Interview: Wilson Ho
2011 Medard W. Welch Award Recipient
Interviewed by Paul Holloway, November 2, 2011
HOLLOWAY: My name is Paul Holloway. I’m a member of the AVS History Committee. Today is Wednesday, November 2, 2011, and we’re at the 58th International Symposium of the AVS in Nashville, Tennessee. Today I have the privilege and pleasure of interviewing Dr. Wilson Ho of the University of California Irvine. He’s the 2011 Medard W. Welch Award winner. And his citation reads, “For the development and application of atomic scale inelastic electron tunneling with a scanning tunneling microscope.” So Wilson, thank you for agreeing to do the interview and congratulations on the Welch Award.
HO: Thank you. Thank you, Paul.
HOLLOWAY: So could we start by giving me your birth date and birth place?
HO: I was born on February 5, 1953 in a city in the middle of Taiwan. And then we immigrated to the United States when I was fourteen years old.
HOLLOWAY: So do you consider yourself a citizen of the United States?
HO: Yes, because most of my life is spent in this country. But I still have cultural and personal connections to people in Taiwan.
HOLLOWAY: Right. So some of your extended family is still in Taiwan?
HO: Yes.
HOLLOWAY: And you go back to visit them periodically?
HO: Periodically. Most of the time, though, it’s associated with meetings in Taiwan. So that’s how it goes.
HOLLOWAY: So give us a little bit of an indication of your educational background. So you were educated in Taiwan, to begin with?
HO: Yes. So for the first fourteen years, I spoke no English. Not a word. And the education in Taiwan, when I was there, it was a primary school education, and I distinctly remember the emphasis that was placed in mathematics. And we spent a lot of time, the teacher did, and we would just do problems after problems. We would basically do a lot of problems, and that really has some advantages in that you really get to establish a good foundation in how to solve these problems, which I found was quite useful in my later career.
HOLLOWAY: Good. So where did you go after primary education in Taiwan?
HO: We then spent two years in Japan. My father was helping to establish a Chinese school in Japan. We spent two years there before we immigrated to the United States. It was sort of like a stepping-stone in coming over to the United States.
HOLLOWAY: So what city were you in in Japan?
HO: We lived in a city called Rokko, which is very close to Kobe. And my father’s involvement in this school was in Osaka, Japan.
HOLLOWAY: What year was that?
HO: That was 1965-’67. So that’s the two-year period that we spent in Japan. The whole family was there.
HOLLOWAY: And then the whole family came to the United States?
HO: And then the whole family came to the United States, and I distinctly remember that we took a Pan Am flight from Tokyo to San Francisco. And my first impression when I landed in San Francisco and driving to my aunt’s place was the beautiful houses that we saw when we drove from the airport to my aunt’s place.
HOLLOWAY: Slightly bigger than Kobe?
HO: Yes. Bigger, and also not as crowded, and not as many people. And the houses just looked, at that time, very beautiful.
HOLLOWAY: And so you attended university in California?
HO: Yes. I went to junior high school and high school in San Francisco. And I was always interested in the sciences, and applied to three schools. Unlike students these days, who apply to a lot of schools, I applied to three schools: Caltech, MIT, and at that time as a backup, UC Berkeley. So I was fortunate to have the choices where I can go. And I decided to go to Caltech as an undergraduate. That was starting in 1971, and to ’75.
HOLLOWAY: So you got your Bachelor of Science?
HO: I actually also received a Master of Science since I did a lot of research, and that was one thing that I thought was very good at Caltech.
HOLLOWAY: As an undergraduate?
HO: As an undergraduate. I started research right away. And I distinctly remember the four professors I worked with: Aron Kuppermann, Bill Goddard, Wilse Robinson, and then finally Henry Weinberg. And so each year I worked with a different professor. I thought there was no better school than Caltech at that time, as far as the sciences are concerned.
HOLLOWAY: All of those names are very familiar to the AVS.
HO: Oh, I see. Okay.
HOLLOWAY: So you had good mentors then.
HO: Yes. Absolutely, yes. And Caltech was great, from that perspective.
HOLLOWAY: So you got your degree in physics or chemistry?
HO: Actually, I was majoring in chemistry, and so I got a bachelor’s. At that time, the professor thought I had enough credits and had enough publications, so they also awarded me a master’s degree at the same time. I actually had the distinction of publishing eleven papers.
HOLLOWAY: Eleven papers?
HO: As an undergraduate. They were all theory papers; not experiment. So in that sense, it was perhaps easier to get publications.
HOLLOWAY: Well, but eleven publications is really impressive no matter what.
HO: I mentioned that because I think that might be a record. [Laughs]
HOLLOWAY: It’s a record as far as I have any recollection.
HO: But things just happened that way. And the last research I did was with Henry Weinberg, the last year I was at Caltech. At that time I was looking for research in graduate school, and surface science was almost at the very beginning back in 1974. It was not at the very beginning, but close to the beginning.
HOLLOWAY: Very close.
HO: Yes, close to the beginning. And I thought that was an interesting and exciting area to get into. So I chose a graduate school based on the people that were doing research in this area. I finally chose to go to the University of Pennsylvania to go work with Ward Plummer. And at that time there were theory and experimental groups there with Bob Schrieffer, Paul Soven, Ward Plummer, Togny Gustafsson. And they had a whole bunch of people coming from Europe at that time. So I thought that was a very exciting place.
HOLLOWAY: Very cosmopolitan.
HO: Yes, very cosmopolitan. I also wanted to have the experience living on the East Coast for a change. It was not a major decision factor, but turns out at that time, I thought the best place to do surface science work was in Philadelphia.
HOLLOWAY: Certainly the collection of people were very well respected and well recognized at that time.
HO: Yes. Right.
HOLLOWAY: Let me take you back for a moment, back to the other three. You mentioned Henry Weinberg. Did you do catalysis work with him?
HO: I did calculations.
HOLLOWAY: Calculations?
HO: Right. I was fortunate enough to work with a postdoc by the name of Steve Cunningham, and we were doing some band structure calculations by Green’s function method, and we used the tight binding Green’s function method, and we put atoms on the surface, and then calculated the electronic state of adding an atom to a semiconductor and also to a metal surface. These were models, but we were able to see states that split off from the bands due to the adsorption of these atomic absorbates.
HOLLOWAY: So was that a collaboration with Goddard and the others?
HO: No. It was just with Henry Weinberg. And with Goddard I did generalized valence bond calculations on the molecule cyclobutadiene. These are quantum chemistry calculations with this particular method that Goddard developed called the generalized valence bond method. And I learned correlation effects, configuration interactions and different ways of calculating electronic states of molecules.
HOLLOWAY: So you did something similar with Kuppermann and Robinson?
HO: With Aron Kuppermann, I did research in the first year as a freshman. The project I worked on was to try to make life in a test tube.
HOLLOWAY: Oh, yeah.
HO: So we used the Lyman-alpha line to zap a gas mixture containing ammonia and hydrogen. The exact mixture I forgot, but something like those molecules in a gas bulb, and we zapped it with the Lyman-alpha line to try to make amino acids. Sort of like the Miller experiment.
HOLLOWAY: So did you publish that?
HO: That, we didn’t.
HOLLOWAY: So what did you do after your freshman year?
HO: The second year was with Wilse Robinson, and that was at the very beginning of the ultrafast laser development. And we did some picosecond two-photon absorption experiments in liquids. And that was a lot of fun, because I remember that I was able to have the liberty of using the whole laser system by myself as a sophomore. So I have very strong memories of that experience. That interest in ultrafast lasers I picked up again at Cornell University.
HOLLOWAY: So how did you switch from essentially experimental for freshman and sophomore to theoretical for junior and senior?
HO: Yes, it’s just to try different things. To do experiments, you always have to understand in detail what your results are. So I thought that it would be good to gain some theoretical experience. I tried to understand how to model experimental results. I also took a course from Goddard that involved different methods of quantum chemistry calculations. I wanted to apply the classroom knowledge to actually try to do some research with it.
HOLLOWAY: So you did an extended homework problem and it turned into a publication, huh? [Laughs]
HO: [Laughs] Yes, that’s right.
HOLLOWAY: So then you moved on to Ward Plummer at Penn?
HO: Yes, the University of Pennsylvania.
HOLLOWAY: And what did you do with him, then?
HO: That was a very interesting and exciting period in which I did some theory and experiment. Theory with Bob Schrieffer. He had a graduate student named Jim Davenport. And we did some theory connected to the experiment that we were designing and trying to perform. This was the subject of electron energy loss spectroscopy to probe vibrations of molecules on surfaces. This was a technique that was discovered by Harold Ibach about three years before we started, and he was able to show that by scattering electrons from a solid surface, you can actually obtain a vibration spectrum for the molecules adsorbed on the surface. So that was a very powerful method for obtaining chemical information of molecules on surfaces. And at that time, not many surface techniques could do that, could really give you chemical information of molecules. And at the sensitivity of about one percent for some absorbates—one percent of a monolayer you could detect. And Ward Plummer instilled in me a respect and love for instrumentation, because he always liked to build things.
HOLLOWAY: He loved to build things, didn’t he?
HO: Yes, design things and building things. And with that, you can do new types of experiments and try to discover new science. So that has stayed with me for the rest of my career, ever since my graduate school, to develop new instrumentation, and with that, try to do new science. So with Plummer, I really learned that approach to do science.
HOLLOWAY: So how much did you interact with Bob Schrieffer?
HO: A lot. Bob was very interactive. He had casual meetings with the people around him, and you could talk to him any time. Sometimes I would run into him in the library, and he would chat with me, with a beginning graduate student. He would still take time out to chat with you.
HOLLOWAY: Was he a Nobel laureate at that time?
HO: At that time, yes. Yes, he was. I was at Penn from 1975-’79. I forgot exactly the year that he received the Nobel Prize. But definitely I don’t remember the celebration at Penn, if it had happened during the time I was there. And so it has to have happened before I got to the University of Pennsylvania. So that was a very good place, with a combination of theory and experiments. I really liked that.
HOLLOWAY: So for your dissertation, you looked at inelastic scattering of electrons?
HO: Yes, inelastic electron energy loss spectroscopy or EELS. And we actually discovered impact scattering for the system of hydrogen on tungsten-100 surface. Up to that time, electron energy spectroscopy was always carried out in the specular direction, where the dipole scattering mechanism dominates. But you don’t see all the modes; you only see certain modes that are dipole active. So what we did was to measure off-specular scattering. That is, to measure the electron energy loss spectrum at different scattering angles. And from this, we were able to demonstrate that you can see all of the normal modes, at least for hydrogen on tungsten. In addition to the dipole allowed modes, we were able to see so-called impact scattering modes. For example, in the case of hydrogen on tungsten-100 surface, simple atomic hydrogen, there should be three vibrational modes: vibrational motions in the x-, y-, and z-direction. In the dipole mechanism, where you collect the electrons in the specular direction, you only see the mode vibrating perpendicular to the surface. By collecting the electrons away from the specular direction at large-angle scattering, we were able to resolve all of the three modes: the x, y, and z modes. So that was my thesis work, and for the theory, together with Jim Davenport and Bob Schrieffer, we did a calculation of the so-called negative ion resonance scattering. That was a good thesis to show that you can obtain new information by developing new capabilities for this spectroscopy. Before we did this experiment, all of the measurements were done in the specular direction.
HOLLOWAY: Now, did you work exclusively with hydrogen on tungsten, or did you do any multi-atomic molecular studies?
HO: Right. So we also tried to extend this. After you make the preliminary measurements, we tried to extend this to other systems to show that it’s not a fluke only for a hydrogen atom. I remember that we actually had to synthesize some molecules. I was involved in the synthesis by collaborating with chemists at the University of Pennsylvania. Larry Sneddon was the professor. And we synthesized some organic molecules. We tried to put different functional groups in the molecule, and then see whether we could understand the mechanism of electron scattering and inelastic electron scattering.
HOLLOWAY: So was it different, or did it reinforce what you had learned?
HO: It reinforced. So for the modes, which are dipole inactive, we can really pick it up in the off-specular measurements.
HOLLOWAY: Okay. Can you tell the orientation of the molecule on the surface from that, or that came later?
HO: Qualitatively, yes. You don’t get the exact angles, but you can say that the molecule’s tilted either parallel or perpendicular to the surface, but not the angles. Not the quantitative angles. And so for that, I would imagine you’d have to do careful calculations to do that.
HOLLOWAY: So after your Ph.D. at Penn, where did you go?
HO: So after my Ph.D., I was looking to see what’s next. That’s the question that everybody asks themselves. So I thought I would go to a place where I would spend a few years, and then try to switch to a university and get a tenured position right away, without going through the assistant professor step. I was looking at a few places, and finally I went to Bell Labs at Murray Hill, with the intention that I would stay there for a period of five years, and then go to a university with tenure. So I went to Bell Labs and started talking to people and tried to see what experiments I should be getting into, and particularly I was looking at the possibility of coupling lasers to surface science because they had the possibility of doing time resolved experiments on surfaces. These are all before the start of femtochemistry some years later, because lasers were not as developed.
HOLLOWAY: This was 1979?
HO: This was 1979 to 1980. I went to Bell Labs and started designing and thinking about experiments, but the campus life became too attractive. I was single at that time, and Murray Hill is in the suburbs of New Jersey. People come in, they work, but most of the time, people go home at dinner time. Of course, they have families, they go home. So evenings were pretty quiet. So I got a little bit lonely there. I mean there were a lot of good people there, but socially and the surroundings, I just missed the campus. After nine months, probably another record I set, I left Bell Labs and went to Cornell. People at Bell Labs at the time still talk about how fast I left the place. [Laughs] I still have many good friends from that period of time.
HOLLOWAY: Well records are meant to be broken, so maybe somebody…
HO: [Laughs] Yes.
HOLLOWAY: So which department did you go into at Cornell?
HO: At Cornell, I was in physics. John Wilkins, who is now at Ohio State, was the person who pushed through my appointment at Cornell. I didn’t really interview. I did not interview in the sense that nowadays people send out applications and all that stuff. There are many places that they apply. But at that time, it just happens that there’s a good match between my interests and Cornell University.
HOLLOWAY: So you did inelastic scattering again?
HO: Yes. At that time, I was interested in time resolved experiments. So we started developing the instrumentation of trying to get the speed into the instrument and we incorporated multi-channel detectors into the electron analyzers. The first one we built had 96 detectors.
HOLLOWAY: Oh, is that right?
HO: Yes. So we were able to record the vibrational spectrum in the millisecond timescale, and we were able to do chemical kinetics experiments on that timescale.
HOLLOWAY: So you coupled this with lasers again at this time?
HO: This time we started with using pressure jumps by using pulsed gas and observed changes on the surface: adsorption, desorption, reaction, and kinetics on the surface at different temperatures. So we would do pressure jump and temperature jump experiments, which are not fast, in the millisecond timescale comparable to the time that we can record a vibrational spectrum, electron energy loss spectrum.
HOLLOWAY: Now there were a number of other surface sensitive spectroscopies, the electron spectroscopies. Did you use those as complimentary tools?
HO: Yes. We used, at that time, low energy electron diffraction, Auger spectroscopy, but not ultraviolet photoelectron spectroscopy. The main technique was the High Resolution Electron Energy Loss Spectroscopy (HREELS), and we used the other ones to make sure that the surface was ordered and that there were no impurities on the surface.
HOLLOWAY: That was true for Cornell as well as for Penn? Did you use those as complimentary techniques at Penn as well?
HO: Yes. That’s right.
HOLLOWAY: One question that I have is you mentioned early on that Harold Ibach was sort of the father of scattering.
HO: Yes. That’s right.
HOLLOWAY: Did you interact with him? Did he visit with you?
HO: Yes. At that time, not as much. But later on, more and more.
HOLLOWAY: Sort of parallel paths.
HO: That’s right. Later on, more and more, and up to these days. He has retired as one of the directors at Jülich , Germany. He and Dr. Mills of UC Irvine wrote a definitive book on high resolution electron energy loss spectroscopy, and he would come to Irvine now and then.
HOLLOWAY: Is that right?
HO: That’s right.
HOLLOWAY: So how long did you stay at Cornell?
HO: I spent twenty years at Cornell. Survived twenty winters. [Laughs] It was a very beautiful place, Cornell. I was attracted to that place, in part by the surroundings, and but also by the very strong Physics and Chemistry Departments there. So it was a very exciting place at that time. In a very short period of time around 1971, a few years before I got there, there were simultaneous discoveries of the renormalization group theory and superfluid helium-3. So the place was just bubbling with excitement. Very energetic. They were able to attract excellent graduate students. It was just a superb place at that time.
HOLLOWAY: So who did you collaborate with there?
HO: There we collaborated with John Wilkins, who did the theory of some of these vibrational modes and phonons. We also had the distinction of collaborating with Roald Hoffmann, who is a theorist doing electronic structure calculations of molecules and molecules absorbed on surfaces. So he was a very dynamic and good person to chat and discuss things. He had such great insights into the electronic properties of molecules. I was very fortunate to be able to interact and collaborate with him. We actually wrote some papers together. And Roald had a very interesting way of running his group. He would require his students to spend one year in a laboratory. So I had a few of his students who spent one year in my laboratory.
HOLLOWAY: So even though he was a theorist, he wanted them rounded well in experimental approach?
HO: That’s right. So actually having the student working full time in an experimental lab.
HOLLOWAY: How many times did you find one of the experimentalists with two left hands?
HO: [Laughs] Two left hands. You mean the book he wrote?
HOLLOWAY: No, what I’m asking is sometimes people do theory because their capability with
their hands with experimental procedures is not so good.
HO: Ah, yes, I see. [Laughs]
HOLLOWAY: I know when you get more senior as a professor, the students don’t like to see you come into the lab and mess with their experiments sometimes.
HO: [Laughs] Yes.
HOLLOWAY: You did work at both elevated temperature and room temperature and low temperature, I believe. How much of that was driven by low temperature work at that time?
HO: Low temperature work didn’t go that low at that time. Basically, at the beginning, we used liquid nitrogen for cooling. Towards the end, we started using liquid helium. And especially the last phase of my work when we got involved with the scanning tunneling microscope, and then we started using liquid helium to go to lower and lower temperatures, in particular to do high resolution spectroscopy. For the scanning tunneling microscope (STM), it’s much better to go to lower and lower temperatures to get higher energy resolution, and also to observe interesting quantum mechanical phenomena at low temperatures.
HOLLOWAY: So talk about the STM and the impact that had upon your field. Because when you were going through Penn, that technique didn’t exist, right?
HO: That’s right. So the technique was invented in 1981 in Switzerland. At the beginning I did not want to get into it, because there were many, many good people who jumped into it and started doing many, many good things. And I thought that I was a little bit behind in getting into this field. Including my first graduate student, Joe Stroscio, who went to do a postdoc in this area of using scanning tunneling microscopes with Randy Feenstra, who was at IBM at that time. And that was back in 1985-’86, very close to the beginning of the scanning tunneling microscope. So Joe, as you know, was a fantastic student. He had been working with scanning tunneling microscopes ever since he got his Ph.D. And I thought that I would not be competitive in this new field. I was a little bit late to get into it. People jumped to it in the 1980s, and here I was in the 1990s, ten years after them, what was there to discover? [Laughs] But then before the STM, we had fifteen years of interest in lasers, femtosecond lasers to carry out time resolve experiments. You know, with time resolved electron energy loss spectroscopy, the time resolution was milliseconds. We wanted to go faster, because many things happen at a much faster time scales. So we developed and actually built femtosecond lasers. Over a fifteen-years period before we got involved with the scanning tunneling microscopy, we worked with femtosecond lasers and looked at many, many interesting things. The reason that we started to work with the STM, the scanning tunneling microscope, was that we wanted to know more about a molecule when it was dissociated by photons—photochemistry and photodissociation of molecules. We want to know, where are the products? How far do they move on the surface? What’s there on the surface before you irradiate a surface with photons? Because there are many experiments that study molecules on surfaces, but not really obtaining a real space view of where they are. What are their environments? And I thought that with the scanning tunneling microscope, we could use it to get some information if a molecule breaks up on a surface. How far do the fragments travel on the surface? And I thought that can be obtained by actually imaging where the reactants are, and where the products are. So that was the reason that we got into the STM. And we did that. We did the photodissociation of an oxygen molecule, and actually can measure the two oxygen atoms, and how far they can move away from each other because of the excess energy that is released in the breaking of this O-O bond, the oxygen bond. So we built the STM. At that time we did not have money or any project that was directly associated with the STM because we don’t have any track record in it. As you know, it’s difficult to get money, and it also takes some time to get the money. We used the NSF funds because the STM was tied to the surface photochemistry that NSF was supporting and could enhance it greatly. And of course, at that time, I was not sure whether there were commercial instruments or not. Maybe there were some. Park Instruments probably has some commercial products. But again, from this interest that we have in instrumentation development, we started to build the scanning tunneling microscope ourselves. At the beginning, we wanted to go low temperature, because the system is more stable at low temperatures. And the molecules are not as mobile on the surface at low temperatures. And so we designed the system to be able to study phenomena at low temperatures.
HOLLOWAY: Did you collaborate with IBM during that process? Or was it completely on your own?
HO: No, it was on our own. But before we started the design, because there were different types of microscopes, different mechanisms of sample movement and tip movement, we sent two students to three different labs, maybe four different labs, to look at their designs and talked to them. And also we asked whether they were willing to share their knowledge and experience and even the programs with us. Some were willing. Some were not as open. [Laughs] At that time, I remember that we visited Joe Strossio’s lab, Phil First’s at Georgia Tech, Bob Wolkow at Bell Labs, and Mo, I forgot his first name, at IBM. These were the four labs that two of my students who were interested in this work went and visited, spending a day with each of them, and tried to understand the different types of the scanning tunneling microscopes, and which approach was, in our opinion, the best at low temperature. Some did not operate at low temperature at the time, but we definitely wanted to be at low temperature because it would be more novel and also the molecules would be fixed on the surface and not as mobile.
HOLLOWAY: Did you complement that with atomic force microscopy (AFM) at all?
HO: No. At that time, no. The AFM was invented in 1985, about four years after the STM. But at that time we did not. We were interested in metals and semiconductors and the tunneling microscope because of its ability to reach atomic resolution at that time. Now, you can reach atomic resolution with the AFM, but at that time it was not clear. STM also involves electron tunneling, and electrons do interesting chemistry. So just like electron scattering experiments, we felt more comfortable with electrons at that time. It turned out our homemade STM was much more stable than we expected, the one that we built. Amazingly, we were able to fix the tip over a molecule for a long time, and had control on the molecule in the sense that if there were two molecules right next to each other, we could control the dissociation of only one of them, not the other. And we could also park the tip over a single atom on the silicon surface and be able to watch its motion as it jumps out, jumps back, and all these switching motions. And so that was an eye-opener, the STM was very, very stable, being able to be very stable and have control on what you want to do on a particular molecule on the surface or an atom on the surface. So we decided to try vibrational spectroscopy with the STM, because the STM, up to that time, was about imaging. Imaging and looking at the electronic structure, and it was not really chemically sensitive, not able to distinguish different chemical species on the surface. And there was always a lot of interest, even from the very start of the STM development and research, of obtaining vibrational spectroscopy on a single molecule. And we thought that the microscope that we developed was stable enough, and we attempted to do that. After about six months of trying this and trying that, we finally succeeded in obtaining the vibrational spectrum from a single molecule with the STM with atomic resolution.
HOLLOWAY: That must have seemed like a long time to you at the time, but it’s remarkably short time, actually.
HO: Yes. We actually benefited a lot from my thesis work, including the calculations that I did with Jim Davenport and Bob Schrieffer. At that time Bob Schrieffer was interested in tunneling spectroscopy from the macroscopic junctions, the metal-oxide-metal tunnel junctions. If you have molecules in these junctions, then you can actually record a vibrational spectrum.
HOLLOWAY: They couple into the vibrational bands.
HO: Yes. And again, however, for many, many molecules in the junction. And also the orientations of the molecules are not known, exactly what molecules you have in the junctions. But the technique of measuring the vibrational spectrum is the same between the two methods. It’s just that the STM allows you to obtain a spectrum of a molecule on the surface, and you can take a very detailed image of what you have. Also the sensitivity is that of a single bond in a single molecule. You can even measure the vibration of a single hydrogen atom on the surface.
HOLLOWAY: It’s really truly remarkable when you think about that possibility.
HO: My thesis work was on electron scattering from hydrogen atoms on tungsten, which you need about 1013 atoms per cm2 to get a spectrum. And so with the STM you can reduce that from 1013 to one, a reduction by 13 orders of magnitude. So that was quite exciting.
HOLLOWAY: Now you had, before the STM came along, set up the lasers. Did you couple the lasers with the STM work as well?
HO: Yes, at that time. Initially, we looked at the photo-induced dissociation of the molecule on the surface, and we used the STM to image the products. Then we started to couple femtosecond lasers to the STM with the idea that perhaps we could do simultaneous time and spatially resolved experiments. And that’s continuing these days in our lab, to see whether one can measure on the femtosecond timescale and also simultaneously with Ångström spatial resolution. So that’s a new challenge.
HOLLOWAY: So once you have accomplished the single molecule vibrational spectroscopy, where do you want to go to the next stage?
HO: That’s right, the question is always what is the most exciting thing to do at the next stage? It’s often the unexpected that is most exciting. The STM, to this day, still gives results that are surprising to us, that we did not anticipate when we started to do the measurements. That it gives results far more exciting than we anticipated.
HOLLOWAY: So you discovered that there was a lot to be discovered.
HO: [Laughs] Yes. That’s right.
HOLLOWAY: So you’ve also coupled the STM with magnetic fields?
HO: Yes. That’s the more recent work of going up to nine tesla magnetic fields. One tesla is 105 Gauss magnetic field. And with that we were able to detect a single electron’s spin. If there was an unpaired electron in a molecule, we were able to get a spectrum of that single spin. This is called the Zeeman spectroscopy, where instead of vibrational excitation, we are able to measure single spin-flip excitation between the Zeeman levels. This is another inelastic effect, but in this time it’s the spin excitation, where the spin is excited from a state with its magnetic moment aligned with the field to a state with the moment against the field. And that energy is very small. In a ten tesla magnetic field, the energy splitting is about one millivolt, or about eight wave numbers, whereas vibrational energies are in the order of a hundred millivolts, or about 800 wave numbers. So that requires us going down to even lower temperatures, because otherwise the Boltzmann factor would make it not possible to do this experiment since we need to have the spin initially in its ground state. So these experiments are done about less than one K in a nine tesla magnetic field.
HOLLOWAY: Now, this was done while you were at Cornell?
HO: This was done at UC Irvine.
HOLLOWAY: So when did you move to UC Irvine?
HO: We moved in 2000. So after twenty years, or twenty winters, however you count it, twenty winters in Ithaca, we moved to UC Irvine. So that was back in 2000. So it’s been eleven years now. So we started building this apparatus to be able to do experiments at 600 millikelvin and nine tesla magnetic fields.
HOLLOWAY: So you build your own magnet?
HO: No, we bought the magnet. We worked with Janis to build the cryostat that included the magnet, but the rest of the system we built ourselves: the microscope and all of the controls for the experiment and the microscope. And now when you get down to these low temperatures, high fields, and very high resolution spectroscopy, you have to worry about many other things. Like the electrical noises, which became much more important to minimize because the energy resolution now is down to about a hundred microvolt, which is below one wave number resolution. Before, it was about a few millivolts that we were working with, but now we could get down to sub-wave number energy resolution. So that opened up a lot of interesting things. And the most exciting thing happening right now is the ability to obtain magnetic information, spin information, even for molecules without unpaired spins. So in electron spin resonance (ESR) spectroscopy, you need unpaired electrons. But with tunneling spectroscopy, we discovered that even if there were no unpaired electrons, you could still measure the Zeeman spin states, because it was based on tunneling spectroscopy rather than absorption spectroscopy, as in the electron spin resonance experiment. So this is quite exciting. So I would say there are three most exciting experiments that I am most proud of. The first is my thesis work, the discovery of the non-dipole impact scattering in vibrational spectroscopy or high resolution electron energy loss spectroscopy. The second is the detection of the single vibrational mode by the STM, vibrational spectroscopy of a single bond in a single molecule. And the third is the most recent experiment of the spin spectroscopy - spin vibronic spectroscopy - spin spectroscopy of molecules that do not have unpaired electrons. And this was observed when the spin couples with the vibronic state of the molecule. In that the spin somehow —we don’t understand the mechanism yet— somehow it couples to the vibrational modes and also to the electronic state. And the coupling between electronic states and vibrational modes is referred to as the vibronic states. And eachvibronic state was observed to split into two states in a magnetic field, even though there is no unpaired electron. So for some reason, all of these three favorite experiments involve vibrations of molecules.
HOLLOWAY: You resonate with these molecules, that’s what it is. [Laughter]
HO: [Laughs] Yes, that’s right. Resonate.
HOLLOWAY: So you’re a professor in both physics and chemistry at Irvine, I believe?
HO: Yes.
HOLLOWAY: Tell us how you wound up at Irvine.
HO: It was mostly for family reasons. My parents and my wife’s parents and all of my sisters’ families are in California. And Irvine is a different type of school. It’s very young school compared to Cornell. It was established in 1965, so it’s not even fifty years old. But I felt that it was on an upward trajectory, in that there’s the excitement of expansion for the university, and so I found that exciting. And also, another reason at that time was that it would be satisfying to teach at a public school, deriving a sense of service to society. Cornell, as you know, is a private school. So those are some of the reasons. But another reason is that the weather is better. [Laughs] So much easier to go and move around. And my wife doesn’t like anything that falls from the sky. [Laughs] That includes rain, snow, and sleet, of which Ithaca has plenty.
HOLLOWAY: She doesn’t worry about the earthquakes though, huh?
HO: She doesn’t. Somehow she doesn’t worry as much. I find it interesting that you mention earthquakes. We didn’t have the big ones, but we often have these small tremors, but our STMs still work when it tremors at Richters of three, four, or five.
HOLLOWAY: So can you use that as a detector?
HO: Yes. The STM will respond to the tremors, but it was not sufficient to perturb the system on the other hand.
HOLLOWAY: Permanently.
HO: Yes. We could actually take the spectrum. We could sense it. There might be some noise in our data, but it came and went.
HOLLOWAY: So do you have colleagues in the departments that you work with there at Irvine?
HO: Yes. Dr. Mills, who does theory, has a long interest in electron energy loss spectroscopy of vibrations and phonons and also in magnetism. In chemistry I have a number of interactions. John Hemminger is one. So there are quite a few people with whom we interact with at UC Irvine.
HOLLOWAY: I noticed that at the end of your short write-up in the awards that you said that you’re particularly proud of the fact that your research results have found their way into textbooks.
HO: Yes. For example, particle in the box is a textbook problem. And tunneling, of course, is another one. And in chemistry, the STM was used to manipulate atoms and molecules to form new chemical bonds. You can see in real space quantum mechanical phenomena and the formation of chemical bonds, and these results have now appeared in textbooks. The image of atoms on the silicon surface has actually appeared in the California elementary science textbooks.
HOLLOWAY: Is that right?
HO: Yes. And the more advanced topics like particle in the box and seeing the nodes in the wave functions and for different excited states, the making of molecules, these appeared in the college textbook. This is quite exciting to have research results to appear in textbooks.
HOLLOWAY: The other thing that you mentioned is that you’re particularly satisfied with the success from your former students.
HO: Yes..
HOLLOWAY: Are there particular ones that stand out?
HO: Well my first graduate student, Joe Strossio, is now a fellow at NIST. He is one of the stars in the research with the scanning tunneling microscopes. He is a master in instrumentation and has made many contributions to the field. Lloyd Whitman is now the deputy director of the nano center at NIST. NIST has five of my former students there who are doing cutting edge work and measurements. And I have students who have gone to universities, and we still keep in contact. Lincoln Lauhon is a professor at Northwestern University. And more recently, I had Chinese students. At Cornell I had mostly American students who were born here, but at UC Irvine in the past few years I have had the fortune of working with Chinese students, and they now have gone back to China and have been very successful in the top universities in China, like Tsinghua University, Fudan University, and Peking University. And also at the National Nanoscience Center in Beijing and the Institute of Physics of the Chinese Academy of Sciences. I have former students who now are leaders in their fields.
HOLLOWAY: This is Academia Sinica?
HO: Yes. There is one person who has gone back there and now is on the scientific staff. So that is exciting, to see them return to become the new generation of leaders of science in China.
HOLLOWAY: Were they mainly experimentalists?
HO: They are all experimentalists. I haven’t done theory since graduate school. Now we tend to collaborate with theorists. But the students had different degrees of interest in theory. Like Joe Strossio did actually a lot of theory for his thesis. And another student, Frank Zimmerman, who is now a professor at Rutgers, did excellent theory. So it depends on the student’s interests in what they want to do.
HOLLOWAY: I tell my students that it’s my job to keep them from going too far off the path, but I’m not going to define the path for them. They’re going to have to find it themselves.
HO: Yes. So it’s exciting. I think that’s one of the reasons that we are in the educational field is the possibility of working with these young people, who develop in their capability in front of you, and then they go off and do better than what you can do.
HOLLOWAY: Our job is to make them better than us.
HO: That’s absolutely right. Yes.
HOLLOWAY: We’ve covered quite a few topics. Are there other topics you would like to add to the interview?
HO: Well, the American Vacuum Society has instilled a deep impression on me. I remember that I gave my first talk at the AVS meeting.
HOLLOWAY: Is that right?
HO: Yes. It was on this research that I did with Henry Weinberg. I was a senior at Caltech, and there was an AVS meeting in Anaheim that year, and I gave a talk at the post-deadline surface science meeting.
HOLLOWAY: That’s quite an accomplishment.
HO: Yes. So AVS always has a special place in my heart. The AVS was the place where I gave my first professional talk.
HOLLOWAY: So you presumably encourage your students to participate in societies in the same fashion?
HO: That’s right. So they are encouraged to go to meetings to present their results to many people, to see what others are doing, instead of just being confined to the problems that they face every day. Experiments are in general not easy. Research is not easy. So it’s good for them to see other people in this field and how other people struggle besides them. [Laughs]
HOLLOWAY: Good. Anything else you’d like to add?
HO: No. Thank you very much.
HOLLOWAY: Well congratulations again on the Welch Award. Well deserved.
HO: Thank you, Paul.
HOLLOWAY: Thank you.