Electron Diffraction: Fifty Years Ago
Total Page：16
File Type：pdf, Size：1020Kb
Electron diffraction: fifty years ago A look back at the experiment that established the wave nature of the electron, at the events that led up to the discovery, and at the principal investigators, Clinton Davisson and Lester Germer. Richard K. Gehrenbeck An article that appeared in the December Before finishing his undergraduate discovery of electron diffraction; it was 1927 issue of Physical Review, "Diffrac- degree at Chicago, he became a part-time also at this time that he was joined by a tion of Electrons by a Crystal of Nickel," instructor in physics at Princeton Uni- young colleague, Lester Halbert Germer, has been referred to in countless articles, versity, where he came under the influ- just discharged from active service. monographs and textbooks as having es- ence of the British physicist Owen Rich- tablished the wave nature of the elec- ardson, who was directing electronic re- An adventurous New Yorker tron—in principle, of all matter.1 Now, search there. Davisson's PhD thesis at Germer was born on 10 October 1896, fifty years later, it is fitting to look back at Princeton, in 1911, extended Richardson's the first of two children of Hermann the events that led up to this historical research on the positive ions emitted from Gustav and Marcia Halbert Germer, in discovery and at the discoverers, Clinton salts of alkaline metals. Davisson later Chicago, where Dr Germer was practicing Davisson and Lester Germer. Figure 1 credited his own success to having caught medicine. In 1898 the family moved to shows them in their lab in 1927, together "the physicist's point of view—his habit Canastota in upper New York state, the with their assistant Chester Calbick. of mind—his way of looking at things" childhood home of Mrs Germer. Ger- from such men as Millikan and Richard- mer's father became a prominent citizen A shy midwesterner son. in the little town on the Erie canal, serving Clinton Joseph Davisson, the senior in- After completing his degree, Davisson as mayor, president of the board of edu- vestigator, was born in Bloomington, Il- married Richardson's sister, Charlotte, cation and elder in the Presbyterian linois, on 22 October 1881, the first of two who had come from England to visit her church. children. His father, Joseph, who had brother. After a honeymoon in Maine Germer attended school in Canastota settled in Bloomington after serving in the Davisson joined the Carnegie Institute of and won a four-year scholarship to Cor- Civil War, was a contract painter and Technology in Pittsburgh as an instructor nell University, graduating from there in paperhanger by trade. His mother, in physics. The 18-hour-per-week the spring of 1917, six weeks early because Mary, occasionally taught in the Bloom- teaching load left little time for research, of the outbreak of the war. The local ington school system. Their home was, and in six years there he published only newspaper, after applauding 18-year-old as Davisson's sister, Carrie, characterized three short theoretical notes. One nota- Lester for working as a laborer for the it, "a happy congenial one—plenty of love ble break during this period was the local paving contractors during his sum- but short on money." summer of 1913, when Davisson worked mer vacation, proceeded to ridicule his Davisson, slight of frame and frail with J. J. Thomson at the Cavendish lazier contemporaries for sitting "day throughout his life, graduated from high laboratory in England. after day in the lounging places of the school at age 20. For his proficiency in In 1917, after he was refused enlistment village," saying there is "nothin' doin' " mathematics and physics he received a in the military service because of his and that "a young feller has no chanst in one-year scholarship to the University of frailty, Davisson obtained a leave of ab- this durn town." (Lester, must have Chicago; his six-year career there was in- sence from Carnegie Tech to do war-re- taken a bit of ribbing from the "idle boys" terrupted several times for lack of funds. lated research at the Western Electric after this appeared!) Germer's studies at He acquired his love and respect for Company, the manufacturing arm of the Cornell were partly self-directed; in their physics from Robert Millikan; Davisson American Telephone and Telegraph junior year he and two classmates, finding was "delighted to find that physics was Company, in New York City. His work themselves "unsatisfied with the course the concise, orderly science [he) had im- was to develop and test oxide-coated in electricity and magnetism given ... agined it to be, and that a physicist [Mil- nickel filaments to serve as substitutes for bought a more advanced text and met likan] could be so openly and earnestly the oxide-coated platinum filaments then regularly in the vacant class room .. and concerned about such matters as colliding in use. At the end of World War I he really learned something." bodies." turned down an offered promotion at LTpon graduation from Cornell, Germer Carnegie Tech to accept a permanent obtained a research position at Western position at Western Electric. It was at Electric, which he held for about two Richard K. Gehrenbeck is an assistant professor this time that he began the sequence of of physics and astronomy at Rhode Island Col- months before volunteering for the Army lege, Providence, Rhode Island. investigations that ultimately led to the (aviation section of the signal corps). He 34 PHYSICS TODAY / JANUARY 1978 0031-9228/78/3101-34 $0.50 © 1978 American of Physics apparently made no contact with Davis- years 1913 to 1916, before Davisson and Physical Review in 1920, concluding that son then. Lieutenant Germer, among Germer appeared on the scene. Never- positive-ion bombardment has a negligi- those piloting the first group of airplanes theless, because AT&T was deeply con- ble effect on the electron emission from on the Western Front, was officially cerned about the efficiency and effec- oxide-coated cathodes.' credited with having brought down four tiveness of its triode amplifiers—key With this problem settled, a related German warplanes. Discharged on 5 components in its recently constructed question came up: What is the nature of February 1919, Germer was treated in transcontinental telephone lines—Arnold secondary electron emission from grids New York City for severe headache, ner- assigned Davisson and Germer the task of and plates subjected to electron bom- vousness, restlessness and loss of sleep, conducting tests on oxide-coated cath- bardment? Davisson was assigned this conditions attributed to his military odes. They published their results in the new task and given an assistant. Charles campaigns, but he refused to file for compensation because "others were worse BELL LABORATORIES off." After three weeks of rest, he was re-hired by Western Electric—and had as his first assignment the preparation of an annotated bibliography for a new project being directed by his new supervisor, Davisson. That fall Germer married his Cornell sweetheart, Ruth Woodard of Glens Falls, New York. Electron emission—in court The assignment that engaged Davisson and Germer in their first joint effort re- flects one of the chief interests of the parent company, AT&T, at this time: to conduct a fundamental investigation into the role of positive-ion bombardment in electron emission from oxide-coated cathodes. Although Germer later re- membered this project as having been directly related to the famous Arnold- Langmuir patent suit, that occupied Western Electric (Harold Arnold) and General Electric (Irving Langmuir) from 1916 until it was finally settled2 by the US Supreme Court (in favor of Western Electric) in 1931, a careful examination of the documents makes it clear that Dav- isson and Germer's project could have related to it only in a very indirect way. The patent case concerned improvements Davisson, Germer and Calbick in 1927, the year they demonstrated electron diffraction. In their New York City laboratory are Clinton Davisson, age 46; Lester Germer, age 31, and their assistant to the earliest deForest triode tubes with Chester Calbick, age 23. Germer, seated at the observer's desk, appears ready to read and record metallic (tungsten or tantalum) cathodes; electron current from the galvanometer (seen beside his head); the banks of dry cells behind Davisson it dealt with evidence obtained in the supplied the current for the experiments. Figure 1 PHYSICS TODAY , JANUARY 1978 35 link together their data, the model and the predictions were anything but defi- nite—quite out of keeping with the Rutherford-Geiger-Marsden tradition. Although Davisson (and Kunsman) must have been somewhat disappointed at the limited success of their initial ven- ture, they pressed on with additional ex- periments. In the next two years they built several new tubes, tried five other metals (in addition to nickel) as targets, developed rather sophisticated experi- mental techniques at high vacuum ("the pressure became less than could be mea- sured, i.e., less than 10~* mm Hg,") and made valiant theoretical attempts to ac- count for the observed scattering inten- 50 100 150 200 sities. The results were uniformly un- ENERGY OF SCATTERED ELECTRONS (eV) impressive; several of the studies were not Electron-scattering peak. The energy of the scattered electrons varies from almost zero to that even published. In fact, the generally of the incident beam (indicated by the arrow). This is a reconstruction of the type of observation disheartened atmosphere that seems to that led Davisson and Charles Kunsman to conclude that some electrons were being scattered have prevailed by the end of 1923 is indi- elastically. Davisson saw these as possible probes of the electronic structure of the atom, in analogy cated by the fact that Kunsman left the to Rutherford's use of alpha particles to explore the nucleus.