Crossed-beam study of the reactions H3+(Ar,H2)ArH+ and ArH+(H2,Ar)H3+ 

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  • Crossedbeam study of the reactions H3 +(Ar,H2)ArH+ and ArH+(H2,Ar)H3 +C. R. Blakley, M. L. Vestal, and J. H. Futrell Citation: The Journal of Chemical Physics 66, 2392 (1977); doi: 10.1063/1.434276 View online: http://dx.doi.org/10.1063/1.434276 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/66/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Crossedbeam study of the reaction of van der Waals molecule H+(NO)2 J. Chem. Phys. 92, 1657 (1990); 10.1063/1.458100 Crossedbeam study of the reaction H2 + (CO, H)HCO+ at 1.89 eV J. Chem. Phys. 71, 4166 (1979); 10.1063/1.438189 Crossedbeam study of the reaction H3 +(D2,H2)D2H+ J. Chem. Phys. 64, 2094 (1976); 10.1063/1.432434 Crossedbeam studies of the hydrogen exchange reaction: The reaction of H atoms with T2 molecules J. Chem. Phys. 59, 3421 (1973); 10.1063/1.1680488 CrossedMolecularBeam Measurements of the Total Cross Sections of ArN2, ArNe, ArHe, and ArH2 atThermal Energies J. Chem. Phys. 45, 240 (1966); 10.1063/1.1727317

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  • Crossed-beam study of the reactions H3+ (Ar ,H2)ArH + and ArH+(H2,Ar)Ha+ *

    c. R. Blakley,t M. L. Vestal,t and J. H. Futrell Department of Chemistry. University of Utah. Salt Lake City. Utah 84112 (Received 20 September 1976)

    The dynamics of the reactions Hi(Ar.H2)ArH+ and ArH+(H2.Ar)Hi were studied over the initial relative translational energy from 0.87 to 9.7 eV and from 0.18 to 6.7 eV. respectively. The reactions were found to proceed via direct mechanisms at all energies studied. Energetic data are presented that suggest Hi is not rapidly relaxed vibrationally upon collision with H2 as previously thought. Rather than being relaxed within a few collisions. the lower vibrational states probably are relaxed with a rate constant on the order of 10- 12 cm3 molecule-I.sec- I.

    INTRODUCTION

    Proton transfer reactions comprise a class of very simple ion-molecule reactions which have received much attention in the literature. 1 Most information about these reactions consists of rate constants, branching ratios, and heats of formation as determined by mass spectrometry, drift tube, and ion cyclotron resonance techniques. Kinematic information, how-ever, can only be inferred from these data. A more direct approach is to use crossed-beam techniques to directly measure the product angular and energy dis-tributions.

    In this paper we report kinematic data on the reac-tions

    Hi +Ar- ArH+ +H2'

    ArW +H2- Hi +Ar.

    (R1 )

    (R2)

    These reactions are of some interest because they per-mit the comparison study of a simple exothermic pro-ton transfer reaction with its endothermic reverse re-action. Pragmatically, the reactions of Hi are of in-terest because of its use as a reagent ion in chemical ionization mass spectrometry.

    EXPERIMENTAL

    The crossed-beam apparatus, shown schematically in Fig. 1, is described in detail elsewhere. 2 Basically, it consists of an ion gun which produces a nearly mono-energetic beam of the desired ion which is then crossed at right angles with a beam of neutral molecules. Ion-ic reaction products are energy and mass analyzed in a moveable detector assembly which consists of a 90 cylindrical electrostatic energy analyzer, a quadrupole mass filter, and a secondary electron multiplier. A chemical ionization source is used to generate the re-actant ions and a supersonic nozzle to produce the neu-tral beam. Time averaged, phase sensitive pulse count-ing is used to obtain the. energy spectra of the mass and angular resolved product ions which are then processed off line on a computer.

    The high pressure source can be used to produce vi-brationally relaxed reactant ions. The supersonic mo-lecular beam source produces internally relaxed intense neutral beams with narrow velocity and angular spreads and can be "seeded" to produce accelerated beams of heavy neutrals. A neutral beam ionizer has been added

    2392 The Journal of Chemical Physics, Vol. 66, No.6, 15 March 1977

    so that accurate neutral velocity and angular distribu-tions can be measured with the moveable detector.

    REDUCTION OF EXPERIMENTAL DATA

    Details of the data reduction have also been described previously.3 Briefly, the experimental data are sum-marized in two forms. In the first of these the contour map of relative intensities normalized for polar cen-ter-of-mass coordinates is plotted superimposed on a Newton diagram4 according to the transformation

    I(U, X,

  • ~Iakley, Vestal, and Futrell: Reactions H; (Ar, H2 ) ArW and ArW (H 2 ,Ar) H; 2393

    it is tedious to obtain angular and energy distributions from them.

    As an alternative we use the reaction translational exoergicity Q and the barycentric scattering angle X as rectangular coordinates. The appropriate intensity transformation is given by

    I(Q, X) =(Tt!T)1/2I(T, e), (2)

    where Tf is the relative translational energy of the products, and the translational exoergicity Q is given by

    (3)

    T; is the initial relative translational energy, t:.Hr is the heat of reaction, and t:.E1nt is the increase in the internal energy of the products over the internal energy initially in the reactants. In this representation the plotted intensities are proportional to the differential cross sections for scattering of products into a solid angle increment on(X) with translational exoergicity between Q and Q+ oQ. The prinCipal advantage of this type of plot is that the angular distributions at each Q and the distributions over Q at each scattering angle are directly displayed; furthermore, by integrating the distribution of intensities over the scattering angle we may obtain the overall distribution of Q values, I(Q), or by integrating over Q at each angle we obtain the overall angular distribution, J(X).

    The computer programs for analyzing data from crossed-beam experiments have recently been modi-fied to improve the accuracy and resolution of the data. These modifications include corrections for known de-tector discrimination and application of deconvolution techniques to remove distortions of the data caused by the transmission function of the apparatus. 7

    Briefly, the known discriminations that are corrected include the change in reaction volume with laboratory observation angle, the change in luminosity of the elec-trostatic prism with energy, and the change in duty time if the energy scan range is changed during the ex-periment. The effective reaction volume changes with observation angle since it is the intersection of two un-equal conical beams.

    The general deconvolution problem consists of mea-suring some function f(x) which is related to the func-tion h(x) we would like to determine by the integral

    f(x) = i: h(x')g(x-x')dx'=h*g, (4) where g(x) is the "apparatus" function which is pre-sumed to be known. For this work, the apparatus func-tion for the cylindrical energy plate energy analyzer was assumed to be Gaussian with a calculated standard deviation equal to 1. 8%. The method employed is the Van Cittert solution8,9 for obtaining an approximate in-version of Eq. (4). This is accomplished by calculating the functions

    C;(x) = f(x) -1: hi_1(X')g(x - x") dx', (5) and the ith estimate of the desired function is given by

    The procedure is begun by taking as the zeroth esti-mate for h(x), the measured functionf(x); that is,

    ha(x) = f(x). (7)

    The procedure may be continued until

    (8)

    where E is the maximum acceptable error. As is shown by Wertheim, 9 the maximum number of iterations which is useful is limited by noise in the data.

    A typical experiment consists of 255 point energy dis-tributions obtained at 20-30 laboratory angles. To speed the construction of the intenSity contour plots, they are generated as 42x42 element arrays. This re-duces the total number of data pOints from about 6000 to 1764, thereby speeding the data reduction process, and permits the plot to be printed on a high speed line printer. The slight loss of resolution caused by the re-duction of the number of pOints is generally more than offset by the speed and ease with which the data are handled. Since the number of elements is held con-stant, the scale of the array in velocity or X, Q space is adjusted to contain the data. Each of the 255 data pOints of the raw energy distributions are allocated to the appropriate array element. Usually, more than one datum is assigned to a