SynopsisElectrostatic effects are believed to determine the molecular structure and function of macromolecules in many ways. In metallo-based enzymes and in metal-macromolecule interactions in solution, these effects may predominate. In order to tackle metal ion-nucleic acid interactions theoretically, we propose a modification of Debye's distance-dependent dielectric function first proposed more than 50 years ago. This function more closely approximates physical reality a t small interatomic separations. Our theory yields a dielectric function that gives reasonable agreement with experimental data in preliminary calculations.

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The energy loss and differential probability of energy losses is calculated to all multipole orders for a charged particle moved uniformly past a sphere. The sphere's response is characterized by use of a local dielectric function.PACS numbers: 71.45. Gm, 79.20.Kz A particle approaching a polarizable body may engender collective excitations which act back upon the particle. Both virtual and real excitations occur, and the resulting interation thereby has both conservative and dissipative components. Excitations may include surface plasmons, 1 surface optical phonons, 2 surface excitons, helicons, ripplons, 3 and other collective effects including, for example, orientation of permanent molecular dipoles. 4 Similar effects may be found in atomic and nuclear physics (as in Coulomb excitations of nuclei). For the case of a planebounded medium, a large body of literature exists 1,2 ' 5 ; while for the case of a spherically symmetric body, the literature is somewhat less extensive. 6 Because of its general applicability, the case of a point charge moving past a spherically symmetric, polarizable body has not only been of continuing interest in fundamental physics, but also has attracted interest from fields such as materials science and electronics. The advent of modern scanning-transmission electron microscopes (STEM) has allowed probing of nanometer-sized bodies with a 0.2-nm beam of electrons. Energy analysis of the transmitted beam has been accomplished in this case. 7,8 In other experiments, the radiative decay of Coulomb-stimulated surface plasmons has been used to diagnose the spectrum of energy losses. 9 Despite the general and high level of interest in the problem, the formulation of the energy-loss probability for a charged particle passing near a general dielectric polarizable sphere is unsatisfactory to date. In particular, for ever closer trajectories one expects ever higher contributions from higher multipoles, since one tends toward the planar limit. The calculations for the dipole modes are quite straightforward, but little work has been done in the general case. The well-known solution of electrodynamics for the case of a uniformly moving charge and a harmonically bound charge 10 is the equivalent of the dipole approximation used for a sphere. Schmeits, 11 for example, has considered the dipole and linear quadrupole excitations, but higher multipoles, including the polar and the azimuthal indices, are equally important in most experiments. The classical theory of impact-parameter-dependent energy losses for planar interfaces has been shown 8 to account for some of the observations on small spheres of oxide-coated Al and of silver. Wheatley, Howie, and McMullan 8 used the planar results for spheres by replacing the impact parameter with respect to the sphere by the instantaneous value at each point of the track and resolving the force in the track direction. While this ad hoc procedure is easy to apply and might provide useful results, clearly one needs a more comprehensive approach which yields a good ...

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