BERNSTEIN, RICHARD BARRY

New York, New York, 31 October 1923; d. Helsinki, Finland, 8 July 1990)

chemistry.

Bernstein was born to an immigrant family that attached great importance to education. His father, Simon Bernstein, was a lawyer and businessman; his mother, Stella Grossman Bernstein, owned a small dress shop and, after her children had grown, became a psychologist. A gifted pianist, Bernstein considered a career in music but decided instead to pursue science. He graduated from high school at the age of fifteen and enrolled at Columbia University, where he studied chemistry and mathematics.In 1942, Bernstein began work in the Synthetic Alloy Materials (SAM) Laboratory at Columbia, a part of the Manhattan Project. The laboratory, which was supervised by Harold C. Urey, carried out studies of isotope separation by gaseous diffusion and centrifugation. After receiving his AB with honors in 1943, Bernstein was inducted into the U.S. Army Corps of Engineers; he was first assigned to Oak Ridge, Tennessee, and then was posted back to Columbia in 1945. Before leaving the army in 1946, he was a participant in the first tests of fission weapons at Bikini Atoll.

Bernstein received an MS degree from Columbia in 1946 and began to carry out doctoral research with T. I. Taylor on gas-phase processes for isotopic separation. He was awarded a PhD in physical chemistry in 1948 and accepted a position in Chicago as assistant professor of chemistry at the Illinois Institute of Technology (IIT). In the same year he married Norma Olivier; they had three daughters and a son.

While at IIT, Bernstein carried out research on gas-phase spectroscopy and chemical kinetics. In 1953, he moved to the Department of Chemistry at the University of Michigan. This was the first of his many moves. He once jokingly noted that he changed positions every ten years. In fact, on average, he moved more frequently.

Early Studies of Molecular Beams. It was at Michigan that he began his research on molecular beams, which brought him fame and which would occupy him for the rest of his life. Scattering techniques had been employed as a tool in nuclear and atomic physics for many years, but the applications in chemistry that require crossing two molecular beams did not begin until the mid-1950s. In the address that he presented when he received the Robert A. Welch Foundation Award in 1988, Bernstein recounted that in 1953–1954 he became aware of the progress that his friend Sheldon Datz had made in studying chemical reactions with molecular beams. (Datz had also worked at the SAM in Columbia.) Earlier attempts at using beams in chemistry had been unsuccessful, largely because of the lack of sensitive detection, but ionization detectors could be employed to examine reactions involving alkali metals. Bernstein decided to begin a molecular beam program in Michigan early in 1955 and set out to measure the systematics of total collision cross sections for the interactions of atoms with molecules. His first paper (with Edward W. Rothe) applying beam techniques appeared in 1959.

At this time Bernstein and several others recognized that beam experiments offered an opportunity to provide a molecular-scale understanding of the kinetics of chemical reactions. He reasoned, however, that before one could understand reactive scattering (the result of collisions between molecules that can react with each other) it was necessary to understand nonreactive collisions. He therefore began with both experimental and theoretical examinations of elastic scattering. It was nearly a decade later, in 1965, that his first study of reactive scattering was published. By that time he had been at the University of Wisconsin for two years, where he was named W. W. Daniels Professor of Chemistry in 1967.

The path of his research with molecular beams, which he followed until his death, led through appointments at the University of Texas as W. T. Doherty Professor of Chemistry and Physics; to Columbia, where he returned in 1977 as Higgins Professor of Natural Science; and to the position of senior vice-president of the Occidental Research Corporation in Irvine, California, in 1983. Two years after Bernstein’s arrival, the company decided to reduce its commitment to fundamental research, and he joined the faculty in chemistry at the University of California, Los Angeles (UCLA). His career path also traced the refinements in the technique—velocity and state selection, alignment and laser pump and probe—that made it possible to examine in detail the molecular nature of chemical reactivity.

Chemical Reactions at the Molecular Scale. In the early 2000s, even biochemical processes were being probed at the molecular level. But until the mid-twentieth century, there was mostly indirect evidence for how molecules interact and react. Matter was studied in the bulk, and from observing the macroscopic properties and their rate of change, inferences were made about the underlying molecular-level mechanism. Because only bulk averages could be studied—not the truly elementary events—there was always the possibility that the assumed mechanism was consistent with the rate as observed but was not, in fact, correct. Scattering techniques seek to undo the averaging inherent in bulk chemical kinetics.

An example and an experiment that brought Dick Bernstein worldwide recognition is the direct demonstration of the steric effect in chemical reactions. The understanding that chemical forces have a strong directional character goes all the way back to Jacobus van't Hoff. Molecules therefore have a shape, and how they react must depend on their relative orientation as they approach one another. During the first part of the twentieth century, organic chemists provided compelling but indirect evidence for the importance of steric requirements of chemical reactions. The three-dimensional, shape-selective nature of chemical processes is inherent in the proposed “lock and key” mechanism for enzyme selectivity that takes the steric requirements as a key concept. When the reaction takes place in the bulk, all orientations of the approaching reactants are possible. The steric requirements mean that not all collisions are fruitful; those that are not do not have a favorable orientation. Bernstein took it upon himself to provide a direct experimental demonstration of the validity of the concept that chemical reactions have steric requirements. The story is well known. It is described in Bernstein’s own words in his Welch lecture (1989), and the experimental and conceptual progress built upon his pioneering effort is well represented by the proceedings of an international conference held a short time after his death and dedicated to his memory (Levine, Zewail, and El-Sayed, 1991).

The Mechanics of Molecular Collisions. At the beginning of his lifelong study of elementary chemical reactions, Bernstein decided to start with the simplest questions and build from there. So his earliest experiments dealt not with chemical reactivity but with a prerequisite. In order for two molecules to react, they need to come together and collide. Bernstein’s early experiments dealt with the rate of collisions of molecules, regardless of whether these collisions resulted in a reaction. Such experiments determine the total collision cross section (“total” because all possible outcomes are counted). Curiously, classical and quantum mechanics differ in what they regard as the allowed outcomes. In classical mechanics, any time a force acts between two molecules, it may be said that a collision took place. By Newton’s first law, a collision takes place whenever the molecules deviate from a straight-line motion. But in quantum mechanics, very tiny deflections cannot be detected in principle because such detection violates the uncertainty principle. (For the same wave-diffraction reason, a shadow is never absolutely sharp.)

Bernstein was fascinated by these essentially quantal features and showed how their experimental observation provides a signature of the attraction that is always present between any two atoms or molecules. By these early experiments, which lasted throughout the first decade of his foray into the new field, Bernstein established the insights and detail that the scattering technique can provide. With the perception provided by several decades of hindsight, it was in the early 2000s possible to suggest on the one hand that with this careful setting of the stage, he delayed his exploration of the more chemical questions and left some key issues to be ably addressed by others and later comers such as Dudley Herschbach. On the other hand, these early pioneers, who styled themselves the “lunatic fringe,” had the entire field of chemical reactivity, a field that many regard as the sine qua non of chemistry, to explore. There were plenty of fundamental questions for all; one remarkable aspect of the emerging field of molecular reaction dynamics is the friendly and collegial relations between its participants. By 1974, the field was sufficiently well mapped that Levine and Bernstein could write a primer for it (Molecular Reaction Dynamics, 1974). By the beginning of the twenty-first century, almost all textbooks of physical chemistry had a chapter (or more) on reaction dynamics.

From physics and Newton’s first law, the small molecular beams community took the idea of the change in the direction of the velocity, the deflection, as a probe of the collision. What is more characteristic of chemical physics is the change in the magnitude of the velocity. For molecular collisions the velocity can change in an essentially continuous manner; this occurs because the total energy is conserved. After the collision is over, the total energy is the sum of the internal energy of the products and of the kinetic energy of their motion. Strictly speaking, the internal energy is quantized, and so it can only assume discrete values. But as compared to the kinetic energy, the energy spacings between different internal energy states are typically small. So the final kinetic energy of the motion of the collision products can take a continuous range of values.

Analyzing the velocity with which the products recede from the collision was pioneered by Bernstein in the study of inelastic collisions (collisions in which the chemical identity is maintained but the internal state is changed). It was, however, in the study of chemical reactions, where Bernstein and his coworkers determined both the direction and the magnitude of the velocity of the products, that the real benefits were reaped. Contour maps of the final velocity are a polar coordinate representation where the polar angle is the angle of deflection and the radius is the magnitude of the velocity. Polar maps from Bernstein’s laboratory can be found in the early 2000s in textbooks of chemical kinetics and even in introductory textbooks of physical chemistry. An overview of the set of tools that he pioneered and perfected can be found in his Hinshelwood Lectures at Oxford (published as Chemical Dynamics via Molecular Beam and Laser Techniques, 1982).

Bernstein regarded theory as part of the tools that experimentalists must have at their command; to this end, he edited a handbook, Atom-Molecule Collision Theory: A Guide for the Experimentalist (1979). It is interesting to note that this ability to be equally at home in experiment and theory is common to many other pioneers of the field of reaction dynamics.

Lasers as a Probe of the Transition between Reactants and Products. Very early on, Bernstein added another experimental technique that was to revolutionize the field: lasers as both promoters of the chemical change and probes of the products. In 1990, he spent a sabbatical leave as a Sherman Fairchild Distinguished Scholar at the California Institute of Technology, where he collaborated with Ahmed Zewail to devise ways of using lasers to probe the “transition state,” the transitory configuration leading from reactants to products of a chemical reaction. At the same time he continued to lead his group at UCLA through a rather active period of looking at the steric effect of processes happening at the interface of a solid. Later that year, while on a scientific visit to Moscow, he suffered a heart attack. He was airlifted to a hospital in Finland but did not recover from the secondary attack.

Richard Bernstein would publish experimental results only if he had been present in the laboratory while the experiment was carried out. The scientific community celebrated his integrity, seriousness of purpose, innovation of scientific questions, the novelty of experimental design, and the quality of the results with many awards and honors. Principal among them are his early elections to membership in the National Academy of Sciences in 1968 and as a fellow of the American Academy of Arts and Sciences in 1970. He received the American Chemical Society’s Peter Debye Award in Physical Chemistry in 1981, the National Academy of Sciences Award in the Chemical

Sciences in 1985, and the American Chemical Society’s Irving Langmuir Award in Chemical Physics in 1986, as well as an honorary doctorate in science from the University of Chicago. In 1988, he was the recipient of the Robert A. Welch Award in Chemistry, the American Chemical Society’s Willard Gibbs Medal, and the National Medal of Science. With his wife Norma at his side he also edited the journal Chemical Physics Letters for more than a decade.

BIBLIOGRAPHY

A complete bibliography is included in the Journal of Physical Chemistry, R. B. Bernstein Memorial Issue (see last citation below).

WORKS BY BERNSTEIN

With Edward W. Rothe. “Total Collision Cross Sections for the Interaction of Atomic Beam of Alkali Metals with Gases.” Journal of Chemical Physics31 (1959): 1619–1627.

With Hans U. Hostettler. “Observation of Quantum Effects in the Scattering of a Monoenergetic Li Beam by a Crossed Hg Beam. Physical Review Letters5 (1960): 318.

“Extrema in Velocity Dependence of Total Elastic Cross Sections for Atomic Beam Scattering: Relations to Di-atom Bound States.” Journal of Chemical Physics 37 (1962): 1880–1881. With Arthur E. Grosser and Anthony R. Blythe. “Internal Energy of Reaction Products by Velocity Analysis. I. Scattered Br from the Crossed Molecular Beam Reactions K + Br.” Journal of Chemical Physics 42 (1965): 1268–1273.

“Quantum Effects in Elastic Molecular Scattering.” In Advances in Chemical Physics, edited by Ilya Prigogine, vol. 10, p. 75. New York: Wiley-Interscience, 1966.

With R. J. Beuler and K. H. Kramer. “Observation of the Reactive Asymmetry of Methyl Iodide. Crossed-beam Study of the Reaction of Rubidium with Oriented Methyl Iodide Molecules.” Journal of the American Chemical Society88 (1966): 5331–5332.

With Raphael D. Levine, B. R. Johnson, and J. T. Muckerman. “Computational Investigation of Internal Excitation in Non-reactive Molecular Collisions: Resonances in Rotational Excitation.” Journal of Chemical Physics 49 (1968): 56–64. With Robert J. LeRoy. “Dissociation Energy and Long-range Potential of Diatomic Molecules from Vibrational Spacings of Higher Levels.” Journal of Chemical Physics 52 (1970): 3869–3879.

With Keith T. Gillen and Alan M. Rulis. “Molecular Beam Study of the K + I₂ Reaction: Differential Cross Section and Energy Dependence.” Journal of Chemical Physics 54 (1971): 2831–2851.

With Alan M. Rulis. “Translational Energy Dependence of Product Energy and Angular Distribution for the K + CH₃ I Reaction.” Faraday Special Discussions of the Chemical Society55 (1973): 293.

With Raphael D. Levine. Molecular Reaction Dynamics. Oxford: Clarendon Press, 1974

. ———. “Energy Disposal and Energy Consumption in Elementary Chemical Reactions: The Information Theoretic Approach.” Accounts of Chemical Research7 (1974): 393–400.

With S. Stolte and A. E. Proctor. “Translational Energy Dependence of the Branching Fraction and Cross Sections for the Decay of Collision Complexes: K + CsF, RbF.” Journal of Chemical Physics 65 (1976): 4990–5008.

As editor. Atom-Molecule Collision Theory: A Guide for theExperimentalist. New York: Plenum Press, 1979.

With Richard N. Zare. “State-to-State Reaction Dynamics.” Physics Today 33 (1980): 43.

Chemical Dynamics via Molecular Beam and Laser Techniques. London: Oxford University Press, 1982.

“Systematic of Multiphoton Ionization-Fragmentation of Polyatomic Molecules.” Journal of Physical Chemistry86 (1982): 1178–1184.

With Raphael D. Levine. Molecular Reaction Dynamics andChemical Reactivity. London: Oxford University Press, 1987.

With N. F. Scherer, L. R. Khundakar, and Ahmed H. Zewail. “Real-Time Picosecond Clocking of the Collision Complex in a Bimolecular Reaction. The Birth of OH from H + CO₂.”Journal of Chemical Physics 87 (1987): 1451–1453.

With Dudly R. Herschbach and Raphael D. Levine. “Dynamical Aspects of Stereochemistry.” Journal of Physical Chemistry91 (1987): 5365–5377.

“Selectivity in Elementary Chemical Reactions.” Presented at the Selectivity in Chemical Reactions: NATO Advanced Research Workshop, Sept. 1987, Browness-on-Windermere, U.K.

With Ahmed H. Zewail. “Real-Time Laser Femtochemistry: Viewing the Transition from Reagents to Products.” Chemical Engineering News 66 (1988): 24–43.

“Molecular Beams in Chemistry: A Subjective Account.” In Welch Conference on Chemical Research XXXII: Valency, pp. 157–197. Houston: Robert A. Welch Foundation, 1989.

With D. H. Parker. “Oriented Molecule Beams via the Electrostatic Hexapole: Preparation, Characterization and Reactive Scattering.” Annual Review of Physical Chemistry40 (1989): 561.

OTHER SOURCE

Levine, Raphael D., Ahmed H. Zewail, and Mostafa A. ElSayed, eds. Journal of Physical Chemistry(R. B. Bernstein Memorial Issue) 95 (1991): 7961–8421.

Raphael Levine

Charles Knobler