Structure In Energy Loss Spectra Research Articles

Trajectory-dependent energy loss for swift He atoms axially scattered off a silver surface

Angle- and energy-loss-resolved distributions of helium atoms grazingly scattered from a Ag(110) surface along low indexed crystallographic directions are investigated considering impact energies in the few keV range. Final projectile distributions are evaluated within a semi-classical formalism that includes dissipative effects due to electron–hole excitations through a friction force. For mono-energetic beams impinging along the [11¯0],[11¯2] and [001] directions, the model predicts the presence of multiple peak structures in energy-loss spectra. Such structures provide detailed information about the trajectory-dependent energy loss. However, when the experimental dispersion of the incident beam is taken into account, these energy-loss peaks are completely washed out, giving rise to a smooth energy-loss distribution, in fairly good agreement with available experimental data.

Application of electron optical techniques to the study of amorphous materials

Despite the sub-angstrom resolution capability of modern transmission electron microscopes, elucidation of the structure of amorphous materials by direct-imaging methods is still challenged by problems of projection and data presentation identified over 20 years ago. Classical diffraction analysis, leading to the radial distribution function not only retains its essential importance but has been extended to deal with small scattering volumes in inhomogeneous samples by the use of focused electron probe illumination but retaining orientation averaging. Fluctuation microscopy provides a significant advance in the detection and statistical representation of medium-scale spatial variations in amorphous structure. Issues of scattering coherence are crucial in both these approaches. Further opportunities for use of focused probes include the study of angular correlations in nanodiffraction patterns as well as fine structure in energy loss spectra. Finally, there may be a return to the direct imaging approach thanks to recent advances in electron tomography and in aberration-corrected depth sectioning.

Energy losses and straggling of light ions transmitted through thin carbon foils

The energy losses of 10 MeV/amu 3 He 2+,1+ , 6 Li 3+,2+ , 12 C 6+,5+,4+ and 16 O 8+,7+,6+,5+ ions passing through carbon foils of 5–110 μg/cm 2 in thickness were measured with a high resolution magnetic spectrograph. The measurement was carried out for ions emerging from foils in the same charge state as the incident beam. The measured fixed-charge stopping powers have been reported in our previous papers. In this paper, the energy loss straggling of partially clothed ions in the charge-state nonequilibrium region is reported for the first time. A double peak structure in energy loss spectra of 3 He 1+ and 6 Li 2+ ions transmitted through carbon foils of 40–50 μg/cm 2, which is due to charge changing contributions, was also described before. The present study shows that these contributions also bring about a characteristic behavior in the foil thickness dependence of the straggling values. For partially clothed C and O ions, a linear relation between the foil thickness and the straggling does not hold in the thicker foil region and the straggling exhibits an abrupt increase, although the double peak structure did not appear in the energy loss spectra. A Monte Carlo simulation taking account of the charge exchange can well reproduce the energy loss straggling of 3 He 1+ and 6 Li 2+ emerging from thicker foils.

Multiple-peak structures in energy-loss spectra of swift ions

Generally valid expressions have been developed for the energy-loss spectrum of ions penetrating matter, separated into contributions with different numbers of charge-changing events. The theoretical procedure is based on series expansion of the generalized Bothe–Landau formula in terms of the probability for charge exchange and is particularly suited for analysis of multiple-peak structures. Numerical results are given for 32 MeV He ions penetrating carbon, a system studied experimentally by Ogawa and coworkers. Accurate analytical expressions for the partial spectra of swift ions are found based upon a modification of the Landau approach.

Collisional time-correlation functions in the semiclassical limit. III. Application to vibrational–rotational energy transfer in collisions of Li+ with N2

We apply a previously developed treatment of energy tranfer based on collisional time-correlation functions (TCFs) to Li+ scattered from N2 at hyperthermal velocities. Double differential cross sections are calculated using a previously available, realistic potential energy surface for this system. After sucessfully testing the TCF results against independent calculations, we present an in-depth study of vibrational–rotational energy transfer over a wide range of scattering angles (θ=10–175 deg) and collision energies (E=4–17 eV). We also carry out a comprehensive comparison with time-of-flight experiments and explain the observed bimodal structure of energy loss spectra. This has led to a modification of the available translation–vibration coupling potential. The calculations are based on the conditions of slow rotational motion and low vibrational excitation, and involve classical trajectories for relative motions generated at each possible orientation of N2. Cross sections were obtained from interpolations of Tables of more than 60 000 trajectories. Vibrational and rotational motions were described quantum mechanically within a previous treatment based on operator algebras.