Experimental reaction cross sections

J. V. Dugan, Jr., Comparison of numerical capture cross sections with experimental reaction cross sections for NHa + NH3, Chem. Phys. Letters 8, 198-200 (1971). 

Faubel M, Martinez-Haya R, Rusin L Y, Tappe U and Toennies J P 1997 Experimental absolute cross sections for the reaction F + D2 at collision energies 90-240 meV J. Phys. Chem. A 101 6415-28... 

After identifying the reactant ion, reaction cross-sections were measured as a function of average reactant ion kinetic energy. Q experimental is measured for given values of (eEl) 1/2 in the spectrometer, and experimental values of k... 

Intramolecular isotope effect studies on the systems HD+ + He, HD+ + Ne, Ar+ + HD, and Kr + + HD (12) suggest that the E l dependence of reaction cross-section at higher reactant ion kinetic energy may be fortuitous. In these experiments the velocity dependence of the ratio of XH f /XD + cross-sections was determined. The experimental results are presented in summary in Figures 5 and 6. The G-S model makes no predictions concerning these competitive processes. The masses of the respective ions and reduced masses of the respective complex reacting systems are identical for both H and D product ions. Consequently, the intramolecular isotope effect study illuminates those... 

Reactions of Complex Ions. For reactions of systems containing H2 or HD the failure to observe an E 1/2 dependence of reaction cross-section was probably the result of the failure to include all products of ion-molecule reaction in the calculation of the experimental cross-sections. For reactions of complex molecule ions where electron impact ionization probably produces a distribution of vibrationally excited states, kinetic energy transfer can readily open channels which yield products obscured by primary ionization processes. In such cases an E n dependence of cross-section may be determined frequently n = 1 has been found. 

Using a as an experimental variable, information concerning other reaction cross-sections will also become available. Some of the values of the cross-sections obtained by this technique are summarized in Table I, clearly demonstrating that much useful information can be obtained from the detailed studies of these simple discharge systems. 

Experiments have also played a critical role in the development of potential energy surfaces and reaction dynamics. In the earliest days of quantum chemistry, experimentally determined thermal rate constants were available to test and improve dynamical theories. Much more detailed information can now be obtained by experimental measurement. Today experimentalists routinely use molecular beam and laser techniques to examine how reaction cross-sections depend upon collision energies, the states of the reactants and products, and scattering angles. 

For harpoon reactions of alkaline metal atoms with iodine molecule I2, the interaction radii, Re, calculated using the formula Re = (ajji) 12 from the experimentally measured cross-sections a, are compared in Table 3 with the distances, Ru, calculated with the help of eqn. (40) and the sums of the gas-kinetic radii i M + i l2 of the reagents. In these calculations, effective radii of alkaline metal atoms have been used as RM, while the radii of the molecule I2, calculated from the data on the viscosity of I2 vapour at T > co and at T = 273 K, have been used as i l2 (the values of RM + i ,2 given in brackets correspond to the latter) [71], It is seen that the values of Re exceed Rm + Rh, i.e. electron transfer occurs at large impact parameters. 

The effect of rotational energy on a reaction cross section has been studied experimentally in only one reaction, namely, Ar+(H2, H)ArH + (Table III). For this exoergic particle-transfer process, an inverse dependence of the cross section on rotational energy was observed. 

The classical theory makes especially clear the inherent ambiguity of data analysis with the optical model, and this ambiguity carries over into the quantum model. If we wish to use experimental differential cross sections to gain information about V0(r) and P(b) or T(r), we must assume a reasonable parametric form for V0(r) that determines the shape of the cross section in the absence of reaction. The value P(b) is then determined [or T(r) chosen] by what is essentially an extrapolation of this parametric form. In the classical picture a V0(r) with a less steep repulsive wall yields a lower reaction probability from the same experimental cross-section data. The pair of functions V0 r), P b) or VQ(r), T(r) is thus underdetermined. The ambiguity may be relieved somewhat (to what extent is not yet known) by fitting several sets of data at different collision energies and, especially, by fitting other types of data such as total elastic and/or reactive cross sections simultaneously. 

Finally, some experimental observations are discussed in which charge transfer to surface states is important. The emphasis is on methods to be quantitative in describing the role of surface states by determining their density and reaction cross sections. Some previously published observations as well as preliminary new results are used to illustrate the role of surface bound species as charge transfer surface states. 

Having obtained reaction cross sections as a function of relative kinetic energy, the data must be analyzed to extract the onset for the endothermic process of interest. Because of the extensive distributions of energies, accurate location of this onset is a difficult process. In our work, we choose to model the experimental cross sections with a mathematical expression that is theoretically justified [22-24], Eq. (3). In addition, this expression has proven to yield accurate thermochemistry in a number of systems, providing a reasonable empirical justification for its use ... 

---