniques. Within the experimental uncertainties, all of the rate coefficients are found to be temperature-independent in this range. The results are discussed in relation to the most recent theoretical models describing the influence of the quadrupole moment on the temperature dependence of ion-molecule reaction rate coefficients.