Recoil Velocity Distributions Research Articles

The question of energy randomization in the decomposition of chemically activated C2H4F

The question of whether energy randomization occurs in the decomposition of chemically activated C2H4F is further examined with the crossed molecular beam technique by measuring the collision energy dependence of the C2H3F product recoil energy distributions for the reaction F+C2H4. The symmetric angular distributions of product molecules indicate that the lifetime of C2H4F remains longer than a rotational period even at the highest collision energy studied. The effect of the exit channel barrier becomes less important at higher collision energies since the fraction of the total energy associated with the barrier becomes smaller as the collision energy increases. The experimental recoil velocity distributions yield average product translational energies which remain almost constant at ∼50% of the total available energy: These results are shown to be inconsistent with phase space theory and the recent ’’Tight Transition State’’ theory of Marcus. Our experiments and the infrared chemiluminescence experiments of McDonald and co-workers indicate that the C2H3F product energy distribution is nonstatistical, a conclusion seemingly at variance with Rowland’s determination of lifetimes from stabilization of chemically activated C2H4F produced in hot-atom experiments. We discuss possible explanations for differences among the experimental data on this system.

Molecular Beam Study of the K+CH3I Reaction: Energy Dependence of the Detailed Differential Reactive Cross Section

A molecular beam study has been carried out on the reaction of a velocity-selected K beam with a crossed beam of CH3I (from a glass capillary-array source). Velocity analysis of the angular distribution of the reactively scattered KI has yielded detailed differential cross sections at three incident relative kinetic energies Ē= 1.77, 2.80, and 3.71 kcal/mole. The center-of-mass (c.m.) differential cross sections were obtained from the lab data by a least-squares computer ``inversion'' program providing for deconvolution of beam velocity distributions. The c.m. cross section is ``backward'' peaked (KI recoiling opposite to the incident K, in accord with the literature) with a very sharp (30%–40% FWHM) recoil velocity distribution which maximizes at 80% of the energetically allowed KI recoil velocity. This distribution shifts systematically to higher velocities with increasing incident collision energy. Averaging over all angles yields an overall product c.m. translational energy distribution which shifts upwards as the incident translation energy is increased. The average energy released into relative product translation varies from 15 to 17.5 kcal/mole over the above range of Ē; the average fraction of the total available energy released is ≈ 0.58. thus incident translational energy is efficiently converted to relative translational energy of product recoil. The detailed differential cross sections at each energy were scaled relative to one another and suitably integrated to yield relative values of the total reactive cross section. The present results confirm previous observations of an increase in reaction cross section with increasing Ē up to ≈ 4 kcal/mole.

Reactive scattering of a supersonic alkali atom beam: K + Br2, BrCN, SnCl4, PCl3, CCl4, CH3I

Angular distribution measurements of reactive scattering of a supersonic potassium atom beam by a series of molecules are reported with initial kinetic energy (∼ 5 kcal mole-1) above the thermal energy range. The narrow Laval nozzle velocity distribution gives improved resolution over thermal energy measurements. Total reaction cross sections are found to decrease with energy. Differential reaction cross sections for Br2, BrCN and CCl4 show increased forward scattering compared with thermal energies but for CH3I there is no change to within experimental error. The BrCN scattering is compatible with spectator-stripping dynamics, though this limit has not been reached in the Br2 scattering. The SnCl4 differential reaction cross section appears not to be compatible with a single peak in the centre of mass recoil velocity distribution. It is suggested that high and low velocity contributions to the intensity may arise from a long-lived collision complex dissociating by two reaction paths.

Molecular Beam Kinetics: Differential Cross Sections of the Reactions Cl+I2 and Cl+IBr

The chemical reactions Cl+I2 and Cl+IBr were studied using a crossed molecular beam apparatus. Angular and recoil velocity distributions were obtained for all reaction product molecules. In the center of mass system the forward to backward scattering ratios for the reactions Cl+IBr, Cl+I2, Cl+Br2, [N. Blais and J. Cross, J. Chem. Phys. 52, 3580 (1970)] and Cl+BrI were found to be 4, 6, 8, and 12, respectively. The recoil energy distributions were determined by the chemical bond being formed rather than the one which was broken. For ICl, 50% of the total available energy was found in internal excitation with a very broad distribution. For BrCl, 80% of the total available energy was found in internal excitation with a rather narrow distribution. We conclude that the angular distributions are consistent with those arising from short range attractive forces and that the reaction time is less than one rotational period.

Molecular Beam Study of the K+I2 Reaction: Differential Cross Section and Energy Dependence

A molecular beam study of the reactive and nonreactive scattering of a velocity-selected K beam crossed with a thermal I2 beam is described. Velocity analysis yielded the recoil velocity-angle distribution of product KI flux. The energy dependence of this flux distribution was determined from measurements at several incident K beam velocities, corresponding to a range of collision energies Ē from 1.9 to 3.6 kcal/mole. Computational methods were developed to extract the c.m. differential reactive cross section functions (angular and recoil-energy distributions) from the laboratory data. In accord with previous literature on related systems, the reactive cross section is “forward peaked” and most of the exoergicity (some 40 kcal/mole) goes into internal excitation of the KI product. However, there is also significant KI scattering at wide angles and with large recoil energies. The c.m. angular and recoil-velocity distributions are some-what coupled (i.e., nonfactorizable). An increase in Ē from 1.9 to 3.6 kcal/mole produces only a slight change in the shape of the c.m. differential cross section, accompanied by a small decrease (<̃ 20%) in the magnitude of the reaction cross section. Measurements were also made on the angular and velocity distributions of the nonreactively scattered K over the same energy range.

Angular Distributions of Products from Reactions of Copper Isotopes withHe4Ions

Angular distributions of the recoil products of the ${\mathrm{Cu}}^{63}(\ensuremath{\alpha},n)$ reaction as well as of the ($\ensuremath{\alpha},2n$), ($\ensuremath{\alpha},3n$), ($\ensuremath{\alpha},2p$), and ($\ensuremath{\alpha},\ensuremath{\alpha}n$) reactions of ${\mathrm{Cu}}^{65}$ have been measured over the energy range of 20-43 MeV. The results have been compared with a Monte Carlo evaporation calculation and good agreement has been obtained for a reasonable choice of the level-density parameter for all but the ($\ensuremath{\alpha},n$) and ($\ensuremath{\alpha},\ensuremath{\alpha}n$) reactions at the higher energies. The discrepancies observed for the latter are indicative of a direct-interaction mechanism. Average properties of the angular distributions have been used to obtain the average values of particle and photon energies for the various reactions. The information obtainable from transformations of the angular distributions between the laboratory and center-of-mass systems has been explored. It was found that, on the assumption that the distribution of evaporation recoil velocities is isotropic and Gaussian, the laboratory distributions could be characterized by two parameters, namely the most probable evaporation velocity and a Gaussian width parameter. The latter could in turn be used to obtain average nucleon and photon energies.

The electron-neutrino angular correlation in beta decay of helium 6

The electron-neutrino angular correlation in decay of He 6 has been determined by measuring the distribution of electron energies and recoil velocities without selection of angle. The correlation parameter α was found to be −0.353 −0.053 +0.033, confirming current belief that the Gamow-Teller interaction is of axial vector type. The maximum admixture of a tensor interaction is given by |C T| 2/|C A| 2 < 0.08.

The Recoil of Electrons from Scattered X-rays

Quantum theory of the recoil of electrons from scattered x-rays.---This is an extension of the quantum theory of scattering suggested by Compton, which assumes that each directed x-ray quantum is scattered by a single electron. Expressions for the distribution of recoil velocities, of energies and of ranges are developed for each of two postulates, assuming (1) the scattered radiation consists of directed quanta, and (2) the scattered radiation proceeds as spherical waves. On the first postulate the maximum recoil energy is shown to be ${E}_{m}=h{\ensuremath{\nu}}_{0}\ifmmode\times\else\texttimes\fi{}\frac{2\ensuremath{\alpha}}{(1+2\ensuremath{\alpha})}$, where $\ensuremath{\alpha}=\frac{h}{\mathrm{mc}{\ensuremath{\lambda}}_{0}}$; the recoil electrons are shown to be concentrated at angles near the direction of the primary beam; and from the distribution of energy, using a relation given by C. T. R. Wilson, the distribution of ranges is found to be such that two-thirds have tracks shorter than half the maximum range. The maximum range increases rapidly with frequency. The values for the maximum ranges in the case of x-rays (.34 to.48 A) are computed to be about one-third of those observed by Wilson for his fish tracts, but the difference may be due to the lack of homogeneity of the rays used. The relative number of recoil electrons to photo-electrons increases with the frequency and is in agreement with observations by Wilson. The second postulate, however, leads to a value for ${E}_{m}$ only one-fourth that given above, a value which is inconsistent with that derived from a consideration of radiation pressure and which leads to values for maximum ranges one-fiftieth of those observed by Wilson. Other experimental observations are cited which also lead to the conclusion that the first postulate is much more likely to be true than the second, hence, that each quantum of scattered radiation is probably emitted in a definite direction.