Chris Greene analyzes the physical behavior of atoms and molecules, including nucleic acids. For nearly five years, he has studied the interactions of femtosecond laser pulses with atomic clusters of xenon. In 2002, researchers at Hamburg, Germany's TESLA Test Facility discovered that if you aim a pulsed beam of soft X-rays at cold (20 K) clusters of 1000 or more xenon atoms, the clusters explode. They are completely destroyed, and researchers can detect xenon ions that have lost anywhere from two to eight electrons. Because xenon is a noble gas, the ease with which the X-ray pulses dislodge electrons was completely unexpected, and, initially, inexplicable.

However, Greene and his colleagues figured out what was happening: The first wave of photons knocks out one or two electrons from xenon atoms in the cluster. Then, those free electrons continue to absorb energy from the laser's electric field. When there are enough high-energy free electrons in the cluster, they begin to recollide with xenon ions, knocking more electrons out of their outer shells. Eventually, some of the electrons get so hot and energetic, they leave the clusters, and the clusters explode.

Greene and his group had to develop a new approach to explain the observed laser-cluster interactions at high photon energies. They discovered that the high efficiency of high-energy photon absorption in xenon clusters is due to the absorption of a photon by an electron in the field of a nucleus in combination with atomic-structure and plasma-screening effects. The researchers continue to develop their theory to explain the experimental behavior of atomic clusters subjected to high-energy femtosecond laser pulses. At the present time, theoreticians and experimentalists at JILA and elsewhere are leapfrogging over one another to understand this unusual behavior.

Greene and his group recently began a project to understand DNA damage caused by low-energy electrons, which are knocked out of DNA molecules by high-energy radiation. These low-energy electrons get captured by the DNA bases, temporarily forming a negatively charged molecule, or anion. The anion lasts just long enough to transfer its excess energy to the weakest nearly chemical bond, often breaking it. In DNA, the weakest link is the carbon-oxygen bond between the sugar and phosphate groups that form its backbone. These bonds can break in either strand of the DNA double helix, increasing the likelihood that the genetic code will be misread, causing permanent, and potentially catastrophic, biological changes.

The Greene group performed a theoretical analysis of electron resonances within DNA bases at the moment of electron capture. The researchers discovered that the electrons stay long enough inside the ring structure of the bases to lead to bond breaking. Then the group analyzed low-energy electron capture by chemical models for the sugar-phosphate DNA backbone. The next step is to find out more about the mechanism that allows captured electrons to leave the ring structure and transfer their energy to the DNA backbone.