Special Issue Preface pubs.acs.org/JPCA
The Ties That Bind: An Autobiographical Sketch of Peter B. Armentrout
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along the street afterward with Jack asking me pointed questions about my research. I later was accepted to graduate school at Caltech, where I had a memorable visit. He and his group took me out to dinner, where we had sushi (a first for this solidly Midwestern boy), followed by a trip to the Loch Ness Monster Pub. Needless to say, this visit cemented my desire to attend Caltech, where the intimacy of the campus, faculty, and student body was a real attraction. I received a B.S. degree with highest honors in 1975 from CWRU. Jack offered me a summer position doing research, which was really tempting, but I had other plans. I convinced my girlfriend, Mary Ann White, who was also going to attend graduate school in chemistry at Caltech, to bicycle across the country with me. We carried all our own gear, bought by mail order from REI (well before it had expanded across the country from Seattle). I was overly ambitious in my preparations, carefully cutting the maps of our route up to the bare minimum in order to save weight. 2200 miles and two months later, we arrived in Pasadena. Bicycling is a great way to see the country and you get to meet lots of people, I think partly because you are completely unthreatening and rather vulnerable . We enjoyed the trip so much we also did a reprise once we finished our Ph.D.’s, traveling in the Pacific Northwest through Glacier National Park up to Banff and Jasper in Canada. Although I was attracted to the research being done by Prof. Peter Dervan, my interest in ion chemistry prevailed, and I joined Prof. Jack Beauchamp’s group my first year at Caltech. (Jack’s charisma helped here too.) Although I did a little ICR work, Jack had me spending most of my time working with Ron Hodges on an instrument originally designed to examine ion chemistry in a small cell at variable pressures, with analysis provided by a quadrupole mass filter. In that apparatus, alkali cations were formed by surface ionization with the alkali put on the filament in the form of a small glass bead. Jack now wanted to extend this approach to other metals, having higher ionization energies, for which the glass bead approach was no longer viable. Hence, I spent much of my first year developing a surface ionization source capable of higher filament temperatures that used metal salts held in a reservoir. My central project involved examining the chemistry of uranium, in association with trying to develop new tools for isotope separation for the Department of Energy. In contrast to the previous work performed using this instrument, we also began to examine the kinetic energy dependence of the ion− molecule reaction chemistry. Although there were some literature studies of this phenomenon (for example, by Chantry, Tiernan, Mahan, Koski, and others), an understanding of the kinetic energy dependence of chemical reactions was not particularly well developed. So my first paper on this subject (“Endothermic Reactions of Uranium Ions with N2, D2 and
was born in Dayton, Ohio on March 13, 1953, a Friday. So much for superstitions. My dad had never finished high school and was a manager at Dimco Gray Corporation, who makes plastic knobs, timers, and shuffleboard equipment. My mom, who had attended two years of college, was mostly a stay-at home mom although she also did interior decorating on the side. I had a sister, Judy who was two years younger than me. I grew up in the community of Oakwood, which my parents chose because of its excellent schools. I rapidly gravitated toward math and science. I had decided fairly early that a career in chemistry was a good choice, shortly after abandoning the possibility of being a pony express rider, a decision that was fostered by having excellent chemistry and physics teachers at Oakwood High School. I enjoyed a well-rounded high school experience by participating in theater and debate. I fondly remember having a minor role in the play “Mame” and, in my senior year, played the judge in “The Crucible” and the romantic lead in the musical “Once upon a Mattress.” For a science geek, the opportunity to break out of my high school type-casting was very refreshing. When I went to college in 1971, I attended Case Western Reserve University (CWRU) in Cleveland Ohio and originally double majored in mathematics and chemistry, but abandoned the math degree after a couple of years. In my junior year, I approached Prof. Fred Urbach about doing undergraduate research. He accepted me into his group, but the only thing I remember doing was washing glassware. Fortunately, the next year, Prof. Rob Dunbar took me into his group, where I got to do some real research. I took some photodissociation spectra on some organic ions in his ICR, which netted me an acknowledgment in a paper. He then had me resurrect an old time-of-flight mass spectrometer to do photodissociation. This was a real beast, with mercury diffusion pumps, no less, but the good news was that you could tune the mass spectrometer up on the Hg+ peaks. Eventually, I set about to measure photodissociation of p-iodotoluene cation using a flashlamp-pumped dye-laser, dutifully recording the results with an oscilloscope. The paper, which I did help write this time, was submitted to the International Journal of Mass Spectrometry and Ion Physics. I recall that the paper had a difficult time in the review process as there was not a lot of new science, but Prof. Dunbar successfully argued its merits making “Laser Photodissociation of Ions in a Bendix Time-of-Flight Mass Spectrometer” my first publication (1977). Among the significant events during my early research days was our group meetings. Dunbar had decided that it would be valuable to make our way through “Unimolecular Reactions” by Robinson and Holbrook,1 which had just been published. I remember the excitement when, after substantial grappling, we finally understood a key concept. This year also marked Rob’s granting of tenure, which was celebrated by a group dinner at a nice restaurant. I was pleased to be included, but did not have much appreciation for what getting tenure meant at the time. In my senior year, Prof. Dunbar invited his colleague and friend, Prof. Jack Beauchamp, another young professor doing ion chemistry at Caltech, for a seminar at CWRU. I recall walking © 2013 American Chemical Society
Special Issue: Peter B. Armentrout Festschrift Published: January 10, 2013 970
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CD4”) contains rudimentary ramblings about the forms this dependence might take. (Notably, all the data taken for these early papers was acquired by hand, writing down the intensities of the reactant and product ions after manually changing the energy.) Although the surface ionization source was relatively clean (producing mainly atomic metal ions), for some systems of interest, it became clear that we needed to mass select before our reaction chamber. Transition metal cations were the focus of these studies, largely inspired by the seminal work of Doug Ridge on their reactions with hydrocarbons. Jack scrounged a magnetic sector that was being thrown away by a colleague in another department, and I coupled it with the reaction cell/quadrupole instrument, creating our ion-beam tandem mass spectrometer. Data acquisition was improved. (Jack and I picked up a second-hand CAT, computer of average transients, an early multi(16 or 32)channel scaler that allowed acquisition and averaging of the entire mass spectrum at a single kinetic energy!) I worked hard on better understanding the form of the kinetic energy dependence of these reactions, developing the foundations of the approaches we still use today. We also measured a lot of metal ligand bond energies, really the first time such an approach had been applied systematically. At the time, theory was not available to “check” things, but when it was (work driven largely by Jack collaborating with Prof. Bill Goddard), our values turned out to be robust. In many respects, I believe we were successful in this endeavor because I was unfamiliar with the literature. Substantive studies had shown that the average energy available to the system in a bimolecular collision does not equal the center-of-mass collision energy but is much lower. Given these observations, one could conclude that on average, reaction chemistry is displaced to higher energies, meaning you should not be able to measure a thermodynamic threshold reliably. Thankfully, I did not know this and blithely forged ahead measuring thresholds. In retrospect, this works just fine because at threshold, one measures those unique events in which all the energy available to the system is used to overcome the endothermicity of the reaction (albeit with zero probability). As one increases the energy, the distribution of energies that can induce chemistry above the threshold increases and so does the cross section. By modeling this increase and including the distributions of energies, accurate thermodynamic threshold information is accessible, as we have now demonstrated for a wide variety of systems. Among the bumps in my road to a Ph.D., the story of my candidacy exam is notable (and apparently legendary at Caltech now). At that time, a second-year student had to defend their research to date as well as three original proposals. My committee focused on a proposal in which I thought wavelength specific photoionization of UF6 could be used for isotope separation. My committee thought I was overlooking hyperfine splittings, which I definitely had because I had never heard of them before. So we proceeded to develop the background needed for this from scratch by starting with the Schrö dinger equation. For reasons that should be apparent to most people who have gone through such a stressful oral exam in front of five people that you know are much smarter than you are, I drew a complete blank. Therefore, in my infinite wisdom, I decided to buy some more time by asking “Relativistic or not?” This was not a very bright move on my part as it granted me very little time and I had no idea whatsoever how to write down the relativistic form of the Schrödinger equation, even if I had been able to recall the nonrelativistic form. Their bemused response was “Whichever” and then they thankfully dragged me through reconstructing the nonrelativistic form of the equation, followed
by the Born−Oppenheimer approximation and its foibles. Eventually I was required to write up a paper on whether my suggested approach was viable or not. Upon doing so, I proved it would NOT work in less than a page, which I’m not sure was exactly what the committee expected of me, but was accepted anyway. Upon defending my thesis in 1979, I scheduled the public seminar and defense, giving the departmental secretary the title “Getting It Up in the Gas-Phase”. She thought this was a little risqué and asked me to revise. We ended up going with “Getting Up in the Gas-Phase” as a suitable alternative. Somewhere during this time, Jack took me to lunch to discuss my future. I recall him telling me that he thought I could probably get an academic job somewhere if I wanted but probably not at the very best institutions, such as Berkeley, Stanford, MIT, etc. While deciding what I wanted to do with my future, I interviewed fairly widely, actually getting job offers at places like Bell Laboratories in their applied division. I also had the opportunity to postdoc with Dick Bernstein at Columbia, where I would have worked on neutral molecular beams. Instead, I chose to became a postdoctoral member of staff at Bell Laboratories in Murray Hill, New Jersey, working on fundamental science with Dr. Robert S. Freund, joining his group in January of 1980. This move was a calculated one on my part as Freund was looking at the electron impact ionization of metastable atoms and molecules, which were created by neutralization of mass selected ion beams. My thought was to find out whether this approach could be generalized so that hyperthermal beams of neutrals could be routinely produced and their chemistry examined, i.e., applying the same approach I had developed for ions to neutrals. I rapidly learned that although a sufficient signal of neutrals could be generated for analysis by ionization, the loss of signal resulting from a reaction of those neutrals made the overall experiment impossible except for specialized cases. Nevertheless, I did manage to measure the first (and I believe still the only) electron impact ionization cross section of a molecular metastable, N2 (A Σu+). Furthermore, my experience at Bell convinced me that I wanted to stay in academia and that my thirst for learning more thermochemistry had a place in the research world. I applied for positions in both physical and inorganic chemistry across the country, securing interviews at several places including Princeton (inorganic), Oregon State (physical), Rochester (physical), Illinois (both inorganic and physical, on subsequent days), and Berkeley (physical). I remember that Jim Lisy, Jim Weisshaar, and I kept following each other around the country interviewing for many of the same positions. I was fortunate to land the position at the University of California at Berkeley, starting in July of 1981. For what I wanted to do, Berkeley was an ideal place, for several reasons. First, the students are excellent. I was fortunate to have four join my group that first year (and indeed had to turn away others). Throughout my career, these have been joined by many other super students, who have been instrumental in allowing our research to progress as it has. Second, my start up package included a lot of equipment from Prof. Bruce Mahan’s laboratory, which was not only directly useful but also held detailed mechanical drawings of the equipment, which importantly, held an understanding of the right way to build a complex apparatus. Among this equipment was a magnet, designed and hand-wound by Yuan T. Lee when he was a graduate student in Mahan’s laboratory. (We still use that magnet to this day.) Third, the machine shop was excellent, and my first six months at Berkeley were spent drawing and then working with the machinists to implement my concepts. 971
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the students who had worked so hard and come so far. I made a point of going to each of my physical chemistry colleagues afterward and getting their insights into my failures. The most interesting conversation took place with George Pimentel, one of the fathers of that incredibly strong department. He invited me back into his office, an inner office located in the basement, and as such, had no windows. After we sat, I told him the purpose of my visit. He looked at me and then abruptly switched off the lights. It was pitch dark and I was wondering what I had gotten myself into, when he said “There’s the problem. You don’t glow in the dark.” George was renowned as an educator, and his “explanation” helped provide me with the fortitude to continue my work. It is an interesting moment in your career when you are publicly told you are not good enough. I applied for jobs at several institutions across the country, receiving interviews at quite a few places (thanks again), which helped assuage my tattered self-esteem. I also was interviewed at IBM for a position in the group of Joe Jasinski, where I would have had the opportunity to continue our cluster work unfettered by teaching responsibilities. Ultimately, I still relished those responsibilities and the satisfaction that comes with teaching, both in the classroom and the laboratory. Jack Simons, chair of the Department of Chemistry at the University of Utah at the time, had apparently targeted me along with several other young physical chemists as potential hires. He had presciently invited me to give a series of lectures at Utah and spend a week or so at Utah, and had done so months before the tenure decision. When I was invited back for an interview, I already knew this was a place where there were kindred research spirits, Jack, Michael Morse, Bill Breckenridge, and my friend from Caltech and another Beauchamp alum, Chuck Wight. It was an easy decision to make, although at the time, the offer was without tenure. I felt confident that this would take care of itself soon enough and indeed, I was tenured there a year later. I was also pleased to learn that the students at Utah were generally just as good as those at Berkeley, and I was able to attract outstanding postdocs. At Utah, my students and I developed statistical methods for analyzing the thresholds of collision-induced dissociation data, a tremendously important advance in handling increasingly large systems. We had an ideal system to examine these effects, the bond energies of Li+ bound to a series of alcohols, for which relative values were available from the work of Taft and coworkers. Through many series of adjustments, we jointly worked out how to handle such data properly. We still use these methods to date, and they have only grown more powerful as the years pass. We also performed dynamics experiments that demonstrated that the threshold model I had chosen to use many years ago was robust and could also model the kinetic energy deposition in collision-induced dissociation reactions. We continued to expand our repertoire of ion sources to include flow tube and electrospray ionization and, most recently, ion mobility, in all cases being particularly cognizant of our need to know the internal energy content of the ions formed. I continue to find immense pleasure in doing the “same thing” I did as a graduate student, but gradually expanding the chemical territory involved. Indeed, nature holds a near infinite variety of chemical interactions for which quantitative information provides a powerful key to using them in productive ways. Being a small part of that enterprise is both gratifying and humbling. I’ve been fortunate to have really outstanding students work with me through the years. (Whenever one of them would suggest that they worked for me, I was quick to correct them.)
Fourth, during my graduate career, I became aware of the 1977 paper by Teloy and co-workers in which exquisitely precise energy-dependent cross sections for the reaction of N+ with CO were determined. This utilized a radiofrequency “ion beam guide” developed earlier by Teloy and Gerlich. At the time, I told myself that if I ever got the chance to build my own ion beam instrument, it should definitely incorporate such an ion guide. Remarkably, when I arrived at Berkeley some years later, Gerlich had just finished postdocing with Prof. Y. T. Lee, where he helped build a photoionization ion beam guide instrument. He and graduate student Scott Anderson had used the instrument to examine several interesting ion−molecule systems. Although Dieter was gone by the time I arrived, my conversations with Scott, who was very generous with information, enabled me to include an ion guide in our tandem mass spectrometer (originally driven by a ham radio transmitter). This was actually the first guided ion beam tandem mass spectrometer, as the previous instruments built by Gerlich with Teloy and Lee did not incorporate a mass selective device for the reactants. As Jim Farrar kindly noted in 1988 in a chapter reviewing ion-beam methods:2 “The addition of mass selection of the primary ion beam by Ervin and Armentrout... has been a key extension of the guided-beam method, opening up the possibility of studying a much wider range of chemical phenomena than had been possible heretofore.” In one of our earliest studies using this new machine, we decided to examine the Ar+ + H2 reaction, as this reaction had been studied by ion beam methods more than any other, as well as having thermal rate constant and hyperthermal drift tube data available. Originally, the idea was to calibrate our instrument using these previous results to enable us to measure absolute cross sections. We soon found that the guided ion beam approach allowed us to study this reaction over a much wider energy range (spanning the gap between the drift tube and the lowest energy ion beam results, as well as extending to higher energies) than had been possible previously. Further, our results were more precise than previous results such that a straightforward calculation of the reaction cell length (taking into account end effects) provided the most accurate absolute cross sections available. When these agreed nicely with the thermal and drift tube rate constants as well as collision theory, the ability of the method was cemented. This system remains a prototype for the determination of absolute cross sections to this day. Somewhere during this hectic time, Mary Ann agreed to marry me. We have been blessed with three amazing children. This past year, Matt obtained his Ph.D. in geology from the University of California, Los Angeles (UCLA), where he focused on spectroscopic studies of metals at very high pressures using diamond anvil cells. Patty received her Doctorate of Physical Therapy from the University of Southern California (USC) and is obtaining a specialization in neurological cases. Erin was awarded her BA in biology from Kenyon college and has started Ph.D. studies in molecular and microbiology at my alma mater, CWRU. Meanwhile, back at Berkeley, we continued to refine the apparatus and our methods of data analysis, and extended the applications to the examination of metal clusters, the effects on electronic excitation, and an ever increasing litany of thermodynamic information. It seemed like things were going well. I had good funding (NSF and Air Force grants) and a good number of publications (over 30 from my time at Berkeley). However, when my tenure clock struck, my career hit an interesting glitch. Seems not everyone was impressed with the array of information we were acquiring, and I was denied tenure. I was disappointed and felt that the decision was an injustice to 972
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Listed on the following pages, they have helped me grow and learn in ways that have always kept what we do interesting and fun. I would have liked to have named them all and recounted their individual contributions, but was forced to leave some room for the papers that follow in this issue. I’m proud to have been associated with all of the students that have passed through my laboratory, continue to be pleased to hear of their many accomplishments, and delighted that many were able to contribute to this issue. I am also lucky to have been able to collaborate with many worldclass scientists, also named on the following pages. In particular, I have enjoyed long-term collaborations with the likes of Mike Bowers, Helmut Schwarz (who I first learned of when he dropped me a postcard reminding me that there was no “t” in his last name), Detlef Schröder, Chuck Wight, Michael Morse, Jos Oomens, Kent Ervin, and Mary Rodgers. I should also thank Mary for forcing me to take a look at biologically related systems when she joined my group as well as including me in the PIRE grant supporting several groups’ IRMPD studies, both areas of much satisfaction over the past decade. In thinking about what to name this short diatribe, “the ties that bind” seemed appropriate as it can refer to chemical bonds, which I have made a career breaking and investigating quantitatively, and to the many people that have made that career possible. This simple phrase evokes many thoughts about the common interests and relationships we have with family, friends, and colleagues. In seeking the origin of this powerful notion, I came across one possible and ancient source, the philosopher Plato, who reportedly stated: “If the study of all these sciences which we have enumerated, should ever bring us to their mutual association and relationship, and teach us the nature of the ties which bind them together, I believe that the diligent treatment of them will forward the objects which we have in view, and that the labor, which otherwise would be fruitless, will be well bestowed.” I am very grateful to have had so much help in all my labors and hope that they continue to be well bestowed for all of us. Thanks to those of you who have contributed to this very special issue and to Mary and Kent for their hard work in assembling it.
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Peter B. Armentrout REFERENCES
(1) Robinson, P. J.; Holbrook, K.A. Unimolecular Reactions; WileyInterscience: London, 1972. (2) Farrar, J. M. Ion-Beam Methods. In Techniques for the Study of IonMolecule Reactions; Farrar, J. M., Saunders, W. H., Eds.; Wiley: New York, 1988.
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