Research Interests

The problem of determining the energy loss suffered by charged particles as they pass through material media has presented a long-standing and continuing challenge to both theorist and experimentalist. A knowledge of the energy loss of energetic ions in matter is, in addition to its fundamental interest, necessary for many applications. These include corrections for finite thickness in nuclear physics, the design of reactors, and the ion-beam surface-layer analysis of materials among other applications.

My research program is dealing with:

• Time-Dependent collision processes, in particular, scattering, bond breakin and bond making, chemical reactions, ro-vibrational processes, excitations and ionization. ALl of this processes analyzed simultaneusly (coupled) within the same framework.i
• Stopping in atoms and molecules, with especial emphasis on aggregation effects such as deviations from the Bragg rule, solid/gas sample phase effects, and stopping in large molecules
• The study of energy deposition requieres the knowledgement of the differential cross section as well as the energy loss as a function of the scattering angle (or impact parameter b).

Time-dependent atomic and molecular collisions

Dr. Ohrn's group has developed in the last years a methodology to study time-dependent processes called Electron-Nuclear Dynamics (END) based on the time dependent variational principle and a coherent state representation of the wave function.
As part of my research, I have implemented semiclassical corrections to the classical nuclei by means of the Schiff approximation to scattering processes.

Among the properties we treat with reasonable confidence are:
 

• Direct and charge transfer differential cross section for any combination of ion-atom-molecule system.
• Kinetic energy loss of the prjectile.
• Energy absoption by the target into ro-vibrational, translational, and electronic degrees of freedom.
• Chemical reactivity of the collision.
• Dissociation and bond breaking.

• Ionization (free centers)
• Spontaneus and stimulated photon emission (electromagnetic fields).

• H⁺ and H -> H, He, C, N, O, F, Ne
• H⁺ and H -> H₂, D₂, N₂, O₂, CH₂, C₂H₂, C₂H₆, H₂0
• He⁺ -> Ne
• N⁴⁺ -> H

Atomic and molecular stopping

It is the aim of this project to develop a scheme for calculation of the energy deposition of swift ions in molecules. The attack is two pronged. One prong, direct calculation and integration of the appropriate generalized oscillator strengths (GOS's) using the polarization propagator formalism, is presently feasible only for small molecules, but in principle provides an accurate method to calculate stopping. This scheme will be developed for study of small molecules, such as water or H molecule, to provide a normative comparison for the large molecule scheme, and to gain insight on how the large molecule scheme can be improved. The second prong, a scheme based on the additivity and transferability of properties of chemical bonds, is not as accurate, but is not limited by the size of target molecules, and thus admits calculation of stopping properties for target molecules of a size big enough to be biologically significant.

Among the problems currently under study are:

• calculation of generalized oscillator strength distributions
• calculation of molecular dipole oscillator strength distributions and their energy weighted moments
• studies of deviations from the Bragg Rule
• development of additive fragment stopping functions for use in a cores and bonds generalization of the Bragg Rule