Sheldon Datz

Sheldon Datz, who helped introduce a new field of research, molecular-beam studies of chemical dynamics, died on 15 August 2001 in Oak Ridge, Tennessee, of pneumonia brought on by progressive pulmonary fibrosis.

Sheldon was born in New York City on 21 July 1927. His brilliant career may have been foretold when, at the age of 16, as a student at the city’s Stuyvesant High School, he was involved in research alongside Ellison Taylor at Columbia University’s Synthetic Alloy Materials Laboratory. There, he and Taylor investigated the properties of “tube alloy,” a code name for uranium. At the time, Sheldon was unaware that he was working with uranium. The study later became part of the Manhattan Project.

At age 17, Sheldon joined the US Navy and saw combat in the South Pacific. After World War II ended, while still in the navy, he witnessed nuclear testing in the Bikini Atoll. He then began his studies in chemistry at Columbia, receiving his BS in 1950. Ten years later, he earned his PhD in chemistry from the University of Tennessee at Knoxville. His thesis, entitled “Molecular Association in Alkali Halide Vapors,” was prepared under the guidance of Richard D. Present.

In 1951, Sheldon joined the chemistry division of the Atomic Energy Commission’s Oak Ridge National Laboratory as a research chemist, and again had an opportunity to work with Taylor, who joined ORNL that same year. Sheldon undertook the first of his more than 300 refereed publications in 1955, when he and Taylor wrote an article on chemical reaction mechanisms using molecular beams. The first demonstration of this technique was acknowledged by the 1986 Nobel Prize in Chemistry to Dudley R. Herschbach, Yuan T. Lee, and John C. Polanyi. A year after this initial seminal work, a seemingly modest pair of experimental technique papers written by Sheldon and Taylor were published in the Journal of Chemical Physics . Those papers described the first quantitative study of surface ionization detection, a method that was originally used by I.I. Rabi and his students in atomic and molecular beam research. The method remains the mother lode for an entire industry of beam studies carried out to this day. Further development of the technique led to Sheldon’s inventing the differential surface ionization detector that allowed him to identify chemical reaction products.

In 1962–63, Sheldon, on a Fulbright fellowship, worked at the Institute for Atomic and Molecular Physics in Amsterdam. In 1964, he and a colleague discovered that a low-velocity argon ion would scatter from a single copper atom residing on a solid surface. Laboratories worldwide later adapted this technique to analyze the elemental composition and structure of surfaces. During his stay in Amsterdam, Sheldon became interested in ion channeling, in which an ion could be directed to pass cleanly through a crystal so that the researcher could measure the change in the ion’s energy and trajectory to reveal the crystal’s electronic and structural characteristics.

In 1965, Sheldon became an assistant director of ORNL’s chemistry division, a position he held until 1974. He and his ORNL colleagues demonstrated in 1965 that an atomic ion traveling in the interstitial regions of a crystal, between rows and planes of crystal atoms, undergoes much less energy loss than if the ion had entered the crystal in a random direction. Further development of this ion channeling technique led to the discovery of resonant coherent excitation for multicharged ions with one and two electrons. In this process, an incoming channeled ion is quickly elevated to an excited state when a harmonic of the ion’s perturbation frequency, given by the ion velocity and the spacing of atoms in the crystal, matches the frequency for electronic excitation. Excited ions thus produced are rapidly ionized inside the crystal. One could study the coherent excitation processes inside a crystal by studying the ions’ charge state fraction versus the ions’ velocity.

From 1976 to 1981, Sheldon led the atomic and molecular collisions group and heterogeneous catalysis group in the chemistry division. From 1979 to 1991, Sheldon and collaborators from Stanford University and Lawrence Livermore National Laboratory explored a related but different periodic perturbation effect. They channeled high-velocity electrons and positrons through a crystal to produce channeling radiation—that is, the release of strong, forward-directed x-ray radiation.

In the late 1970s, Sheldon and his ORNL coworkers first measured the dielectronic recombination for ions by merging a multiply-charged ion beam with an electron beam. This method provided cross sections that were needed for modeling energy-loss processes in hydrogen fusion-oriented plasmas. In 1981, Sheldon was named head of the atomic physics group of ORNL’s physics division.

His continued innovations led him, along with coworkers, to conduct experiments using heavy-ion beams at CERN during the early 1990s. The interaction of these ultrarelativistic energy ions in gold foils illuminated, at extraordinary high-collision energy, atomic physics aspects such as ion charge state change and electron-positron pair production. The experimental measurements of the cross sections for charge capture and loss in high-Z collision systems provided the first test of existing theoretical calculations. Because a large capture cross section could lead to reduced storage lifetimes in colliders, it became important to establish design parameters of CERN’s Large Hadron Collider and also of Brookhaven National Laboratory’s Relativistic Heavy Ion Collider, which, at that time, was under construction.

More recently, while heavy-ion storage rings, such as the CRYRING in Stockholm, Sweden, were being built in Europe, Sheldon realized an idea. In those rings, a molecular ion beam is merged with a beam of electrons so that the collision of the electrons with the ions removes energy from the ions. The result is that each ion’s random motion is reduced (that is, cooled). This technique allows the study of collisions at very low energies that earlier had not been possible. Sheldon and his collaborators used this method to study dissociative recombination at near-zero collision energy. The use of heavy-ion storage rings has contributed greatly to an improved understanding of processes in low-temperature plasmas, astrochemistry, and aeronomy.

For his work, Sheldon received numerous honors. In 2000, he received the Enrico Fermi Award, the highest award given by the US Department of Energy, with cowinners Sidney Drell and Herbert York. In 1998, he received the American Physical Society’s Davisson–Germer Prize in Atomic Physics. Sheldon also was a fellow of the American Association for the Advancement of Science.

Those of us who worry how the loss of this giant scientist will affect the progress of collision physics should recall a saying of Sheldon’s. When explaining an approach to a solution he had obviously conceptualized thoroughly but was still mulling over the details, he would say, with gestures for emphasis, “Not to worry.” That will be difficult for many of us to do.