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HOMER DUPRE HAGSTRUM March ~ I, 95-September 7, 1994 BY PHILIP W. ANDERSON AND T H E O D O RE H . GE BAL L E HOMER HAGSTRUM WAS BORN in St. Paul, Minnesota. His father, Andrew, hac! emigrates! there from VarmiancI, Swollen, as a 22-year-oIcl with his oIcler brother, Nels, in ISS9. His mother, Saclie Gertrude Fryckberg, was born in St. Paul in ISS3, the youngest daughter of a family that hac! also migrated from Swollen. Homer was the second of four boys who reached maturity. The home environment was built upon strict Swedish Covenant practices, with a strong emphasis on education. Drinking, dancing, card playing, and movies were considered sinful. Homer continues! to live at home throughout his graclu- ate years at the University of Minnesota, en cl clicl not see a movie until he was 25 years oIcl en cl really to go forth into the woricI. His going to see "Captains Courageous" actually causecl his mother to break clown en cl weep. She hacl gone from high school to work when her father cliecl en cl was cleterminec! that her sons shouic! obtain as much education as they couIcl absorb. His father, with only an elementary school experience in Sweden, became the owner with his brother of a successful men's clothing store, Hagstrum Broth- ers, in St. Paul. The oIclest son, lean, although groomed to become a minister, later became a clistinguishecl professor of English at Northwestern University in Evanston, Illinois, 47

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48 B I O G RA P H I C A L EMOIRS en c! sometime heat! of the National Endowment for the Humanities. The youngest two brothers, Vincent en cl Paul, became executives in the mercantile worIcI. Homer en cl lean hac! the unusual distinction of being awarclec! honorary cle- grees together from their alma mater, the University of Min- nesota, cluring the commencement clay of 1986. Homer roller-skatec! to a local elementary school, where he attenclecl an alpha class that covered two years' work in one year. In middle school he skipped half of his eighth- gracle class. Homer later felt that being much younger than his classmates was a distinct social handicap. At home he showocl a strong mechanical aptitude, with a workshop en cl darkroom in the basement en c! a crystal radio set in the attic. The high school Homer attenclecI, Minnehaha AcacI- emy, was a private religious school in Minneapolis. He hacl begun questioning his famiTy's religious doctrine early on, en cl in high school became engaged in science. He was fortunate to have an excellent science teacher, Henry Schoultz, who profoundly influencer! his life. Even as a fresh- man Homer stayed after school and worked in Henry's labo- ratory. In his senior year Homer and Henry built a 6-inch re- flecting telescope, which remained at the school for many years. At home Homer built an "observatory," a portable wooden structure, to perch on the apex of the roof of his house. Homer sat up observing with star charts en cl flash- light for years afterwards. A photograph shows Homer sit- ting on the steep roof with one of his telescopes in hand. His early interest in science, astronomy, en cl mathematics, en cl in working with his hands, never climinishecl en cl gave him great pleasure for the rest of his life. In later years he arranged vacations so he couIcl observe solar eclipses even when they were in faraway places, such as Africa where he

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HOMER DUPRE HAGSTRUM 49 camper! in the desert with his son, Jonathan en c! South America. Homer entered the University of Minnesota as an elec- trical engineering student, obtaining his B.S.E.E. in 1935 summa cum laucle en cl also his B.A. in 1936. While at the university he clevelopecl a lifelong love of classical music en c! clelightec! later on in relating how he hac! learner! about music while "ushering uncler Ormancly en cl Metropolis." Homer founcl physics to be his natural home en cl went on to graduate school, completing his M.S. in 1939 en c! his Ph.D. in physics en cl mathematics in 1940. He became the last graduate student of John Tate, who is well known, in aciclition to his own scientific achievements, for being the longtime editor of Physical Review. Tate was the second major influence in Homer's scientific life. His first two pa- pers, publisher! with Tate in 1941, were concerned! with the ionization en cl dissociation of molecules by electron im- pact, and with the thermal activation of the oxygen mol- ecuTe. Homer left Minnesota in 1940 to join Bell Telephone Laboratories where he remained for 45 years, his entire professional career. Bell Labs was then at its West Street location in downtown Manhattan, en cl Homer found an apart- ment nearby. He hacl time after work to go out with friends, discovering opera, ballet, ballroom ciancing which he took up with enthusiasm en cl skiing. It was at a ski resort on the slopes of the Berkshires in Massachusetts that he met Bonnie Cairns from Woocistock, Illinois, who was interested! in art en cl sculpture. In contrast with the 25 years it took him to leave his first home, it took only six months before he en c! Bonnie were marries! in 1948. On a trip to Europe in 1955 Homer found a decrepit armilIary sphere on the floor of an antique shop in Florence, Italy. Bonnie pitched in her half of the travel funcis to produce the 60 clollars

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50 B I O G RA P H I C A L EMOIRS neeclec! to buy it. Homer undertook the stucly of armillaries upon returning home en cl repaired en cl reconstructed the instrument, which ciatecl from the sixteenth century. It be- came one of his en c! the famiTy's most prizes! possessions. After the war Homer en cl Bonnie movecl to Summit, New Jersey, near Murray Hill, where the research effort of Bell Labs was relocatecI. They hac! two chiTciren, Melissa en cl Jonathan, en cl raisecl them in a comfortable suburban setting. The chilciren completecl graduate school, each ob- taining a Ph.D.: Jonathan's in geology and Melissa's in an- thropology. A large number of physicists en cl other scientists hacl come from all over the woric! to join Bell Labs. Because of far-flung origins en cl the absence of local family en cl oIcl friends, strong new ties were cemented. The Hagstrums' close friencis incluclec! Joyce en c! Phil Anderson, John en c! Maggie Gait, Ted and Sissy Geballe, Bruce and Joan Hannay, Davicl en cl June Thomas, Peter en cl Cathy Wolff, en cl of course, many others. The group met throughout the year, celebrating major holidays, going on outings with children, and on forays to Manhattan for symphonies and plays off en c! on Broadway. After a few cirinks Homer lover! to recite German poetry and sing Swedish hymns. He enjoyed a good winter hike in the Great Swamp, en cl ice-skating with his en c! other chiTciren. Later in life he especially lover! hiking in high alpine terrains with beautiful nighttime skies. He became a pillar of the Unitarian Church, whose minister lake Trapp was the father of Bernc! Matthias's wife, Joan. Homer was clevotecl to his family. He actively encour- aged Bonnie to clevelop her own talent as a sculptor en cl to travel to northern Italy to study en c! work in the local marble. Bonnie's work in stone is highly regarded. Some was shown in an exhibition at the National Academy of Sciences in the spring of 1991. In contrast to the Bible camp where Homer

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HOMER DUPRE HAGSTRUM 51 spent his youthful summers, his chiTciren went to camps with an outdoor orientation. Upon returning one summer from a camp in New Mexico, Jonathan brought home a wiTc! bull snake for a pet. Initially the snake escapee! regu- larly from its terrarium en cl roamed the house at will. Homer, in particular, was fascinated by the snake's mocle of loco- motion en c! wouic! bring it out to show guests, usually to their great dismay. Jonathan later flew the snake back to its natural habitat. Homer's professional life was spent at only two institu- tions the University of Minnesota, where he was eclucatecI, en cl Bell Labs, where he clicl his pioneering research. His thesis was a characteristically careful en c! definitive investi- gation of the ionization en cl dissociation of molecules by electron impact en cl on the thermal activation of the oxy- gen molecule. His first two publications resulting from this work were publishecl (naturally in Physical Review) with Tate in 1941. Homer went straight to Bell Labs in 1940 en cl joiner! the physical electronics group uncler Jim Fisk. That group was responsible for cleveloping and, at first, manu- facturing the "strapped" microwave magnetron that became the core element in the Allies' racier superiority cluring the war. One of the first clozen the English macle was cleliverecl to the United States by the famous Tizard mission of Sep- tember 1940, en c! within a month Bell Labs hac! uncler- taken the responsibility of producing them. In the whole battle of the black boxes this en cl the early warning racier were most responsible for turning back the Germans in the crucial early battles on which Englancl's survival clepenclecl- in the case of the magnetron the Battle of the Atlantic of 1941. Fisk, Hagstrum, en c! Paul Hartman clescribec! the work in a postwar paper in the Be]] System Technical To urn e] (1946~. Others who were involved include J. R. Pierce, J. C. STater, l. P. Moinar, en c! A. H. White. In later years Homer

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52 B I O G RA P H I C A L EMOIRS enjoyed! telling stories about his trip to Englanc! with John Pierce cluring the war in connection with this work en cl about the games with "fly-powerecl airplanes" the group in- ventec! to relieve the pressure of their work. He remained! close friends with Acl White en cl Julius MoInar. In 1946, with wartime priorities no longer dictating re- search, Homer returnee! to the stucly of dissociation by elec- tron impact measuring the dissociation energies of impor- tant molecules such as nitrogen, oxygen, carbon clioxicle, en c! nitric oxide (1951~. The success of the experiments required ever improving the acivancecl vacuum techniques. Homer en cl H. W. Weinhart publishecl a calibratecl leak macle from a porcelain roe! (1950), but in fact his real con- tribution was in setting, en cl then breaking again en cl again in the course of the years, records for vacuum pressure en cl other measures of cleanliness ~ ~ 976, ~ ~ . In the early 1950s Homer turned his attention to the interaction of ions with metal surfaces (1960~. His first pa- per was on electron ejection from Mo by He+, He++, en c! Her++ (1956~. Homer recognized that this process could be turned into a new kind of spectroscopy: ionization neutral- ization spectroscopy (INS). This requires! new instrumenta- tion, new experimental protocols, en cl new theory, all of which Homer took on en cl succeeclecl in arriving at work- able solutions. This lee! to his first paper, in 1953, on the instrumentation en cl experimental procedure. The theory of the neutralization process at the solicl surface turns out to be a complicates! two-electron quantum problem. As the slow ion approaches the surface some of the large neutral- ization energy (i.e., the negative of the ionization energy of the atom) goes to emitting an electron from the surface while the second electron falls into the ion. The spectro- scopic information is contained in the kinetic energy distri- bution of the electron emitter! from the surface. Homer

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HOMER DUPRE HAGSTRUM 53 consiclerec! two relater! paths (1954, 1961), the first being a direct Auger following the theory of S. S. Shekhter V Exp. Theor. Phys. ~U.S.S.R.] 7~19371:750) en cl the second being more complex, involving resonance neutralization follower! by Auger cle-excitation (H. S. W. Massey, Proc. Camb. Philos. Soc. 26~19301:386~. Many of Homer's wartime associates who were still at Bell hacl been tapped for higher administration, mostly on the technical sicle. During the immediate postwar years he continues! to work in the physical electronics group, which clicl research mostly relatecl to vacuum tubes. But this group was also minecl for administrative talent, in view of the ex- pectation that heater! cathodes en c! vacuum technology wouic! soon be superseclecl by solicI-state crevices. From this group came, for instance, MoInar, K. G. McKay, John Hornbeck, en c! its heacI, Aciclison White, as higher-level managers, sev- eral eventually went on to have very clistinguishecl careers in management. Homer, along with Conyers Herring, represented! to the next generation such as ourselves the possibility of a sec- oncl fruitful track within the expanding Bell Labs, a career staying within the cutting ecige of funciamental science without succumbing either to the blanclishments of academia or the technical management route. But in 1954 the Bell acI- ministrators recognizes! that surface physics was becoming an ever more important frontier in science en cl in technol- ogy. New en cl improved methods for characterizing surfaces were neeclec! in semiconductor physics en c! technology, as well as in heterogeneous catalysis en cl biology. One of the first interclisciplinary research departments with expertise in physics, chemistry, en c! metallurgy but focuses! on a single subfielcI, surface physics was organized with Homer as department heacI. Siciney MilIman, his perceptive labora- tory director, undertook to relieve Homer of the most bur-

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54 B I O G RA P H I C A L EMOIRS densome administrative duties in order to free his research time. Within a few years there were other interdisciplinary departments such as biophysics. Homer recognized that INS had potentially more sur- face sensitivity than the more widely used sensitive soft X- ray scattering (SXS) and photoemission spectroscopies (PES). But more accurate INS data were required. It took Homer five years of sustained research to design and construct a new apparatus that pioneered by incorporating a low-en- ergy electron-diffraction insert for being able to investigate the surface symmetry and reconstruction. In the course of this work he introduced the concept of a turret within which the sample could be maneuvered to allow a number of dif- ferent probes or coatings to be applied to the same surface, a methodology that was widely applied. The apparatus for INS and the procedures for the data analysis are described by Homer in a comprehensive article ~ ~ 966, ~ ~ . He was awarded the Medard W. Welch Award by the American Vacuum Society in ~ 974. In ~ 976 he was elected to the National Academy of Sciences and was awarded the Davisson- Germer Prize by the American Physical Society. With a minimum number of assumptions the relative probability that an electron at a given band energy in the solid will be involved in the neutralization process the tran- sition probability is calculated. It depends upon the ini- tial and final state densities and upon the transition matrix elements and final state interactions much as in PES. It depends upon wave functions outside the surface and is thus more surface sensitive than the other spectroscopies. It is also amenable to studies of surfaces containing foreign atoms. In a series of investigations with Y. Takeishi (1965), G. E. Becker ~966, ~973), E. G. McRae ~976), and T. Sakurai (1976, 1979) that continued until his last working day at Bell, Homer kept taking data. He and his collabora-

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HOMER DUPRE HAGSTRUM 55 tors stucliec! pure well-characterizec! surfaces of the metals Fe, Co. Ni, Ag, Cu. W. and Mo and the semiconductors Si, Ge, en cl GaAs. The studies of these surfaces with acisorbecl oxide, sulficles en c! other chaTcogenicles, en c! hyciricles were macle, as well as with alkali metals. Among many other re- sults of interest was the observation of band narrowing at the surface for Cu en c! Ni, the first observation of surface resonances in acisorbecl atoms en cl of the kinetics of acI- sorption. At the time of his retirement he was making the first measurements of magnetic resonant states in acisorbec! atoms, though this work was not completecI. Homer was that rare type of scientist who enjoyocl work- ing on all aspects of a carefully thought out research pro- gram, from his initial iclea to the design en cl construction of the neeclecl apparatus, to the taking of ciata en cl then mollifying the theory when necessary to obtain a cletailec! unclerstancling. In the goIclen age of research at Bell Labs at that time it was possible for Homer to take five years J 1 from his research to built! the apparatus. Even though he succeeclecl in establishing INS as a valuccl spectroscopy, it has not become a standard laboratory practice. The con- fluence of new clevelopments renclerec! INS of less value than Homer hacl envisioned. In particular, a spectacular array of new scanning tunneling probes has come into be- ing following Hans Rohrer en c! Gerc! Binnig's revolutionary demonstration of scanning tunneling microscopy in 1984. There the electron tunneling is by means of wave functions that extent! from the surface much as in INS. In all other respects the tunneling tip, which can be accurately con- trollecl in all three dimensions, is much superior to the moving ion. INS has been macle obsolete after only two short clecacles of existence. If there is a lesson to be Earned, it is that science moves ahead on many fronts en cl the most

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56 BIOGRAPHICAL MEMOIRS acimirable achievements are not necessarily the most en- during. Homer kept taking data right up until the day he re- tirec! from Bell. He planner! to be engages! in analysis in the years ahead. Unfortunately, about that time Homer suf- ferecl a series of small strokes that ciamagecl his short-term memory en c! impairec! his ability to concentrate. Nothing that has transpired detracts from Homer's achievements. While INS will not be remembered as a milestone of twenti- eth-century physics, Homer Hagstrum will be rememberer! as a pioneer who created many of the icleas en cl techniques of moclern surface physics.

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HOMER DUPRE HAGSTRUM SELECTED BIBLIOGRAPHY 1941 57 With J. T. Tate. Ionization and dissociation of diatomic molecules by electron impact. Phys. Rev. 59:354. With J. T. Tate. On the thermal activation of the oxygen molecule. Phys. Rev. 59:509. 1946 With J. B. Fisk and P. L. Hartman. The magnetron as a generator of centimeter waves. Bell Syst. Tech. I. 25:167. 1950 With H. W. Weinhart. A new porcelain rod leak. Rev. Sci. Instrum. 21:394. 1951 Ionization by electron impact in CO, N2, NO, and O2. Rev. Mod. Phys. 23:185. 1953 Instrumentation and experimental procedure for studies of elec- tron ejection by ions and ionization by electron impact. Rev. Sci. Instrum. 23:1122. 1954 Theory of Auger ejection of electrons from metals by ions. Phys. Rev. 96:336. 1956 Electron ejection from metals by ions. Bell Labs Rec. 34~2~:63. 1960 With C. D'Amico. Production and demonstration of atomically clean metal surfaces. 7. Appl. Phys. 31:715. 1961 Theory of Auger neutralization of ions at the surface of a diamond- type semiconductor. Phys. Rev. 122:83.

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58 B I O G RA P H I C A L 1965 EMOIRS With Y. Takeishi. Effect of electron-electron interaction on the ki- netic-energy distribution of electrons ejected from solids by slow ions. Phys. Rev. 137:A304. With Y. Takeishi. Auger-type electron ejection from the (111) face of Ni by slow He+, Ne+, and Ar+ ions. Phys. Rev. 137:A641. 1966 Ion-neutralization spectroscopy of solids and solid surfaces. Phys. Rev. 150:495. With G. E. Becker. Ion-neutralization spectroscopy of copper and nickel. Phys. Rev. Lett. 16:230. 1967 With G. E. Becker. Ion-neutralization spectroscopy of copper and nickel. Phys. Rev. 159:572. 1971 With G. E. Becker. The interrelation of physics and mathematics in ion neutralization spectroscopy. Phys. Rev. B 4:4187-4202. 1973 With G. E. Becker. Folding and nonfolding electron distributions in ion neutralization spectroscopy and evidence for an electronic superlattice at the Si(111~7 surface. Phys. Rev. B 8:1592-1603. With G. E. Becker. Resonance, Auger, and autoionization processes involving He+~2s) and He++ near solid surfaces. Phys. Rev. B 8:107. 1975 With K. C. Pandoy and T. Sakurai. Si~lll):SiH3 A simple new sur- face phase. Phys. Rev. Lett. 35:1728-31. 1976 With E. G. McRae. Surface structure experimental methods. In Treatise on Solid State Chemistry, vol. 6A, ed. N. B. Hannay, pp. 57-163. New York: Plenum. With T. Sakurai. Interplay of the monohydride phase and a newly discovered dihydride phase in chemi-sorption of H and Si(100) 2x1. Phys. Rev. B 14:1593.

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HOMER DUPRE HAGSTRUM 1979 59 With T. Sakurai. Study of clean and CO-covered Ge(111) surfaces by UPS and INS. Phys. Rev. B 20:2423.