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Autobiography of David R. Yarkony: My Life as a Scientist I was born in New York City in 1949, the first of two sons of Ann and Stanley Yarkony. My mother was born on the lower east side of Manhattan. My father was born in Poland. My brother Gary, 4 years my junior, and I were raised in a middleclass jewish environment in Brooklyn, attending public elementary, junior high, and high schools. I was pretty good at high school, graduating first in my class. I started college at Cooper Union in lower Manhattan, expecting to become a chemical engineer. It was 1967, and those were tumultuous times. My decision to go to Cooper Union rather than Columbia University turned out to be a wise one as education at Columbia was interrupted by student protests. However, my decision to pursue chemical engineering was not so great, and after 2.5 years at Cooper Union, I transferred to the State University of New York at Stony Brook, as a chemistry major. I graduated Stony Brook in 1.5 years with a degree, a Bachelor of Arts (BA) in chemistry, summa cum laude. Many years later, I was told by one of my Stony Brook professors, Robert Kerber, that the BA degree I received was created especially for me owing to my reluctance to take laboratory courses. I am not exactly sure what I expected when I took my newly minted BA degree and went off to graduate school at UC Berkeley. I had not done undergraduate research, but my functional analysis professor at Stony Brook told me Berkeley was a great place, so off I went. It was September 1971.
Hopkins. In 1981 I was promoted to Associate Professor. In 1984, I was promoted to Full Professor. In 2001, I was appointed D. Mead Johnson Professor of Chemistry, a position I still proudly hold. At Hopkins, I started working on excited states of diatomic molecules and how they decay radiatively. In those days, multiconfigurational self-consistent field (MCSCF) methods were an emerging area, and I did some work in that regard, although the heavy lifting was done by others. I used MCSCF plus configuration interaction (MCSCF/CI) methods to study ground and excited states of molecules in which multiconfigurational effects were important, molecules including BeO, MgO, and CaO. I worked on diatoms for almost 2 decades until it became clear I could not make a career studying them.
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COMPUTERS When I started graduate school, scientific calculations were performed on large “mainframe” computers maintained by research laboratories and/or universities. When I received my Ph.D. in 1975, the era of the minicomputer had begun. Minicomputer systems were much smaller than the institutional computers but were reserved for usually one or two research groups. The (Bill) Miller−(Fritz) Schaefer groups at Berkeley played a key role in ushering in that era, getting their first minicomputer system in ∼1973. I like to think I played a small but significant role in that revolution since in the preceding year, I contributed greatly to exhausting the Schaefer group’s annual computing budget in a few months! After a few years at Hopkins, I was able to acquire my own minicomputer system, a PerkinElmer 3200. Could a man be more proud of a system with a 20 megabyte disk drive? I doubt it! Over the years, the PerkinElmer has been replaced by an Alliant, then by a series of IBM workstations, and finally by a dedicated cluster of coupled CPUs which we currently use. However, the mainframe, now a large state of the art massively parallel computer system, is alive and well with allocations based more on scientific merit rather than easily exhaustable dollars.
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TRAINING: UC BERKELEY AND MIT At Berkeley, there were then, and are today, many very bright students seeking research advisors. I joined Henry F. (Fritz) Schaefer’s group. Here, my mathematical training at Stony Brook came in handy since I am convinced Fritz allowed me join his group because I knew what a Banach space is. Joining his group turned out to be one of my better decisions in life since over the years Fritz has been as much a friend as a mentor. At Berkeley, I took courses in the physics, chemistry, and mathematics departments. One particular instance stands out in my mind to this day. I was in the back of the room in William H. (Bill) Miller’s (kinetics, I believe) class. On the black board, Professor Miller had just written an equation involving the derivative coupling, ⟨ΨI(r;R)|∇RΨJ(r;R)⟩r = fI,J(R). About a decade later, I would figure out how to calculate that matrix element, and that would change the direction of my career, forever. I finished the requirements for my Berkeley Ph.D. in August 1975 and went back across the country to the Massachusetts Institute of Technology (MIT) to work as a postdoctoral research associate with Robert J. (Bob) Silbey.
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SPIN−ORBIT COUPLING AND SPIN-FORBIDDEN CHEMISTRY While diatoms are small, it was an even smaller system, the Ca atom, and my colleague Paul Dagdigian that pointed me in a new direction, spin-forbidden chemistry, that is, processes where total electron spin is not conserved as a result of the spin−orbit interaction. Paul wanted to understand the mechanism for the radiative decay of the Ca(1D2) state. Although we were able to demonstrate the importance of the spin-forbidden radiative decay channel Ca(1D) → Ca(3P), I had no fully first-principles way to confirm our estimate. That deficiency started me along the path to develop methods to
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GETTING STARTED: JOHNS HOPKINS At Berkeley, Fritz Schaefer taught me how science could be done with state of the art computers. At MIT, Bob Silbey showed me what could be done with pencil, paper, and a hand calculator. In July 1977, I moved to Johns Hopkins University in Baltimore, MD hoping to combine those skills as a new Assistant Professor of Chemistry. I rose through the ranks at © 2014 American Chemical Society
Special Issue: David R. Yarkony Festschrift Published: December 26, 2014 11838
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shell molecule composed of light atoms borrows spin−orbit coupling through overlap with a (closed-shell) heavy atom. Much of the work in this period was done with group members, including graduate students Riad Manaa and Lisa Peterson, postdoctoral research associates Hinne Hettema, Gérard Parlant, and Karl Sohlberg, my collaborators Byron H. Lengsfield and James O. Jensen, and colleague Paul Dagdigian. But by now, it was well into the 1990s, and I must go back in time to describe another direction my research was taking.
treat spin-forbidden radiative decay in polyatomic molecules. This required incorporation of the spin−orbit interaction (in the Breit−Pauli approximation) into my electronic structure codes. At that time, the requisite spin-forbidden transition dipole moments were determined using an intensity borrowing (eigenstate) approach. I rewrote the eigenstate based equations as a system of linear equations. This allowed determination of the contribution from all (millions of) eigenstates rather than just a few. It enabled more accurate determination of the spinforbidden transition moments, in general, and the determination of the radiative lifetime for a state that borrows its intensity from states embedded in a continuum, in particular. Such systems could not be treated with the eigenstate method. Science has its ups and downs. I knew that when I started. But my work in spin-forbidden chemistry made that all too clear, too quckly. All of the programs needed to deal with the spin−orbit interaction had to be written from scratch, about a year’s work for me, then a young professor. The first spinforbidden radiative transition we studied was the 1Σ+0+ → 3Σ−1,0+ transition in NF. This transition is described by two transition moments which, at that time, both experiments and computation (using simple single configuration wave functions) agreed were in a ratio r = 1:7. Our initial calculation was on based on thousands of configurations, certainly much better than the previous calculation, but it gave r ≈ 7:1! The inverse of what was measured. Using an even more elaborate description, we would subsequently refine the ratio to r = 3.87:1. But clearly, I had a problem. At that point, I called the eminent laser spectroscopist, Robert Field at MIT, who I had met during my time as a postdoctoral research associate with Bob Silbey, and asked him if there was any chance the experiment was wrong and could he look over the experimental paper. When I told him one of the authors on the experimental paper was a distinguished spectroscopist, he explained to me that I should look elsewhere, that is, to my own work, for the mistake. To make a long story short, I learned enough spectroscopy to understand the experimental paper and determined that r should have been reported as 7:1 not 1:7! I wrote to the spectroscopist who acknowledged the error. Shortly thereafter, he had the spectrum remeasured and obtained r = 3.6, in excellent agreement with our subsequently recalculated result, noted above. Career saved! I stayed with spin-forbidden radiative decay for several more years, treating molecules including NCl, MgO, CH−, and NO+. The NO+ calculations were, I believe, the first calculations using the microscopic Breit−Pauli spin−orbit interaction to involve over one million configuration state functions and have an interesting history. The NO+ calculations described the spinforbidden decay process a3Σ+1 → X1Σ+0+. After our results appeared, new experiments claimed that our predicted lifetimes were incorrect. More disappointing was the fact that those measured results were in better agreement with calculations based on the presumably less accurate eigenstate expansion approach. It was not until a year or so later that more accurate experiments vindicated our results. During this period, in addition to spin-forbidden radiative decay, I worked on spin-forbidden radiationless decay attributable to spin−orbit-induced coupling of bound states with dissociative states of different spin multiplicity. Again, this work involved diatoms, BH(b3Σ−,a3Π), CH(a4Σ−), MgO(B1Σ+), NH(A3Π, a1Δ, b1Σ+, c1Π), OH+(c1Π), and OH(A2Σ+). Later, I studied another aspect of the spin−orbit interaction, the heavy atom effect, as exemplified in BAr, in which an open-
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SPIN-CONSERVING NONADIABATIC CHEMISTRY By 1980, I had an active collaboration going with Charles W. Bauschlicher, Jr., a quantum chemist, working at the Institute for Computer Applications to Science and Engineering (ICASE), in Langley, Virginia. Charlie had graduated a year after I did from both Stony Brook and Berkeley. In 1980, on one of my trips to Langley, he introduced me to Byron H. Lengsfield III, who had just completed his Ph.D in quantum chemistry. Byron quickly became a leader in MCSCF methods development. So I was quite pleased when he started a permanent position at the Ballistic Research Laboratory (BRL) in Aberdeen, MD in the group of George F. Adams. George assembled a critical mass of quantum chemists at BRL, including Paul Saxe, another Fritz Schaefer Ph.D. and an expert on GUGA CI. I would visit that group frequently in the summer months. One of their contributions was particularly germane. It was an algorithm to calculate the gradient of the energy obtained from a general MCSCF/CI wave function. At that point, remembering Bill Miller’s class, I realized that with some modifications, their algorithm could be used to calculate fI,J(R)! It was 1984, and my career path would change irrevocably. I was about to enter the realm of spin-conserving radiationless (nonadiabatic) decay, where nonadiabatic processes occurred because fI,J(R) induced transitions between Born−Oppenheimer states with the same total electron spin. Byron, Paul, and I wrote several papers describing the efficient computation of fI,J and related quantities. Using these tools, I began to address problems in nonadiabatic chemistry. Our determination of the adiabatic or Born−Oppenheimer diagonal correction in the ground state of LiH resolved a discrepancy between detailed experimental measurements and advanced computational results. The ability to determine fI,J allowed us to extend studies of excited-state decay to include spin-conserving radiationless processes and spectral perturbations due to avoided intersections. We could use the integral of the derivative coupling to determine rigorous diabatic bases in diatoms. Some of these studies combined spin−orbit and derivative coupling interactions. This work was done with group members Riad Manaa, Hinne Hettema, Rovshan Sadygov, and Seungsuk Han (initially a postdoctoral research associate and then a visiting professor) and collaborators James O. Jensen, Guy Taieb, and Jöelle Rostas. While diatomics provided a valuable laboratory for studying (combinations of) nonadiabatic effects, they are an anomaly. In diatomcs, things were simple. States of different symmetry could cross. States of the same symmetry could not. As soon as one went to triatomic molecules, things got more complicated and much more interesting! In triatomics and larger molecules, the true vector nature of the derivative couplings is revealed, and as a consequence, rigorous diabatic states do not exist. As for avoided intersections, well that was a whole new story. The issue to be addressed was where nonadiabatic transitions were most likely to occur. It was clear that transitions occurred 11839
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molecules with a odd number of electrons and a nonnegligible spin−orbit interaction. But the most enduring work to come out of Spiridoula’s time at Hopkins was our work on the location and properties of conical intersections of three states. We showed that conical intersections of three states with little or no symmetry were not rare occurrences, as one might have expected. They were found, for example, in dynamically relevant regions of nuclear coordinate space in five-member nitrogen-containing heterocycles, azolyls. Studies of the properties of conical intersections continued. Seungsuk Han and I investigated the geometric phase effect for three state conical intersections and for two state intersections with odd numbers of electrons and a nonnegligible spin−orbit coupling.
where the potential energy surfaces got close, but how close? Points where the potential energy surfaces actually crossed, conical intersections, were known to be effective in this regard because of their conical topography, but the conventional wisdom at the time was that in the absence of symmetry, which meant most of the time, such crossings were generally avoided even if allowed by the, poorly named, noncrossing rule. It was 1989, and things were about to change, especially for me.
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EXERCISE PAYS OFF In those days, I would swim a mile a day, 5 days a week in the pool at Johns Hopkins. While I would never make the Hopkins swim team, or any swim team for that matter, I considered myself a respectable swimmer. Imagine my annoyance when a woman in the next lane, Kathryn A. McClelland, KAM to her friends, would consistently swim two laps to my one! Kathryn, a Hopkins Ph.D, is now a bone marrow transplant coordinator at Johns Hopkins Hospital, and using the nom de plume Hunter McClelland, author of the novel Men of Gain. She is also my wife of 24 years! We married on 21 July 1990 and have raised three children Julian, Alexander, and Andrea in order of appearance.
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SIMULATIONS AND COUPLED POTENTIAL ENERGY SURFACES Michael Schuurman arrived in 2006 as a postdoctoral research associate from Fritz Schaefer’s group. We agreed that it was time to consider the measurable effects of conical intersections. We turned to anion photodetachment spectroscopy in which the states of the residual (neutral) molecules are coupled by conical intersections. In short time, Michael created a program to determine a photoelectron spectrum within the KDC (for Köppel, Domcke, Cederbaum) vibronic coupling model. This program was so sophisticated that it enabled us to calculate the photoelectron spectrum for pyrazolide, which had previously been declared intractable by experts in the field. One of the keys to our success was a new algorithm Michael created, based on energies, energy gradients, and fI,J, for the determination of the coupled quasidiabatic representation of the adiabatic potential energy surfaces coupled by conical intersections, needed in the KDC formalism. The combination of the vibronic coupling spectral simulation program and the surface fitting algorithm allowed us to determine photoelectron spectra for systems with on the order of 25 internal degrees of freedom. (I had stopped working on diatoms!) One study was particularly gratifying, a study of the isopropoxy radical by a graduate student, Joseph Dillon. Not only could we explain a discrepancy between a measured photoelectron spectrum and a high-resolution dispersed fluorescence spectrum but we used our g−h representation of the branching plane to provide a simple intuitive explanation of our results. The representation of bound adiabatic surfaces coupled by conical intersections is a challenging problem but not nearly as challenging as the next task we undertook and the last scientific project I will mention. The task is to represent adiabatic potential energy surfaces coupled by conical intersections in reactive systems or systems exhibiting large-amplitude motion. This project, formally an extension of our coupled bound state work, has been championed by Xiaolei Zhu, a graduate student/postdoctoral research associate in my group, beginning in 2008. It provides the ability to use state of the art electronic structure results in describing nonadiabatic chemical reactions and photodissociation, offering the possibility of a qualitative change in the accuracy of the description of such processes. It remains to be seen how far this technology can be pushed.
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UBIQUITOUS CONICAL INTERSECTIONS 1990 was also an important year scientifically. In that year, two papers appeared in the Journal of Chemical Physics, one from my group and one from the group of Klaus Ruedenberg. Each described the existence of conical intersections of states of the same symmetry. As the existence of these conical intersections did not require symmetry, their existence could not be anticipated on that basis. Those findings would change the face of nonadiabatic chemistry by changing the paradigm for radiationless decay. No longer was symmetry required for efficient radiationless decay. The geometric phase, or Longuet− Higgins or Berry phase, as it is also known, the signature property of conical intersections, together with new algorithms for locating conical intersections were used to establish the prevalence of same-symmetry conical intersections. By the end of the 20th century, that is, in less than a decade, conical intersections went from an archane theoretical concept to an established paradigm for rapid radiationless decay. I have now worked on the role of conical intersections in nonadiabatic chemistry for over 25 years. My initial work, focused on the properties of conical intersections, the geometric phase, the nonremovable part of the derivative coupling (the reason rigorously diabatic bases exist only for diatoms), the construction of quasi-diabatic states, and mechanistic issues including the locus of the seams of conical intersections and the likely products of an encounter with a conical intersection seam. We used analytic gradient techniques of the type used to determine fI,J to construct the orthogonal g and h vectors that defined Ruedenberg’s phenomenologically significant branching plane for a conical intersection. This work was originally done with Professor Mark Gordan (Iowa State University) and his students and group members Hinne Hettema, Nikita Matsunaga, Riad Manaa, and Rovshan Sadygov and continues to this day. Spiridoula Matsika joined my group as a postdoctoral research associate in 2001. She was an expert on the spin− orbit interaction attributable to her time as a graduate student with Russ Pitzer at The Ohio State University. Together, we developed algorithms to locate conical intersection seams in
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THANKS I have been a scientist for over 40 years. In that time, I have visited laboratories all over the world including Canberra, 11840
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Australia; Bologna, Italy; Heidelberg, Germany; Leiden, The Netherlands; Munich, Germany; Okazaki, Japan; Orsay, France; and Taipei, Taiwan, where I have had the opportunity to meet and work with many bright and talented people, whose hospitality has enriched my life, including Lenz Cederbaum, Michael Collins, Wolfgang Domcke, Mark van Hemert, Horst Köppel, Keiji Morokuma, Paolo Palmieri, Gérard Parlant, Jöelle Rostas, and Guy Taieb. In those 40 years, I hope I’ve made a meaningful contribution to our understanding of electronically nonadiabatic processes. I want to thank the organizations that have supported my research over these many years, including the Air Force Office of Scientific Research, the Department of Energy, Basic Energy Sciences and the National Science Foundation. But most of all, I want to thank my wife Kathryn and children, Aly, Alex, and Julian, for just being there, which has made all the difference. It has been a great ride. I hope it continues a while longer.
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