In 1969 a project to carry out this plan was approved by NBS management. Young was able to employ a technician, Fredric Scire. Starting with the availability of laboratory space in 1970, Scire and Young worked closely together designing, constructing, testing, and publishing the topografiner investigation. John Ward joined the project after the instrument had been substantially constructed. He developed tip preparation methods and performed early experiments on operation of a field emission probe in air. The electronic characteristics and sensitivity of the topografiner, and the observation of metal-vacuum-metal tunneling with it, were described in 1971 [6]. The instrument's first published images were included in Young's review of surface microtopography techniques for Physics Today [8]. In July of 1971, shortly before publication of those articles, NBS management terminated the topografiner project to concentrate effort into a program to provide calibration artifacts to the microcircuit industry. The final design of the instrument, together with the theory of its operation and its actual performance, were afterwards published in the title paper [1].

Today, we have the advantage of many years of instrument development, tens of thousands of publications by researchers all over the world, and the commercial production of SPMs, which have become a ubiquitous and relatively inexpensive tool. But Young and his coworkers were breaking new ground with the topografiner, not following any existing recipe. If first attempts teach anything, then second attempts must be better, so it is no surprise that the topografiner had deficiencies compared to current instruments. But the remarkable thing, from the perspective of a modern SPM researcher, is the soundness of judgement with which Young, Ward, and Scire analyzed the deficiencies of their own instrument and planned improvements to it. For example, the topografiner differed from current implementations of the STM by the absence, in the topografiner, of a logarithmic or otherwise variable amplifier in the feedback control circuit. With the benefit of experience, we can say that this is most likely the reason the topografiner's feedback control became unstable in MVM tunneling mode. The exponential increase of tunnel current with decreasing tip-sample separation means the servo loop gain increases exponentially as the tip enters tunneling range. The high gain coupled with inevitable mechanical resonances results in an unstable amplifier condition. Remarkably, Young had correctly diagnosed the problem. The paper included a graph of servo loop gain for various tipsample separations and described the resulting instability. At the time the project was cancelled Young's proposed program included a task to "Develop improved servo loop with ability to handle variable gain feature

inherent in field emission devices..." [9]. This improvement alone might have made possible the acquisition of images in MVM tunneling mode.

The instrument also had in common with Binnig and Rohrer's first STM a rather soft mechanical loop. The resulting low resonance frequencies exacerbate feedback loop stability problems, forcing the instrument to be operated at lower scan rates. Later instruments raised the mechanical resonance frequency by reducing the amount of mass that had to be moved by the z piezo and making the mechanical loop between the sample and the tip as small as possible. Young et al. commented in their section on the servo loop that "these mechanical resonances, which must be eliminated in the next generation of the instrument, cause phase shifts and thus servo loop instabilities."

As mentioned earlier, the topografiner was rigidly attached to its vacuum chamber. Beginning with Binnig and Rohrer, vibration isolation systems for SPMs include an isolation level between the instrument and its vacuum chamber (if there is one). This greatly reduces the transmission of acoustical noise, picked up by the large vacuum chamber, to the instrument. Instead, the topografiner was surrounded by an acoustical isolation shell. Even without internal vibration isolation, the topografiner demonstrated noise levels as low as 0.3 nm. This level of noise would not have prevented scanning in MVM tunneling mode, and its reduction would have been an evolutionary improvement.

The achievements of the project are better appreciated by reflecting upon some of the difficulties with which the principals had to contend. The topografiner did not have computerized data acquisition, taken for granted in today's instruments. The first integrated circuit computers were only beginning to be available at the time of the project (The PDP 8 was introduced in 1970), so data acquisition was with x-y recorder and storage oscilloscope. Feedback circuitry was a challenge. Severe funding limitations prevented the purchase of modern electronic equipment.

Perhaps chief among their difficulties was the difficulty inherent in being the first in any exploration. One to a certain extent stumbles around in the dark, aware of some goals without knowing precisely how to reach them and perhaps completely unaware of other treasures that may lie within reach. One such was atomic resolution. The possibility of laterally resolving individual atoms by such a technique was not suspected by anyone at that time. Young et al., and later Binnig and Rohrer, at first viewed tips as "a kind of continuous matter with some radius of curvature" [10]. The lateral resolution was therefore expected to be limited to something on the order of the tip radius, at that time. approximately 100 nm for state of the art field emission

tips. Images of atoms were an unexpected gift, a consequence of the existence of minitips or other roughness on the tip surface, together with the strong distance dependence of tunneling, which causes the nearest minitip to dominate.

We should not let speculation about could-have-beens detract from the accomplishment that was. In the space of the two years from first funding to project termination, Young et al built a new kind of microscope which was non-contacting, non-damaging, capable of three dimensional imaging, and which compared favorably in its topographic resolution to the best instruments of the time. They demonstrated a new principle of operation, analogs of which are now making significant contributions to every area of microscopy. And they obtained the first I-V characteristic curves for metal-vacuummetal tunneling.

In addition to the notice, already mentioned, in the 1986 Nobel citation, Russell Young received a Presidential Citation in 1986. In 1992 he received the Gaede-Langmuir Award from the American Vacuum Society "for his invention of the Topografiner, an instrument which led to the development of the scanning tunneling microscope." Today, the topografiner resides at the Smithsonian Institution.

Prepared by J. S. Villarrubia, with assistance from R. D. Young, F. Scire, E. C. Teague, and J. W. Gadzuk.

Bibliography

[1] R. Young, J. Ward, and F. Scire, The Topografiner: An Instrument for Measuring Surface Microtopography, Rev. Sci. Instrum. 43, 999-1011 (1972).

[2] G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, 7×7 Reconstruction on Si(111) Resolved in Real Space, Phys. Rev. Lett. 50, 120-123 (1983).

[3] L. A. Bottomley, Scanning Probe Microscopy, Anal. Chem. 70, 425R-475R (1998).

[4] R. D. Young, Field Emission Ultramicrometer, Rev. Sci. Instrum. 37, 275-278 (1966).

[5] Press release, Royal Swedish Academy of Sciences, October 15, 1986 (Text in brackets not in original).

[6] R. Young, J. Ward, and F. Scire, Observation of Metal-VacuumMetal Tunneling, Field Emission, and the Transition Region, Phys. Rev. Lett. 27, 922-924 (1971).

[7] R. D. Young, Theoretical Total-Energy Distribution of FieldEmitted Electrons, Phys. Rev. 113, 110-114 (1959). [8] R. D. Young, Surface microtopography, Phys. Today 24 (11), 42-49 (1971).

[9] R. D. Young, memorandum to Dr. John Simpson, July 19, 1971. [10] G. Binnig and H. Rohrer, Scanning Tunneling Microscopyfrom Birth to Adolescence, Rev. Mod. Phys. 59, 615-625 (1987).

It has now been almost 30 years since the 1971 review article entitled Electron-Stimulated Desorption as a Tool for Studies of Chemisorption: A Review [1] appeared. At that time, the authors, Theodore Madey and John Yates, were young scientists working together in the Surface Chemistry Section of the Physical Chemistry Division of NBS. Few could have predicted that the subject of this review would multiply and remultiply, until today the electron-stimulated desorption phenomenon and a number of related subjects form a cornerstone of chemistry and physics on surfaces-and that the personal collaboration forged at the time would last for 19 years at NBS, resulting in over 100 joint publications, as well as a deep and lasting friendship.

Surface chemistry is traditionally driven by the thermal excitation of adsorbed species. In areas such as heterogeneous catalysis, thin film deposition by chemical vapor deposition, and corrosion passivation, one often thinks only of thermally-driven surface processes. In the late 1960s, quantitative studies of the thermal desorption of molecules chemisorbed on single crystal surfaces were just beginning. Questions about molecular dissociation and atom recombination processes were being studied on well-defined single crystal surfaces for the first time, both by Yates and Madey at NBS and by other investigators. In this fundamental work simple molecules, such as H2, CO, N2, CH4, and NO, were employed so as not to make the chemistry complex.

The study of desorption processes initiated by electronic excitation instead of thermal excitation represented a departure from conventional research activities at the time and provided a special personal fascination for the authors which has to the present remained strong. The desorption of adsorbed species by nonthermal processes (i.e., electronic activation) became a new focus of the work starting about 1965. As a consequence of their work, Madey and Yates were invited by P.A. Redhead (a pioneer in electron stimulated surface processes, and the editor of the Journal of Vacuum Science and Technology) to write a critical review of what was being done worldwide in the area of electron stimulated desorption, a surface phenomenon which had already received the acronym ESD.

It is interesting to note that the title of the review contains the word "Tool." In 1971, many new surface measurement techniques were being devised in the surface physics community and were being transferred to some degree to the surface chemistry, engineering,

and materials science communities in the form of exciting and useful new tools for research. The workers at NBS were inclined to think of ESD as a new measurement method for the study of adsorption. The review uncovered more than 100 papers dealing with phenomena related to ESD, extending back to 1918. At the time of the writing of the review, the vast majority of the important papers had been written within the previous 10 years. The most influential work had been done independently by Menzel and Gomer [2, 3] and by Redhead [4] in 1964. The basic electronic excitation mechanism for ESD is now termed the MGR mechanism worldwide in honor of their central contribution. The 1971 review summarized the experimental methods that had been employed to date for the study of ESD, the theoretical foundation for thinking about the electronic excitation on the surface, and the subsequent desorption process. The authors also included almost all of the particular chemical systems that had been studied by ESD at the time. Since positive ionic fragments are often liberated in ESD, the review. contains a number of examples of the use of mass spectrometers and other devices as ion detectors. It also contains criticisms of certain mechanistic ideas which were in the literature, as well as a selection of experimental data that were regarded as being the most reliable (including much of the authors' own data).

In addition to electron-impact induced adsorbatesurface bond breaking (desorption), molecules on surfaces may be chemically converted to other species by electron impact, and the review article illustrated this by including one of the authors' own studies in which adsorbed N2 molecules were converted to adsorbed N atoms on a tungsten surface, one of the first examples of this phenomenon.

The review was 30 pages long and contained 152 references. It was not possible in a review of that length to do justice to all of this information. In order to summarize the work in these papers, a table was devised to list the principal findings; this table was arranged chronologically and by the adsorbate/adsorbent system and the experimental method. Not surprisingly, work on tungsten and molybdenum surfaces dominated, since during much of the 10 years prior to the review, it was known that atomically clean surfaces of these elements could be prepared through high temperature heating in ultrahigh vacuum. Much of this work was done without the luxury of Auger spectrometers for surface analysis.

Since 1971, well over 1000 papers on ESD have been published. Both Yates and Madey have published additional review articles in the field [5-8], and in 1991 65 reviews of ESD existed in the literature [6]. The acronym ESD, while still employed, has been replaced somewhat by the more encompassing acronym DIET (Desorption Induced by Electronic Transitions). An international meeting on this topic is held every two and a half years; the eighth meeting, organized in 1999 at Rutgers University by Ted Madey, attracted 80 attendees. The acronym DIET includes electronic excitation by photons as well as electrons, and in recent years photodesorption has become a dominant area in which both authors are working at their separate universities.

The writing of the review article was carried out on two continents. John Yates was working in Britain at the time, and many mailings of manuscript and figures were done in both directions to meet deadlines. One of the critical mailings from Britain was lost (and never found) as a result of the use of the Armed Forces mailing system because of a mail strike in Britain. The authors were caught off guard after publishing the 1971 review, since they were soon notified that it had become a Science Citation Classic by Citation Indices. They later remarked to Paul Redhead, "If we had thought we were writing a Classic, we would have written it in Latin."

One of the most exciting developments emanating from this collaboration had to do with the discovery of the ESDIAD (electron stimulated desorption ion angular distribution) phenomenon in 1974. Here the positive ion desorption products were found to be ejected in sharp beams whose direction was closely related to the direction of the chemical bond being ruptured in the electronic excitation process [9]. Fig. 1 shows the early ESDIAD apparatus and the three authors of the first paper, as well as photographic images of the ion angular distributions obtained.

Both Yates and Madey have left NBS for the academic world, and many honors have come to them, leading to professional recognition not only for the collaborative work done at NBS, but also for more recent work done independently at the University of Pittsburgh and at Rutgers University. The early work at NBS was centrally important to their professional development and to many of the rich opportunities they experienced since leaving government service. Indeed, the period from 1963 to the 1980s was generally one of extraordinary freedom in research at NBS in the field of surface science. Other NBS staff members who were their colleagues or friends and who numbered among the founders of the field include E. W. Plummer, J. W. Gadzuk, R. D. Young, C. J. Powell, A. Melmed, R.

Klein, and M. D. Scheer. The research benefited from the support of the Administration at NBS during this time, both at the Director's level (A. Astin, L. Branscomb, R. Roberts, E. Ambler) and at intermediate levels (M. B. Wallenstein, J. D. Hoffman, J. McNesby, M. D. Scheer, and R. Klein). Without excellent scientific colleagues and visionary administrators, NBS would not be historically recognized for its important role in establishing, and continuously nurturing to this day, the exciting and technologically important field of surface science.

It is appropriate to complete this account by citing recent articles from each of the authors' laboratories which are connected conceptually to the ESD phenomenon. In the first example, a photochemical dissociation process, involving an oriented O2 molecule chemisorbed with its O-O axis parallel to the step-edge sites of a Pt single crystal, is found to preferentially eject an O atom toward a neighboring CO molecule to produce CO2 which then desorbs [10]. This selectivity measured for target CO molecules on the step sites compared to CO molecules located on the terrace sites is termed surface aligned photochemistry (SAP), and is schematically illustrated in Fig. 2. This is the first observation of SAP involving molecular alignment on a stepped single crystal template.

The second example addresses an issue that has long puzzled planetary astronomers: what is the origin of the copious atomic sodium vapor in the rarefied atmosphere of the planet Mercury, and of our Moon? An image of this effect is shown in Fig. 3. A detailed DIET study of a model system (sodium atoms and ions adsorbed on SiO2 surfaces, simulating moonrocks) identified the likely scenario: when ultraviolet light from the sun strikes the lunar surface, it excites the surface electronically and causes desorption of sodium atoms [11]. Electrons in the solar wind can also cause ESD of sodium from the lunar surface, but the more plentiful solar photons are the main culprits.

In conclusion, ESD and other DIET processes today impact a host of scientific issues, including structure and dynamics of adsorbed molecules, quantum stateresolved desorption, dynamics of charge transfer, and surface photochemistry. DIET processes also provide insights into the science and technology of radiation damage, which affects quantitative surface analysis using electron and photon beams, partial pressure measurements, stability of x-ray optics, electron- and photon-beam lithography, and molecular synthesis in interstellar space. ESD and DIET continue to be scientifically exciting, growing fields!

Prepared by Ted Madey and John Yates.