Laboratory for Atomic and Surface Physics

Research > Atomic Collisions

We study the interaction of energetic ions with solids to understand the fundamental mechanisms leading to electronic excitations and how these excitations evolve and lead to the emission of light (luminescence), electrons, atoms and molecules (sputtering), and to radiation damage, chemical changes or heat. We currently study metals, rare gas solids, condensed molecular gases, oxides and minerals. The studies are driven by interest in fundamental phenomena and by applications in semiconductor processing, nuclear fusion, gas discharges, and space exploration. Our studies include:

Plasmon excitation by ion impact

We are interested in many-electron excitations in solids.  We discovered that quantized electron density oscillations (plasmons) can be excited in free-electron metals like Mg and Al by very slow ions (slower than the Fermi velocity of the solids).  The source of the excitation is the potential energy carried by the ion, which is released when it captures an electron from the solid.  A secondary excitation mechanism is caused when fast electrons can be energized in the solid by the incoming projectile.  Some of our studies are done in collaboration with the Universidad Autonoma de Madrid (Spain), and the Univ. of Calabria in Cosenza (Italy).


Ionization and excitation effects in condensed-gas solids

Condensed gas solids are good model systems to study the effect of electronic excitations in insulators. We study by experiments and computer simulations how the electronic energy deposited by fast ions in solid films of Ar, Kr, Xe, O2, CO2, H2O, etc. is dissipated into sputtering, luminescence and electron emission. Irradiation results in an unusual state of matter, not accessible by other methods, where the material is taken extremely far away from thermal equilibrium. The results of the experiments have implications in radiation biology, the behavior of solids in space environments, the operation of detectors for nuclear and elementary particle physics experiments, the doping of semiconductor materials, etc. Example: Phys. Rev. Letters 74 (1995) 1474

Electron emission from solids

In collaboration with Fujitsu (Japan), we studied the emission of electrons from thin MgO films induced by low energy ions (rare gases and sodium).  Our goal is to understand the fundamental physical processes which will be useful in designing materials that can increase the energy efficiency of flat-panel displays, where oxide surfaces are subject to ion bombardment by a plasma. Example: Surface Sci. Lett. 571 (2004) L305

In collaboration with INTEC (Argentina) we study electron emission from graphite.  We discovered structure in the electron energy distribution that signal the formation and decay of valence excitons. Example: Phys. Rev. Lett. 100 (2008) 227604

Charge transfer at surfaces

The way in which electrons are transferred between surfaces and atoms or ions is one of the most basic problems in surface physics. We use electron spectroscopy to learn about basic processes. Besides the plasmon excitation mechanism mentioned above, we study Auger processes outside surfaces and also the formation of doubly-excited (autoionizing) states of Ne in collisions of Ne ions with surfaces. Example: Phys. Rev. Lett. 72 (1996) 4041

Understanding the basic excitation mechanisms allows us to apply it to study the effect of adsorption of atoms (like alkalis or oxygen) on the electronic structure of surfaces. Example: Phys. Rev. Lett. 77 (1996), 408

Electrostatic charging of surfaces

We use a Kelvin probe and measurements of the energy of secondary ions to study the dynamics of electrostatic charging and discharging of thin insulator films. See Nucl. Instrum. Meth. B 268, 2888 (2010) for our work on water ice.

Focused ion beam applications

We use a FIB focused ion beam system to make nanostructured patterns in semiconductors to fix proteins  Example: Langmuir 14 (1988) 6785