Reflections

This section collects the essays from Reflections from the Frontiers (Explorations for the Future: Gordon Research Conferences 1931-2006), GRC's 75th anniversary commemorative publication.

Physical Visions
Aart W. Kleyn
Aart W. Kleyn
Foundation for Fundamental Research on Matter / Institute for Plasma Physics Rijnhuizen
Invented by the Devil

Wolfgang Pauli once said that “surfaces are made by the devil”–still an accurate description for the challenges faced by surface scientists. Condensed-matter physicists do not like breaking the symmetry of three-dimensional solids, which was especially true when surface science first came together as a field in the late 1920s following the demonstration of low-energy electron diffraction by Clinton Davisson and Lester Germer and helium atom diffraction from solid surfaces by Otto Stern.

Experimentally, the main difficulty for surface science was the production of clean, well-ordered surfaces, which require using the far-from-trivial technique of making an ultra-high vacuum (UHV). In 1972, when I started working at the Foundation for Fundamental Research on Matter Laboratory of Atomic and Molecular Physics in Amsterdam, UHV required a major effort to maintain. But once the sample was clean and the vacuum held, we could examine the structure of the surface for longer periods.

The Dynamics at Surfaces Gordon Conference, previously Dynamics of Gas-Surface Interactions, is the primary meeting ground for scientists who study the interactions of impinging particles from the gas phase and later the motion of atoms at the surface. In practice, this research involves “contaminating” the surface on purpose, which seemed initially like a counterintuitive experiment for UHV surface scientists. Techniques for studying the dynamics of the gas-surface interaction were derived from experiments on the dynamics of molecular collisions in the gas phase: discussions at Gordon Conferences facilitated this transfer.

By the late 1960s molecular beams had become an important tool to study structure by helium diffraction and interaction dynamics, picking up on studies on the scattering of heavier particles that were halted when Stern left Germany in the 1930s. Most of the experiments, however, did not yet involve well-characterized surfaces, and many of the experiments gave good insights but not the right data.

A revival came in the late 1970s when UHV molecular-beam surface scattering machines were built in a number of laboratories. At that time I began working as a postdoc at the IBM Almaden Research Center in San Jose, California. In 1980 the first measurements of state-resolved molecular scattering at surfaces were carried out–a novelty for major discussion at the 1981 GRC. For the first time we could observe a molecule rotating and later vibrating when it interacted in a single collision with a solid surface.

The nitric oxide molecule suddenly became the fruit fly of dynamics at surfaces, thanks to its facile state-resolved detection. Nobel laureate John Polanyi triggered a major discussion on “rotational rainbow scattering” at surfaces. I vividly remember him sitting quietly at the edge of the stage while a vigorous debate took place around him. Discussion was interrupted only by a participant asking, “Could the conference chair ask the session chair to stop his debate with the audience and ask the speaker to continue?” Lively discussions of that sort were prevalent in the early Dynamics at Surfaces Gordon Conferences in the 1980s, with the audience often interrupting presentations for long periods.

The field of gas-surface interactions evolved rapidly during the 1980s with several new techniques that for many changed the direction of research interests and laboratory work. The scanning tunneling microscope was invented in the early 1980s, and for the first time surface atoms could be seen and induced to move. For the “beam-ists” this motion was incredibly slow (one image of the atom per second was considered fast), since a molecular collision at a surface takes only picoseconds at most. Another major development was the introduction of time-resolved spectroscopy at the pico- and femto-second timescale. Previously, only scattering experiments could detect fast events, but now lasers could track molecules adsorbed at surfaces. Unprecedented views of dynamics at surfaces resulted, and the Gordon Conference, true to its mission of staying on the leading edge of science, changed its name to Dynamics at Surfaces.

Science races on, leaving some unsolved issues behind as attention turns to novel discoveries. Currently, the holy grail of surface science is to make movies of molecular dynamics at surfaces, taken with atomic-scale time and distance resolution. For dynamics at surfaces, new areas of research include more complex surfaces and interfaces, working catalysts, and the flowing liquids of living cells. Many discoveries are only possible through the intense interactions between scientists of different backgrounds. The Dynamics at Surfaces Gordon Conference has been instrumental on this front. My collaborators always enjoy going to a GRC because of the intense interaction with colleagues, a dynamic that is absent at bigger conferences where scientists, especially young ones, get completely lost. Coming together for a week, with a program structured to include large blocks of free time, allows open discussion between scientists. In turn this is a primary driver for the development of open science.