Diatomic Targets Research Articles

Computed rotational rainbows from realistic potential energy surfaces

The quantal IOS approximation in here employed to study interference structures in the rotationally inelastic, state-to-state differential cross sections for polar diatomic targets (LiH, FH, and CO) interacting with He atoms. Quite realistic expressions are used to describe the relevant potential energy surfaces (PES) which were taken from previous works that tested them against accurate experimental findings for total and partial differential cross sections. Specific features like short-range anisotropy and well depth, long-range attractive regions and overall range of action for each potential employed are analyzed and discussed in relation to their influence on rotational rainbows appearance and on the possible observation of cross section extrema in rotational energy distributions.

The effect of collision energy in polyatomic ion/neutral target collisions over a range of 10–6000 eV

Mass spectra resulting from superthermal collisions between polyatomic ions and atomic or diatomic targets have been acquired as a function of the collision energy over a range of 10–6000 eV for the t-butyl cation and the molecular ions of methane, propane and n-butylbenzene. Changes in relative fragment ion abundances with systematic variation of the collision energy have been monitored in order to assess the effect of this experimental variable on the degree to which the polyatomic ion is excited by collision. The data for the t-butyl cation and the methane molecular ion show an apparent monotonic increase in energy deposition into the ion with increasing collision energy over the entire range investigated. The data for the molecular ions of propane and n-butylbenzene, on the other hand, indicate that maxima in energy deposition into these ions occur at laboratory collision energies of 140 and 70 eV, respectively. The results are interpreted as being due to changes in the relative importance of direct vibrational excitation and electronic excitation with increasing collision energy.

Reactive scattering of rotationally excited target molecules with adiabatic theory

We present a formulation of the three-dimensional quantum mechanical reactive scattering of an atom and a rotationally excited diatomic target molecule within the framework of adiabatic distorted wave theory. This is an extension of previous work where only the rotationally ground initial state was treated while the final molecule could be in any state. The importance of the present formulation lies in the fact that the population of the rotationally excited target molecules is significant under ordinary experimental conditions. A method of obtaining exact and approximate adiabatic wave functions and energies is developed through the use of the body-fixed formulation of atom–diatomic molecule scattering. The integration in transition matrix with rotationally excited adiabatic wave function is again reduced to the three-dimensional integral by separating out the angular variables for the rigid motion of the plane for the atom–molecule system. Explicit integration formula is presented for the reactive transition matrix element. The computational feature of the present formulation is illustrated by evaluating the reactive cross section of the (D,H2) system. For 0→1 rotational transition, present calculations reproduced earlier results. For 1→1 rotational transition, for which no previous result of the adiabatic distorted wave theory is available, the present calculations yield qualitatively similar but quantitatively different angular dependence in the differential cross sections, as in those of the rotationally ground target molecule. Physical significances and further implication of the present formulation are discussed.

Multiple scattering of MeV light ions through thin amorphous anodic SiO2layers formed on silicon single crystals

The multiple-scattering angular distributions of protons, helium, and nitrogen ions between 500 keV and 2 MeV which have passed through anodic Si${\mathrm{O}}_{2}$ films (in the 100-1500-\AA{} thickness range) formed on the top of [110] Si single crystals have been determined. They were obtained by comparing, at different penetration depths, the backscattering yields in Si crystals covered by Si${\mathrm{O}}_{2}$ layers to the azimuthally averaged yields in a bare crystal at various angles of incidence with respect to the [110] axis. The precision and limits of the method used are discussed. The detailed and precise experimental data on channeling in silicon which are required are reported. The experimental distributions are compared to the theoretical predictions obtained by extension of the Bothe's formula to diatomic targets and using a scattering cross section corresponding to the Thomas-Fermi potential. In the particular case of Si${\mathrm{O}}_{2}$, the theoretical distributions are shown to be nearly identical to the distributions calculated for a monoatomic target of atomic number $Z=10$. Very good agreement is observed, for protons and helium, between the experimental and the theoretical distributions when the Thomas-Fermi screening radius of the target atoms is used. This screening radius is well adapted to our experimental cases since, in our energy domain, protons and helium have a very high probability to be completely stripped in matter.

Electron–molecule scattering in momentum space

We examine the Fourier transform of the Schrödinger equation for electron–molecule scattering, treated as potential scattering from a multicenter distribution of charged fixed in space. When the angle ϑ between R↘,the internuclear vector of a diatomic target, and q↘, the momentum transfer, is held fixed during the collision, then the directions of incidence and scattering are fixed relative to R↘. The process is then described as having a dynamical dependence on the magnitude of q↘, q, from which the scattering angle is determined, and a parametric dependence on q↘′s direction relative to R↘. This approximation is used routinely at high energies in the calculation of the Born amplitude. Fixed–nuclei coordinate–space studies suggest that this approximation can be extended to low energies, provided the amplitude is taken from the solution of the integral equation of momentum space rather than from its inhomogeneity, proportional to the Born amplitude. To obtain this solution, we use the above constraint in the inhomogeneity, and we adopt a similar constraint in the evaluation of the kernel of the integral term. That is, we constrain R↘ to be in the same direction relative to q↘′, a virtual momentum transfer belonging to the kernel, as it is to q↘. In this approximation, the process is described as having a dynamical dependence on two magnitudes, q and q′, and a parametric dependence on the single angle ϑR. This causes the equations for the ’’radial’’ or k space partial waves to be diagonal for each value of the orientation angle ϑR. Calculations are performed for the e, H2 scattering in the static approximation, and cross sections averaged over ϑR are shown to be in good agreement with cross sections calculated by use of coupled spherical and coupled spheroidal partial wave theories. The angular distribution in the static approximation is also calculated at an incident energy close to 7 eV, where exchange is relatively unimportant. This result is in reasonably good agreement with that of R matrix theory in the static–exchange approximation. The extension of the theory to treat exchange is formulated and discussed. Also its extension to treat more complicated molecular targets is discussed.

On the scattering of slow protons by diatomic targets

AbstractInelastic collisions leading to rotationally excited molecular targets and involving protons as projectiles are theoretically examined from the point of view of both the scattering equations in the close coupling formalism and the interaction potentials between the partners.A phenomenological approach is suggested for constructing such interactions and computational results are reported for simple diatomic targets. The inadequacy of more traditional ‘static’ approaches, when called for explaining dynamic couplings with open channels and centrifugal effects, is also discussed.