Joachim Stöhr

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ORAL HISTORIES

Interviewed by

Jon Phillips

Interview date

April 28, 2021

Location

Video conference

See catalog record for this interview.

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Interview of Joachim Stohr by Jon Phillips on April 28, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
www.aip.org/history-programs/niels-bohr-library/oral-histories/48025

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Abstract

Interview with Joachim Stöhr, Professor Emeritus of the Photon Science Department at Stanford and former Associate Laboratory Director at SLAC. Stöhr begins the interview recounting his childhood in Germany, his interest in sports, and his introduction to physics in high school. He discusses his undergraduate studies at Bonn University and his Fulbright scholarship to Washington State University where he studied optical spectroscopy. Stöhr then describes his doctoral studies with Rudolf Mossbauer in Munich, and his subsequent postdoctoral position at Lawrence Berkeley National Laboratory. He recounts his time as a staff scientist at the Stanford Synchrotron Radiation Lightsource (SSRL) before transitioning into a position at Exxon and then IBM. Stöhr describes his return to Stanford and SSRL, working as Deputy Director and then Director. He then talks about accepting the opportunity to become the first director of the Linac Coherent Light Source (LCLS) and the challenges that came with building up a new program. The interview concludes with Stöhr’s reflections on his return to the science after his directorship and his work on a forthcoming book.

Transcript

[Editor's note: After the interview, Prof. Stöhr generously took the time to provide extensive written edits to this interview transcript to clarify and expand on points discussed in the spoken interview.]

Phillips:

This is Jon Phillips, the assistant oral historian at the American Institute of Physics. Today is April 28, 2021, and I am speaking with Dr. Joachim Stöhr from SLAC/Stanford. Jo, thank you very much for speaking with me today.

Stöhr:

All right. It’s a pleasure.

Phillips:

So before we get started, can you just give me your current full title and affiliation with SLAC?

Stöhr:

Okay. SLAC is an interesting institution because it’s very much interwoven with Stanford University. As a National Laboratory it has its own staff. Some of the scientists and top managers of the science areas are also professors within the University. It is almost as if SLAC were an eighth school of Stanford University. This is not formally put in place, but effectively the SLAC director typically acts as a Dean of Stanford. It’s a long answer to your question because it’s complicated.

It’s the first time ever, I think, since I was six years old that I missed a winter of skiing. So anyway, sports was my main interest. Coming back to education, I would argue, that although the German school system was rather elitist and I would not recommend it in today’s world, you nevertheless got a fantastic education. There was basically very little choice. Once you got into high school, there were very few electives. You were trained in all of the fundamental disciplines, a concept going back to Alexander von Humboldt, a polymath or “universal man”. Historically that included learning Greek and Latin. By the time I entered Gymnasium, Greek wasn’t required any more but I had to put up with Latin (my least favored subject). I liked learning English and one of my big regrets is that I did not learn French. This happened because I was in a branch that was geared toward the natural sciences, so I had biology, chemistry and physics and a lot of math in high school. This really prepared me well for my later career, although I wasn’t that interested at the time in the sciences.

I have to say, though, that physics actually came very easy for me. I don’t know why. I hated how chemistry was taught, kind of mixing things together and things bubbling up and exploding and stuff—that didn’t interest me. But the subtleties of physics I liked…and I had a very good physics teacher who also, in the end, contributed to me becoming a physicist. I could remember it without really having to study at home. I just felt, “Oh, yeah. Physics is interesting.” [Laughs]

There were nine grades in Gymnasium, and I was 18 when I graduated. Then I went to Bonn University. Like many seniors, I had to do some soul-searching of what am I going to do? I wasn’t going to make a career in sports. [Laughs] At least I didn’t think I could and so I had to figure out something else.

So there was this friend of mine who was a grade ahead of me, and he was the smartest kid I knew. I was always arguing with him because he, of course, knew things that I hadn’t learned yet. He decided to study physics in Bonn, and after one semester—I was still in high school—he told me that he had to work too hard to study physics, so he went into medicine and he became a medical doctor. So I said to myself, “Well, you kind of like physics. Wow, that would be a great challenge. I wonder whether you could do it!” [Laughs] So it’s the typical thing that drives humans—the challenge. I wonder whether I can do that!

So that’s how I got into physics, and my friend was right. The first year was hard. I didn’t study much physics but it was all math. I took all the courses with the mathematicians and there were some brilliant kids. I was okay, but I would also at times think, “God! They make me work so hard doing all these homework problems, and all my other friends who are in law or the humanities get to go see movies and have fun and do things. I just sit here and solve my damn homework problems!”

I was a little frustrated, but there’s one thing about the sciences - it’s always challenging. You cannot sit still. Your mind has to work, and not just work—you have to work hard at it. One of my recognitions in life has been that there are a lot of smart kids around and a lot of people who are capable, but the ones that really succeed will succeed more often than not, because they really put in the work. It’s the work ethic and the diligence, the effort that you put in that at some point will distinguish you from other people. I guess I was willing to work hard. At times I cursed it, of course, but I just stuck with it.

I would say that the interesting part of physics for me or when I really discovered that I loved physics, was not until most of my classes were actually done, and it was the research part where I got to choose a problem and work on it and try to figure out what I was observing. That to me was the beauty of science.

Phillips:

Do you remember what the research problem that you got to work on as an undergraduate was?

Stöhr:

Yeah. We are jumping ahead a little bit.

Phillips:

Oh, sorry.

Stöhr:

So at Bonn University, I was just doing courses, right, and homework problems. Then after my pre-diploma, which is I guess equivalent to a bachelor’s degree, I applied for a scholarship to go to the US because, growing up in Germany after the war, America was envisioned as the “land of unlimited opportunity”. That was the phrase that was floating around, and so I said, “Wow, I wonder! Interesting, I need to check this out.”

Of course by that time I also had realized that European physics, especially German physics—that had been really great with quantum mechanics was no longer at the forefront. After the war, the United States clearly became the superpower in physics, and this was where the biggest innovations occurred. To some degree, of course, they were driven by an influx of European physicists, a lot of them of Jewish origin that had to leave. Some of the most brilliant minds that Germany had, left during the Third Reich and went to the US! All of that contributed to me thinking, “I need to check this out.” So I applied and I got a Fulbright Scholarship to study in the US.

Skiing was very much on my mind too [laughs], so I got out the German “Atlas” map to check out the location of universities that offered a free ride. I ended up at Washington State University in Pullman, Washington - out in the boonies. On the German map – mountain regions were brownish and flatlands were green - it looked like Pullman had mountains close by. [Laughs] For lack of better knowledge I thought WSU was in the Rocky Mountains. But Pullman was in the wheat fields and it was at least a two hours’ drive to go skiing. But anyway, that was one of the motivations why I went there in 1969.

So I did two years at WSU and also did a research problem in optical spectroscopy with a professor, John Gruber. In his younger years, Gruber had been at Caltech and overlapped with Mössbauer there. So he wrote Mössbauer a letter and asked whether I could come to Munich and study with him, and Mössbauer said yes. So in ’71 I went to Munich after my master’s and did my doctorate there.

Mössbauer was famous. After the war, he was the first German to win the Nobel Prize in Physics - I think in ’61 - so he was very well-known. But of course he had a lot of obligations because once you’re famous, you get pulled into many different directions, and about two months after I got there, he became the director of the Institut Laue-Langevin in Grenoble, France and left. A new professor came who had been at Argonne National Laboratory for many years. His name was Michael Kalvius and he brought a little bit of an American flavor with him since he had been at Argonne for quite some time. I called him Mike, which was unusual because in Germany it was “Herr Professor,” right?

He was very easygoing and so I said, “You know, that problem Mössbauer gave me, I looked at it. I don’t like it. I will be here forever because first of all, we have to build an accelerator to do this and that will take at least three or four years, and I hope to be done with my doctorate around that time.” He said, “Oh, that’s okay. Pick your own problem.” [Laughs]

At the time, there was a guest scientist who was also from Argonne and later played another role in my life. His name was Gopal Shenoy. He came with Kalvius from Argonne to Munich. So Gopal and I became good friends and he encouraged me to work on a certain problem, which I did. We don’t need to go into details but it was looking at magnetic impurities in non-magnetic metals, a popular topic at the time. Anyway, that’s the problem I chose and finished, and I got my PhD three years later in ‘74. That was an important year because also my daughter Megan was born.

Earlier that year I had heard David Shirley, a professor of chemistry in Berkeley, give a talk in Uppsala, Sweden at a conference. The talk was mostly on photoelectron spectroscopy - which had been pioneered in the form of ESCA by Kai Siegbahn in the town of the conference, Uppsala. I was so impressed by Shirley’s talk that I said to myself, “Why am I wasting my time with Mössbauer spectroscopy? If you want to learn something about solids, you need to look at the electrons, not at the nucleus.”

So I wrote Dave Shirley a letter and said, “I want to come to Berkeley and work with you and learn photoelectron spectroscopy.” At the time, one of Mössbauer ‘s previous students in Munich, Günter Kaindl, who would become a Professor at Freie Universität, Berlin, was a post-doc in Berkeley with Dave and put in a good word for me. So Dave wrote back, “Okay, fine.” But he also added, “Can you bring some financial support?” [Laughs] So I applied for another scholarship and actually got it and then I went to Berkeley in `75. That was a very, very fortunate event. I always loved Berkeley and the intellectual atmosphere. Even to this day I’m very fond of Berkeley.

When I arrived, something very fortuitous happened. At Lawrence Berkeley Lab, Shirley’s group had done photoemission with a commercial instrument using one fixed photon energy. The use of synchrotron radiation allowed changing the photon energy, which was basically unheard of. So Shirley’s group had just started to do synchrotron radiation experiments at near-by SLAC/Stanford. After hearing about the disastrous earlier trials I said to myself, “This sounds really interesting and challenging. I want to get involved.”

At the time, there was one dedicated synchrotron radiation facility in the US called the Tantalus storage ring at the University of Wisconsin. That was a small ring and so it wasn’t super-expensive to build and operate, and was specifically built to produce synchrotron radiation in the extended ultraviolet region. This is where photoemission with synchrotron radiation was developed by people like Dean Eastman, Neville Smith and Jerry Lapeyre. Very many of the early experiments in photoemission were done there and I read those papers.

At SLAC, we used the SPEAR storage ring built for high energy physics. At the time it was used to study collisions between electrons and positrons that led to the discovery of the J/psi particle by Burton Richter in 1974 and won him the Nobel Prize in 1976. X-ray scientists, notably Ingolf Lindau and his student Piero Pianetta, were allowed to drill through one of the shielding walls and bring out the x-rays produced by the circling electrons. X-ray scientists were so-called “parasitic users”.

At the time, much work at SSRL involved the development of EXAFS (extended x-ray absorption fine structure), pioneered by Farrel Lytle, Ed Stern and their student Dale Sayers who regularly came down from Seattle for experiments at SSRL. EXAFS allowed you to pick out an atom by tuning to its characteristic absorption edge and then determining its local bonding environment, the type and number of neighbors and their bond distances. I often spoke with Farrel Lytle from Boeing and Peter Eisenberger, at the time at Bell Labs, who seemed omnipresent on the experimental floor and played a role in the development of my own research.

As a staff scientist at SSRL, I also had to help outside users with their experiments. Of course, my real love was doing my own and I have to thank then SSRL Director Artie Bienenstock for protecting my strong independent steak. When at Berkeley, I had learned from Dave Shirley (who was a chemist) the importance of molecules containing C, N and O atoms. Their absorption edges were in the soft x-ray spectral region which at the time was not readily accessible due to limitations in beam line optics. The lack of good grating monochromators at the lower end of the spectrum (above 200 eV) and the presence of absorbing Be windows and suitable crystal monochromators at the upper end (below 3000 eV). So I made it one of my tasks to open up the soft x-ray region by optimizing the existing grasshopper grating monochromator beam line and building a new ultra-high vacuum compatible crystal monochromator, named Jumbo.

In the process I applied EXAFS to surfaces, coined SEXAFS by the Bell Labs group of Paul Citrin and Peter Eisenberger. My emphasis, however, was on the surface complexes formed by low-Z atoms, in particular oxygen. It so happened that my first SEXAFS study in 1978 of an oxidized aluminum surface above the O K-edge at 540 eV was also the first EXAFS spectrum taken in the soft x-ray region. I soon realized the importance of using the first part of the SEXAFS spectrum, which especially for molecules, is dominated by huge molecular resonances. This led to the cheeky name NEXAFS (near edge x-ray absorption fine structure). In retrospect, I consider the development of polarization dependent NEXAFS around 1980, my most important science contribution while at SSRL. It also first demonstrated the power of using the polarization of x-rays (dichroism).

Another area that was developed at SSRL, largely by Keith Hodgson’s group at Stanford, was protein crystallography because one could now put a small crystal in the beam and with the increased intensity record a diffraction pattern 100-1000 times faster than with the best rotating anode x-ray tube. Limited beam time also triggered the development of robotics to screen protein crystals. This helped in speeding up the crystal growth and evaluation process. Today whole experiments can be performed remotely from your laptop at home through robotics.

Sure. Yeah.

Stöhr:

We saw that everywhere. We saw it at AT&T Bell Labs in the early ‘80s, Exxon in the mid ‘80s and IBM in the early ‘90s. At some point when the money stream gets tighter, if the competition increases, you need all your resources to focus on how you maintain or increase revenues. So the scientists were increasingly pulled into firefighting problems in development and manufacturing, catalysis at Exxon, networks at AT&T Bell Labs, computer chips and magnetic storage at IBM. So this is when the free spirited think tanks turned into more applied or directed research labs.

Exxon had of course some scientists who made a lot of money for the company. One of them was John Sinfelt. He invented a bimetallic catalyst which made Exxon gobs of money. The scientists that really understood the bread and butter of catalysis were actually quite skeptical that basic research - surface science - was very important. Who cares whether in model systems this molecule bonds to this surface by standing up instead of lying down? They said, “In a real catalytic process, we have a big reactor. We throw all kinds of crap in there and go to high pressure, temperature and stuff like that to convert oil into gasoline.” The process is so complex that in most cases advances come from a trial-and-error engineering rather than a surface science approach.

Also, in surface science you have to work on a specific problem in great detail for some time. The work is then published and available to the whole scientific community. It’s known not just to your company but also the competitors. So why pay for it?

Phillips:

Right. I’m also curious. The beamline that you set up at Brookhaven—how closely integrated into the research community at Brookhaven were you when you did that?

Stöhr:

There were two x-ray proposals on the table: a larger higher energy (hard x-ray) facility which later became the Advanced Photon Source (APS) at Argonne and a lower energy (soft x-ray) ring which later became the Advanced Light Source (ALS) at Berkeley. The latter was pushed by then Berkeley Lab Director David Shirley. Of course, his knowledge came from the early experiments at Stanford. I fully supported the Berkeley proposal and I was invited to present the scientific case for soft x-rays to the Seitz-Eastman Committee.

So while the synchrotron community was fighting within itself which of the two x-ray facilities had priority, another question that the Seitz-Eastman committee was addressing was whether to first build one of the x-ray sources or a new neutron source? Rumor had it that the committee was divided. The x-ray facilities had the advantage that the community was strong and that the brightness could be increased by a whopping factor of 10,000 while the neutron flux could only be increased by a moderate factor of no more than 10 in a steady state neutron reactor source. Construction of a pulsed neutron source (the later Spallation Neutron Source (SNS) at Oak Ridge National Lab) was also on the table but the concept wasn’t sufficiently developed.

While managing the magnetic materials department I had started to collaborate with Stuart Parkin and Dieter Weller, who were experts in magnetic materials, and started to study them with the x-ray dichroism technique. Out of that came the first x-ray picture of magnetic recording bits. I had dreamed about it and remember coming into SSRL one morning and my collaborators showed me the picture they had taken over night. “You must be kidding – so that’s what they look like,” I said. X-ray imaging of magnetic nanostructures later became a major research theme and my postdocs and Stanford students actually made movies of how a magnetic memory device switches.

Our new IBM beamline at the ALS was also ready and there I explored something new for me. The group of Joseph Nordgren at Uppsala University, Sweden, had developed a compact x-ray emission spectrometer, and we installed an improved version on our beam line. The collaboration with several Uppsala scientists had developed over the years, after I had met Nils Mårtensson and Anders Nilsson in ’98 in Kaprun, Austria, at the International Symposium on Surface Science, held at a sports hotel high up on a glacier. We called it the 3S meeting for Surface Science and Skiing. Anyway, Anders Nilsson impressed me right away. He was still a graduate student but understood things at an amazingly deep level. We hit it off and over the next few years became really good friends. The work on our ALS beamline was driven by Anders who moved to Berkeley and made a name for himself by applying x-ray emission spectroscopy to surfaces. He later became a professor of Photon Science at Stanford and is now a professor at KTH Stockholm. Anders had this incredible intuition and was willing to stick out his neck. He has done some amazing work on the properties of water, the most important substance on earth, and he seemed to have already envisioned everything in his head, before proving it through experiments. Anders taught me the importance of imagination in science, famously touted by Einstein.

Around 1997, Tom Russell and Hugh Brown (one of my running buddies), who worked on polymers at Almaden, told me about a fascinating science problem – and it had to do with IBM’s business - yeah! It wasn’t in magnetism; it was in flat-panel displays.

Phillips:

Tell me about it…

Stöhr:

At the time, the highest resolution flat-panel displays were so-called liquid crystal displays that depended on an interesting phenomenon. The liquid crystals (LCs) modulated whether a pixel of the screen was dark or lit up. The LCs where composed of tiny rod-like molecules that in each pixel formed a spiral staircase sandwiched between two polymer layers. The spiral twist enabled transmission of polarized light but you could straighten them out with a voltage and the pixels when dark.

The complete display was very complicated high technology, but it depended on an arcane low tech process to anchor the liquid crystals at right angles on the polymer layers to get the stair case twist. It consisted of rubbing a polymer surface – in practice done by large rolling brushes. When you put the liquid crystals on a rubbed surface their molecular rods aligned along the rubbing direction. So they first thought, “Oh, it’s simple, rubbing causes grooves like plowing a field and the rods just lie down in these grooves.”

The model seemed plausible but I had my doubts. We had earlier developed the NEXAFS technique which through its polarization dependence can detect the orientation of molecular bonds near surfaces. So we investigated a lot of different polymer films that had been rubbed. They showed preferred molecular orientations (e.g. in phenyl rings) that were in some cases perpendicular to the rubbing direction. When you put liquid crystals on these surfaces they did not align along the rubbing direction, but rather along the direction of molecular orientation at the polymer surface. So it wasn’t mechanical grooves. The alignment was rather caused on the atomic level through preferential molecular orientation at the polymer surface.

Liquid crystal alignment was actually worked on at IBM Yorktown together with IBM Japan which had a joint production facility with Toshiba. The rubbing process involved big rollers that had to get cleaned all the time due to debris buildup and they were keen to replace it. People at Yorktown were trying a lot of things – trial and error – and in the process had bombarded one of the typical polymers with a directional ion beam, and liquid crystal alignment occurred.

So my colleague Mahesh Samant and I looked at these surfaces with NEXAFS and found that the alignment had nothing to do with the polymer. It was completely destroyed by the ion bombardment and turned into a very thin amorphous carbon layer. The layer, surprisingly showed a small orientational ordering of carbon-carbon bonds – a very small effect. Larger orientational order is known to occur in so called nematic liquid crystals and the bombarded carbon surface had a similar property. So we suggested, “We don’t need to use polymer films at all. Just deposit a carbon film, run the ion beam over it, and the directional bond breaking will give you the needed alignment template - and - the process can readily be integrated into manufacturing”. They first didn’t believe us but checked it out right away, and it worked beautifully. [Laughs] This was a big thing and we got several patents on the process and it was used to build the highest resolution displays.

Mostly, but there was a twist. I also became a professor at Stanford that allowed me to have students – that was very important to me.

Okay, I forgot a key point about how I ended up at SLAC/Stanford. In the ’90s people started thinking about x-ray free-electron lasers (XFELs). The development story is quite complicated because there were different concepts of free-electron lasers. The first one was developed by John Madey in 1971 at Stanford, but it was based on a storage ring that had a long straight section. You would just make the emitted x-rays go back and forth with mirrors in this section and synchronize it with the circulating electron beam. Every time an electron bunch came by the previously emitted and reflected light would interact with it and create some ordering of the billion electrons in a bunch. The ordered electrons emit x-rays in sync (as in a choir) and the intensity gets higher and higher, similar to a conventional laser. This scheme was limited by the extremely low x-ray reflectivity of mirrors, so that the scheme could not be extended into the x-ray range.

Then in 1980, two Russian scientists, A. M. Kondratenko and E. L. Saldin at Novosibirsk and a little later three Italian scientists, R. Bonifacio, C. Pellegrini and L. Narducci came up with another concept, “Do away with the mirrors and just make an exquisite electron beam”. If you run that tiny compressed electron bunch through a long wiggly section called an undulator, then the radiation that is emitted from the back side of this electron bunch defines a wavelength and electrons in the front of the bunch get modulated by that wavelength, ordering essentially in sheets. The in-phase superposition of the radiation from electron sheets builds up from noise. Pellegrini coined the name for this process `self-amplified spontaneous emission’ or SASE, in analogy to the optical process known as amplified spontaneous emission.

SASE requires a very high-quality linear accelerator (linac), not a circular accelerator as used for making synchrotron radiation. In a linear accelerator the electrons rapidly get up to nearly the speed of light and then basically coast without emitting x-rays. This is good because emission of x-rays causes kickbacks on the electrons leading to blow up of the beam size. So you can avoid this in a linac and instead do all kinds of gymnastics with the relativistic electron bunches – you can squeeze them to a thousand times smaller size than in in ring. Only when you have made the smallest possible bunch is it sent through a long undulator to make x-rays. But you can’t use the electron bunch again and discard it.

This SASE concept was tested first by producing infrared and visible light. It worked. Claudio Pellegrini at UCLA then contacted Herman Winick at SLAC and they are the true fathers of LCLS. Claudio is retired from UCLA and living close to SLAC, where he stills interacts with the accelerator scientists. He’s in his mid-eighties and I’m always keeping my fingers crossed that he wins the Nobel Prize…

In the US, SLAC was a natural choice because of its existing 2 mile long linear accelerator that wasn’t used anymore! [Laughing] So the challenge was to get the project funded by the Department of Energy - DOE. In 1998 a committee was appointed by DOE to address the feasibility and capabilities of novel x-ray sources. The outcome was the so-called Leone Report. It specifically emphasized that the new machine, if built, should produce hard x-rays. This aided SLAC since it had the biggest linac and a facility could be built there much cheaper.

My own take - as a soft x-ray lover - is that around ’98 many laser scientists still believed that upconversion of conventional lasers was a better path of getting to soft x-ray energies. This turned out to be wrong - even today, the upconversion concept is limited and the intensity falls off a cliff around 100 eV – not the envisioned 1000 eV!

Phillips:

Right.

Stöhr:

So it became clear what SLAC had to do, “We need to make the case to build this thing. From a financial point of view, this is a no-brainer, you can’t build it any cheaper anywhere else. The key is the scientific case. We need to tell the DOE why they should cough up a half a billion or a billion dollars”. We didn’t really know at the time how much it would cost, but it was hundreds of millions.

So in late 1998 Keith Hodgson from SLAC/Stanford approached me (I was still at IBM), “Well, Jo, would you be willing to put together the scientific case?” I said, “Wow, this is fascinating and allows me to learn a lot,” so I volunteered. Since DOE likes national laboratories to work together I asked Keith, “How about if we bring in Gopal Shenoy from Argonne to help? I’d love to work with Gopal”. In the meantime, Gopal had also left Munich and had gone back to Argonne to see the light [Laughs]. He had become one of the Directors at the Advanced Photon Source. So Keith agreed, “Fine.”

I said, “Let’s get rolling,” and we organized a lot of meetings and brought in scientists from the synchrotron and laser communities in the US and Europe. The x-ray community was still very ignorant about short intense pulses and so was I. I remember thinking, “What is a femtosecond? Why is this important?” So we x-ray scientists had to first learn a lot of things that the laser guys already knew. [Laughs] In the end, we all realized that we had to come up with x-ray experiments that could the give new information on the combined ultrasmall and ultrafast worlds.

Phillips:

Right.

Stöhr:

So atoms move at the speed of sound, and it turns out that’s about a picosecond or so. But when you want to actually look how electron bonding changes you need to be faster by a factor of about 1000 – a femtosecond. Since electrons are lighter they can move faster. It’s just the old inertia thing: the smaller, the faster. So we finally got it into our heads – “Ah, we can now dream of seeing motions on the intrinsic time scale of the building blocks of matter, the atoms, and the “glue’’, the electron density, that holds them together.”

We wrote a report in 1999. Gopal and I were the editors, but it was the work of many scientists. We weaned the scientific case down to six experiments that we envisioned. This required six different experimental stations – three close to the undulator source, three far away – about 300 m. In the Near Hall the beam was very small and concentrated and it became larger in the Far Hall. So this committee came up with a conceptual plan what needed to be built.

The report clearly contributed to me accepting an offer from Stanford around the same time and I started at Stanford at the ominous Y2K date 1-1-2000. We then had to go to Washington and sell it to a committee called BESAC (for Basic Energy Sciences Advisory Committee) of DOE. I was actually a member of BESAC, so I had to recuse myself [Laughing] and sit in the audience. We had thoroughly practiced the presentations. They had to hit the right points, like how revolutionary this was, why SLAC was the right place to build it, and so on...

To make a long story short, the committee and DOE liked it. I give credit to Pat Dehmer, who was the Director of Basic Energy Sciences in DOE at the time. She supported the idea, cleared bureaucratic hurdles and made it happen. [Chuckles] She said, “We want this. We’re going to do this,” and that’s the beginning of LCLS.

Phillips:

You beat the speed of sound, by which the atoms move, with the speed of light! We all know this phenomenon – we see lightning before we hear the thunder. [Laughs] So this turned out to be a winner, and I’m sure, Nobel Prizes will be won in the future where scientists unravel the structure of membrane proteins. So again, one of the big winners in the development of modern x-ray sources has been protein crystallography.

Another area is catching a glimpse of atoms and molecules in action – which has been termed femto-chemistry. The idea is to make a movie of how chemical bonds are broken or formed. Remember, the smaller the faster and one therefore needs both, ultrashort x-ray pulses to catch the molecule in action and sufficient x-ray intensity to get a good picture in the form of a diffraction pattern or an energy spectrum into the “camera”.

Phillips:

Yeah. Yeah, that would be fascinating, too.

Stöhr:

It’s the trick used when taking a picture with a flash. When you take a picture of a hummingbird in flight, the wings are blurry because the mechanical shutter speed of the camera is too slow. So you use a flash which is faster. The flash-photographs are then assembled into a molecular movie, right?

Phillips:

Yeah! It’s been a long time since I took chemistry or organic chemistry, but that would be fascinating.

Stöhr:

Now when it comes to what is called materials or condensed matter science, we’re still struggling a bit, in my opinion, to find the killer app of XFELs. Like the chemistry case it will most likely involve the study of the temporal evolution of phenomena - dynamics. But when you study materials or solids, the intense beam melts them or blows them up (evaporates them). For molecules and macromolecules you can just inject new ones into x-ray beam, but the sample refreshment process is more difficult for solid samples. In principle, the sample has to be replaced after every shot. So novel schemes and techniques have to be developed. There will certainly be applications, but I still see the use of synchrotron radiation as the main tool for the study of solids.

There is another area where x-ray lasers will undoubtedly have an impact. A question, Lawrence Livermore or Los Alamos National Labs are interested in is what matter looks like in extreme conditions. Such conditions may exist in stars but can be created on earth only for an instance of time, like by explosion of a bomb. [Laughing] Again, we want to catch a glimpse of the newly formed state of matter that only exists for a fleeting moment. You can actually produce such states of matter by zapping a material with an extremely powerful laser or with an XFEL pulse, but to see what you have created you need an XFEL to see the atomic structure. This field of study goes by the name “matter under extreme conditions”.

Phillips:

Are there any comparable facilities to LCLS being planned?

Stöhr:

Oh yeah, we haven’t talked about that yet. I should say that it wasn’t LCLS alone that opened people’s eyes about the capabilities of XFELs. XFELs have been built in Germany, Japan, Italy, Korea and Switzerland and plans exist in other countries. In a sense, LCLS got lucky. It benefitted from the prior existence of the FLASH facility in Hamburg, Germany, which gave us a glimpse of what fast and intense pulses of deep ultraviolet radiation can do. Also the European XFEL facility which was conceived around the same time as LCLS was delayed due to its more ambitious scope and because it was politically more difficult to get the buy in from the different countries.

Phillips:

[Chuckles] Right.

Stöhr: