Its basic approach is to
combine a theoretical effort in the energetics and dynamics
of chemical reactions with an experimental effort in dynamics
and kinetics under chemically isolated conditions and also
under more complex conditions in flames.contributions concerning the dynamics of chemical elementary processes.
This program studies elementary chemical reactions, related
non-reactive energy transfer processes, and coupled kinetics
processes involved in combustion. Its basic approach is to
combine a theoretical effort in the energetics and dynamics
of chemical reactions with an experimental effort in dynamics
and kinetics under chemically isolated conditions and also
under more complex conditions in flames.
The theoretical effort, involving five staff members, embraces both large-scale applications of existing theoretical methods and the development of new methods that efficiently exploit advanced computer architectures. Both electronic structure techniques that determine intermolecular forces and dynamics techniques that determine molecular responses to these forces will be pursued.
Simulations of more complex combustion environments involving coupling kinetics are also being pursued. The experimental effort, involving five staff members, encompasses state-resolved measurements in flow tubes at low temperatures, thermal reaction kinetics measurements in shock tubes at high tempertatures, photoionization measurements of thresholds and state-resolved product distributions, and in situ X-ray scattering measurements of sooting flames. Reaction rates, branching ratios (between different neutral products or between ionic and neutral products),
product distributions, the effect of initial vibrational excitation on reactivity, ion-cycles for thermochemical information, and the morphology and chemistry of soot formation can all be examined. The close coupling between theory and experiment brings a unique combination of expertise to bear on the study of chemical reactivity.
This work is designed to provide a fundamental understanding of both major and trace reactions of importance in combustion.
Many of the projects of our group involve several group members and a mixture of expertise that complicates any attempt to organize our projects by authors or by categories. Nonetheless, in the sections that follow, each of our ten staff will describe their contribution to the group's achievements. To give a flavor of the group's accomplishments, I cite here several illustrative achievements:
Our group initiated and led a theoretical/experimental multi-national-laboratory collaboration that definitively showed that the heat of formation of the OH radical has been overestimated in all standard thermochemical tables by approximately 0.5 kcal/mol.
Our group has concluded by systematic experimental measurements and supportive theoretical calculations that the recombination rate of H+O2 is an order of magnitude faster in water vapor than in other common buffer gases (e.g., rare gases, oxygen, nitrogen, or methane) because of long-range polar-polar electrostatic interactions.
Our group, in collaboration with theoretical and experimental programs at other DOE laboratories, has demonstrated that the addition-elimination process CH3+O ® H2+HCO with a barrier but no saddle point
and no steepest descent reaction path can still account for ~20% of the reaction branching ratio. This is the first documentation of a reaction that can not be modeled by reaction paths.
Utilizing state-of-the-art wave packet propagation techniques, the role of excited state and non-adiabatic dynamics in the O(1D) + H2 ® OH + H reaction was investigated. Extensive calculations, including the ground and two excited electronic states predicted the ratio of the reactive cross sections for rotationally excited and cold H2. The results disagreed with earlier experiments and motivated a new molecular beam experiment that agreed quantitatively with the theoretical predictions.
Our group has developed new ways to investigate the long-time dynamics of nonlinear master equations. This has allowed us to develop rate laws to describe association kinetics and vibrational relaxation. Applications have been made to methyl recombination and the nonlinear vibrational relaxation of oxirane. In both cases, our rate laws model the process correctly while standard rate laws break down when reactant concentrations within inert buffer gases become a few percent or higher.
Our group, in collaboration with computational scientists in the Mathematics and Computer Science Division, has developed a new general way to iteratively solve matrix eigenvalue problems. The method, called SPAM, uses projection operators and a simple matrix that approximates the exact one to accelerate the Davidson iterative method (typically used in electronic structure calculations). The method is general to all eigenvalue problems where physical insight can produce a simple approximate matrix.
Our group, in collaboration with the Carbon Chemistry group within the division, has initiated a program of in situ analysis of nano-scale soot within flames using small angle X-ray scattering (SAXS) at the Advanced Photon Source. This effort, one of the first SAXS applications in the gas-phase, has discovered detailed structure in soot distributions in laminar flames and has led to development of a prototype detector to monitor transient (e.g., droplet) flames with a time resolution of ~10 µs. Such a detector will be useful in many other areas of chemistry. Our group has carried out one of the most detailed state- to-state studies ever performed of vibrational autoionization in a polyatomic molecule, in this case ammonia.
Of all the fundamental or combination normal mode excitations tried, initial excitation of the umbrella mode is found to be the most effective in promoting autoionization and the final products of the process involve a change in either electronic symmetry or rotational quantum number depending on the specific autoionizing level.
These accomplishments and others in the research summaries to follow illustrate that our group has increasingly reached out beyond group boundaries to carry out fundamental studies in chemical reactivity.
We have always had strong experimental-theoretical interactions within the group and an active collaboration with university programs. However, in the last several years we have collaborated more intimately than before with other parts of the national laboratory system. For example, our involvement with the Carbon Chemistry group within our division is expected to be a long-term collaboration driven by a mutual interest in soot chemistry and a complementary background in experimental and theoretical expertise.
Likewise, our involvement with computational scientists in other divisions is also long-term and a recognition of the fact that computational chemistry worldwide is one of the leading consumers of computer hardware resources and both a beneficiary and a source of advanced computer software. Our involvement with other national laboratories, especially the Combustion Research Facility at Sandia National Laboratory and the Environmental Molecular Science Laboratory at Pacific Northwest National Laboratory, reflects the complementary expertise that has become centered at those laboratories. The broader involvement by the group has not only furthered our combustion research program but has also won additional funding outside of Chemical Sciences.
This additional funding includes discretionary (LDRD) funding for the soot studies and Scientific Discovery through Advanced Computing (SciDAC) funding from the Mathematics, Information, and Computer Science (MICS) office in DOE.
While different funding sources do not have identical missions, we believe the additional funding we receive will only augment and accelerate the Chemical Sciences supported program in fundamental combustion research.
In the future, our group intends to continue to pursue experimental and theoretical studies into the details of chemical reactivity manifested in combustion. We feel this is the "golden age" of combustion research in which effective coupling of experiment and theory can be achieved for increasingly complex chemical reactions that are prevalent but still poorly characterized within combustion. However,
the increasing complexity of reactions we are studying and the broader collaborations the group has become involved in suggests future group interests not exclusively tied to gas-phase processes that have been our focus in the past. For example, the soot project will involve us in cluster and agglomeration kinetics that has both gas phase and gas-surface overtones.
Furthermore, the experimental and theoretical techniques we develop for soot studies may well be applicable to studies of complex systems outside of combustion, such as molecular self-assembly or chelation kinetics. Another example of a broader study of chemical reactivity our group is involved in is a new collaboration with university researchers into reaction kinetics under carbon nanotube confinement.
While all these activities are rooted in our experience and expertise in gas phase combustion research, the research itself is leading the group to a future in which broader issues of chemical reactivity can be addressed beyond the context of combustion but within the fundamental research agenda of Chemical Sciences.
I was born in San Jose, California on June 18, 1932, the first of six children of Robert and Dorothy Herschbach. My father was then a building contractor and later a rabbit breeder. His family had lived in this part of California for three generations; although our surname comes from a pair of villages in the Rhine Valley, most of his immediate ancestors were of English or Irish origin. My mother's family had moved to San Jose from Illinois when she was a young girl; most of her known ancestors were of German, Dutch, or French origin.
In my boyhood we lived in what was then a rural area of fruit orchards, only a few miles outside San Jose. For years I milked a cow, fed the pigs and chickens, and during summers picked prunes, apricots, and walnuts. From an early age I loved to read but was also very involved in outdoor activities, scouting, and sports. My interest in science was excited at age nine by an article on astronomy in National Geographic; the author was Donald Menzel of the Harvard Observatory. For the next few years, I regularly made star maps and snuck out at night to make observations from a locust tree in our back yard.
When I attended Campbell High School, I took all the science and mathematics courses offered. Chemistry I found at first puzzling and then most intriguing, thanks to John Meischke, a superb teacher. At the time, I was at least as interested in football and other sports; perhaps that presaged my later pursuit of molecular collisions. Like most of my classmates, I did not expect to attend college; none of my known relatives had graduated from a university. However, my teachers and coaches presumed I would go. Indeed, I received offers of football scholarships from some universities to which I had not even applied for admission.
I entered Stanford University in 1950 and found a new world with vastly broader intellectual horizons than I'd imagined. Although I gladly played freshman football, I had turned down an athletic scholarship in favor of an academic one. This permitted me to give up varsity football after spring practice, in reaction to a dictum by the head coach that we not take any lab courses during the season. By then the lab and library already were for me much the more exciting playground. My chief mentor at Stanford was Harold Johnston, who imbued me with his passion for chemical kinetics. Many other subjects and professors were also compelling and I took up to ten courses a term. Mathematics was especially appealing; I so admired the teaching of Harold Bacon, George Polya, Gabor Szego, and Bob Weinstock that I simply took all the courses they gave. I received the B.S. in mathematics in 1954 and the M.S. in chemistry in 1955. My Master's thesis, done under the direction of Harold Johnston, was titled: "Theoretical Pre-exponential Factors for Bimolecular Reactions." It employed the transition-state theory of Henry Eyring and Michael Polanyi and treated the proportionality factor in the most venerable formula of chemical kinetics, the Arrhenius equation.
My graduate study continued at Harvard, where again I found an exhilerating academic environment. I received the A.M. in Physics in 1956 and the Ph.D. in Chemical Physics in 1958. My Doctoral Thesis, done under E. Bright Wilson, Jr., was titled: "Internal Rotation and Microwave Spectroscopy". This presented theoretical calculations and experiments dealing with hindered internal rotation of methyl groups. The height of the hindering barrier could be accurately determined because the observed spectra were very sensitive to tunneling between equivalent potential mimima. Much that shaped my later research I learned from Bright Wilson and other faculty, especially George Kistiakowsky and Bill Klemperer, or from fellow students, especially Jerry Swalen, Victor Laurie and Larry Krisher. My thesis work also benefited from visits of several months to take spectra at the National Research Council in Ottawa and to compute Mathieu functions at Los Alamos National Laboratory. During 1957-1959, while a Junior Fellow in the Society of Fellows at Harvard, I developed plans for molecular beam studies of elementary chemical reactions.
This work was launched at the University of California at Berkeley, where I was appointed an Assistant Professor of Chemistry in 1959 and became an Associate Professor in 1961. The chief experiments dealt with reactions of alkali atoms with alkyl iodides, systems studied forty years before by Michael Polanyi. Rather simple apparatus sufficed to attain single-collision conditions and revealed that the product molecules emerged with a preferred range of recoil angle and translational energy. The possibility of resolving such features of reaction dynamics encouraged other workers pursuing kindred experiments and fostered an outburst of new theory. My early work thus interacted particularly with that of Richard Bernstein, Sheldon Datz, Ned Greene, John Polanyi, John Ross, and Peter Toennies.
This new field developed rapidly after I returned to Harvard in 1963 as Professor of Chemistry. We studied a wide range of alkali reactions and found several prototype modes of reaction dynamics which could be correlated with the electronic structure of the target molecule. Processes involving abrupt, impulsive bond exchange or formation of a persistent complex comprise the two major categories. In 1967 Yuan Lee joined our group as a postdoctoral fellow and led the construction of a "supermachine". This employed greatly augmented differential pumping, sophisticated mass spectroscopy using ion counting techniques adapted from nuclear physics, and supersonic beam sources advocated by enterprising chemical engineers, especially John Fenn and Jim Anderson. The new machine greatly extended the scope of crossed-beam experiments, taking us "beyond the alkali age". In particular, we were then able to study the same reactions elucidated by John Polanyi with his complementary method of infrared chemiluminescence. This much enhanced the interpretation of reaction dynamics in terms of electronic structure.