Profile of Margaret M. Murnane

Plucking an electron from a molecule and smashing it back into the same molecule is a highly sensitive way to probe the intricacies of molecular dynamics. This process for investigating how atoms move within a molecule, known as high harmonic generation, is initiated and controlled by lasers. In her lifelong study and use of lasers, Margaret M. Murnane, Fellow of JILA (formerly the Joint Institute for Laboratory Astrophysics) and faculty member in the Departments of Physics and Electrical and Computer Engineering at the University of Colorado (Boulder, CO), has moved farther and farther away from the visible region of the light spectrum. Murnane pioneered the development of femtosecond lasers to generate laser-like beams that span from the ultraviolet to the soft x-ray regions of the spectrum. In her Inaugural Article in this issue of PNAS (1), Murnane and her colleagues describe how x-rays generated by a molecule can be used to probe the internal motion within the same molecule. This method shows promise as a way for imaging molecules undergoing ultrafast structural transformations, including the fundamental chemical action: the making and breaking of chemical bonds.

Archimedes and Rural Ireland

Elected to the National Academy of Sciences in 2004, Murnane was born in 1959 in a rural part of County Limerick, Ireland. Her father, an elementary school teacher, never had the opportunity to attend college, but Murnane remembers vividly how he loved science. He had wanted to be a botanist and attempted to teach his young daughter the Latin names of plants. “I can’t remember any of them because I was busy scouring the books he would bring home from the library on astronomy and mathematics,” Murnane says. “My father did create a physicist even though that was not his intent. If I solved a math puzzle, then the reward was chocolate or a new science book from the library.” At 8 years of age, Murnane recalls reading one book in particular, with an illustration of Archimedes in the bathtub. “I had this Archimedes moment,” she says, when she realized that she, too, could learn by observing. “I didn’t know any scientists or exactly what they did,” she says. Although her high school was one of few in Ireland that taught physics, “I didn’t see a female physics professor until I was in grad school,” she says.

The lack of female role models, however, did not dampen Murnane’s enthusiasm for learning. “In Ireland, at the time, high schools were segregated” by gender, she says. Murnane found herself surrounded by girls with ambition. “All of the students in my class were hoping to study medicine, dentistry, or law,” she says. In 1977, Murnane began college at University College Cork (Cork, Ireland) and majored in physics. “Academically, it was very challenging,” she says. She remembers examinations referred to as “unseen” experiments: undergraduates were “locked in a room by oneself,” she says, to perform an experiment while graders watched from behind closed doors. Despite the challenging curriculum, Murnane’s professors were highly supportive. They encouraged her to apply to doctoral programs, something that, at the time, required study in the United States.

Margaret M. Murnane

Murnane felt reluctant to leave Ireland. “I stalled by doing a master’s degree. I found that I really loved the sense of discovery. It made me realize that I really wanted to be a scientist, even if it meant leaving Ireland,” she says. While earning her M.Sc. degree, Murnane’s advisor, Michael Mansfield, took her to Dublin for a conference. At the conference, she attended a talk given by one of Ireland’s few female astronomers at the time. Murnane thought that she gave “a marvelous talk” and recalls that “she was totally comfortable and at ease.”

Long Project, Fast Laser

While considering doctoral study, Murnane, who had never traveled abroad, knew several students who had gone to do programs in the United States. One student at the time, Steven Fahy, had moved the year before with his family to the University of California, Berkeley (Berkeley, CA). He wrote a letter to Murnane, saying he thought she would be comfortable at Berkeley. “I trusted Steven when he said I would like it here,” she says. At Berkeley, Murnane chose to work in the laboratory of Roger Falcone, an assistant professor who had just arrived at the university. She chose his group because of her interest in lasers, and Murnane and Falcone spent the first months ordering parts to set up the laboratory in Birge Hall. “Before we had to cover up the windows to see our dim laser beams, I had a laboratory with a spectacular view of the Golden Gate Bridge,” she says. At Berkeley, Murnane built ultrashort lasers, emitting fast pulses that act like a strobe light and are capable of freezing the motion of some of the fastest processes in nature. Falcone and Murnane thought that these laser pulses could be more powerful in the study of chemical bonding if the lasers produced x-rays instead of visible wavelengths of light (2).

For her doctoral thesis, Murnane built an intense laser that shrank the duration of the pulse to ≈100 fs (10⁻¹³ s), shorter than the explosion time of a solid. As a result, when she irradiated a solid with the laser pulses, it heated up to high temperatures, generating a short-lived plasma that emitted fast bursts of x-rays lasting less than a picosecond in duration (3). Murnane recalls these experiments as a long and challenging undertaking, taking her a year to build the laser, 6 months to refine it, and 2 years to show that it generated fast x-ray pulses.

Murnane graduated with her Ph.D. in physics in 1989, and in 1990 she received the Simon Ramo Award from the American Physical Society for her thesis. While at Berkeley, Murnane met her future husband and lifelong collaborator, Henry Kapteyn, also a Ph.D. student in the program. “Henry pretended to need to discuss one of the homework problems assigned in our lasers class,” she says. She soon realized that Kapteyn was one of the smartest people she had met, and the two began collaborating.

Murnane and Kapteyn married in 1988 and moved to Washington State University (Pullman, WA) in 1990 and set up a joint laboratory. “Many large universities would not have allowed us to do that,” she says, but collaboration in the fast-moving, highly competitive field of ultrafast laser science was critical. “We were developing new femtosecond lasers, so we needed to work together,” she recalls. She and Kapteyn heard at a conference that Wilson Sibbett of St. Andrews University (Aberdeen, Scotland) had developed a new type of femtosecond laser. This new ultrafast laser used crystals of titanium doped in sapphire, similar to the crystals of ruby used in the first laser ever demonstrated. Although she says that most of the field was excited that this laser could produce pulses as short as 60 fs in duration, Murnane and Kapteyn realized that this speed was nowhere near the theoretical limit of <10 fs. Murnane and Kapteyn, and others, began to look for ways to make this laser emit even shorter pulses—the race had begun.

Coherent Choices

Murnane explains that, although many lasers have a single frequency or color, to make a fast light pulse, one needs to add together light waves of many different colors. If different waves are added together coherently, constructive interference between the different waves can create a pulse at one point in time, although destructive interference occurs elsewhere. When a short pulse travels through the material in the laser, however, the red colors move more rapidly than the blue colors, tearing the pulse apart in the process. This phenomenon is known as dispersion and is responsible for the ability of a prism to split white light into its constituent colors.

Murnane and her team realized that, by using the right kinds of prisms, they could keep the light waves of different colors all traveling with the same speed in the laser. They rapidly discerned how to generate an 8-fs pulse directly from a laser and how to identify the new physics that held the secret to the extreme stability of the fastest laser to date (4, 5). Murnane and her group also disseminated detailed instructions on how to reproduce their laser to hundreds of scientists worldwide. “It was a very exciting time because people were breaking records all the time,” she says. At Washington State University, Murnane collaborated with theoretical physicist Ivan Christov at the University of Sofia (Sofia, Bulgaria). Christov became a long-time collaborator and has worked with Murnane and Kapteyn most summers since 1993.

In 1996, Murnane left Washington State University, which she had come to love. “We were recruited by the University of Michigan. It seemed like a great opportunity to move to a large university with many potential collaborators,” she says. Several students moved with Murnane and Kapteyn. “We sort of had a convoy going across the country in the middle of winter in a snowstorm,” she recalls. At the University of Michigan (Ann Arbor, MI), Murnane’s team continued to develop high-power lasers. They also continued to explore the process of x-ray generation from lasers. In her past work at Berkeley, Murnane generated x-rays by focusing a laser beam on a solid to generate a short-lived spark. This process created a burst of x-rays lasting under a picosecond, like the bright flash of a strobe light, expanding in all directions.

Murnane (pointing) and Henry Kapteyn working with undergraduates at JILA: Alix Rivera-albino, Adrienne Van Allen, Edgar Marti-Arbona, Nathan Kirchhofer, Kolt Peightal, and Kyle Douglas.

However, a method of using lasers to generate ultraviolet light with potentially laser-like beam quality had been uncovered in 1987, simultaneously by groups led by Charles Rhodes (6) and Gérard Mainfray (7). In this method, a short laser pulse was focused into a dilute gas. The electric field of the laser ripped the electron from an atom, accelerated the electron away from the ion, and, when the laser field reversed, crashed the electron back into the ion, releasing a short x-ray burst via rescattering. This process, called high harmonic generation, appealed to Murnane and Kapteyn, because it allowed them to generate much faster x-ray bursts than before. The possibility also existed that the beams generated from this process would be truly laser-like and directed.

Unfortunately, the method was inefficient and could not generate short-wavelength x-ray beams, only ultraviolet beams. Murnane and Kapteyn therefore worked to apply the new ultrafast lasers they had developed to improve the conversion of laser light to x-ray light. The exact physical mechanism of the high harmonic generation process was still being unraveled, and the use of few optical cycle pulses made it possible for them to devise new experiments that provided crucial confirmation of the rescattering mechanism (8–10). A breakthrough came at 2:00 a.m. one night when, by filling a glass fiber with gas, Murnane and Kapteyn succeeded in forcing the laser light and x-ray light to travel with the same speed over a long distance. The advance allowed them to increase the efficiency of the laser-to-x-ray conversion (11) and to extend the wavelength range of the laser-like x-ray beams to much shorter wavelengths (9). In collaboration with Christov, they also proposed a method to generate subfemtosecond, or attosecond (10⁻¹⁸ fs), pulses by using an extremely short (5-fs) laser pulse to drive the high harmonic generation process (12).

After 3.5 years in Michigan, another move was in order. “Much of our research requires very specialized, one-of-a-kind instrumentation and electronics,” she says. But in Michigan, their laboratories were 2 miles from the central campus and from the fabricators in other departments whose expertise they needed. So, JILA at the University of Colorado provided Murnane and Kapteyn with an opportunity in 1999 to take advantage of “world-renowned technical support systems,” she says. In 2000, Murnane became a John D. and Catherine T. MacArthur Fellow. “I was very, very surprised to get that award,” she says. She used the funds to leverage matching funds for federal grants. “It really helped us get off to a great start,” she says. The nonmonetary cachet of the award has also come in handy. Murnane recalls how the accolade lent credibility to her support of a new interdisciplinary focus in the university’s physics graduate program, enabling physics students to more easily study biophysics, geophysics, and other interdisciplinary research areas.

From Laser to Laser-Like

One of the fundamental properties of the laser is its ability to produce spatially coherent beams that can be focused to a small spot size comparable to the wavelength. Short-wavelength light sources like synchrotrons and x-ray lasers do not use resonators and, as a result, generate only partially coherent light. At JILA, Murnane and Kapteyn continued their work on high harmonic generation, demonstrating how to create bright, fully coherent laser-like beams at short wavelengths. By containing the gas in a waveguide, the laser beam is guided, allowing for a long interaction length between the laser and generated x-ray beams. As a result, the x-rays that emerge are well directed and truly laser-like. Murnane says that this achievement demonstrated for the first time that any x-ray source could generate a perfect, coherent beam without the use of external optics to improve the beam after the x-ray generation process (13, 14).

Murnane also has pioneered the use of lasers to study physical processes on shorter timescales than femtoseconds, i.e., attoseconds. The physics of high harmonic generation involves complex electron motion that occurs on time scales that are short compared with the time scale of a single oscillation of visible light. In an innovative experiment, Murnane and Kapteyn showed that the physics of this process makes it possible to manipulate the high harmonic generation process by changing the exact shape in time of the laser light driving the process. Murnane points out that this experiment was the first instance demonstrating a process of control of a physical process in the attosecond timescale (15). Murnane and Kapteyn also have pursued a number of real-world applications of coherent short-wavelength light, including using it to see the motion of molecules on a catalytic surface (16) and to see acoustic oscillations in materials and in nanostructures (17).

In her PNAS Inaugural Article (1), Murnane explores how the x-rays generated by a molecule’s own electron can be used to probe how atoms move within that molecule. X-ray and electron diffraction are two powerful techniques long used to explore the nanoworld. X-rays have wavelengths comparable to the spacing between atoms in molecules or crystals; electrons, because of the wave-particle duality, also have short wavelengths. Usually, an external source of electrons or x-rays is used to acquire a diffraction pattern, for example, of a crystal. In Murnane’s work, the electric field from an intense, optical laser pulse first plucks electrons away from the molecule and then accelerates them back toward it. The highly energetic electrons then scatter from the molecule. Rather than measuring the scattered electrons, as might be done in an electron-diffraction experiment, the team measured the x-ray bursts that are emitted when the electrons recollide with the molecule. The wavelength of the recolliding electrons is comparable to distances between atoms in a molecule, and, thus, the x-rays emitted are highly sensitive to the instantaneous position of the atoms within the molecule (1).

“It was a very exciting time because people were breaking records all the time.”

Environment Is Key

Currently, the joint laboratory run by Murnane and Kapteyn is a large international group of researchers with diverse backgrounds. Students from physics, engineering, and chemistry work together on a wide range of projects. “We try to provide an alternative to a top-down mentality and prefer to encourage teamwork. Collaboration and a positive environment is very important to me. I don’t think of science as something one pursues by oneself at one’s desk late at night. Students need to develop good teamwork and communication skills, as well as scientific and technical knowledge, to achieve their goals,” Murnane says.

Murnane is active in getting women involved in science and in improving the academic environment for female scientists. Murnane has served on and chaired the American Physical Society Committee on the Status of Women in Physics. She has also participated on the Site Visit Team to Improve the Climate for Women in Physics for a decade, and chaired the program for 3 years. The goal of this program is to assist department chairs in recruiting and retaining physics students and faculty who are women, a goal that Murnane believes is important for academia if the United States is to take full advantage of its intellectual capital. She recalls that participating in the site visits to other institutions could often be “extremely instructive and extremely frustrating.” Instructive because she found that she was not alone in the problems that she experienced, but frustrating because many women experience the same roadblocks. Murnane thinks that academia should be more concerned with good management practices and metrics, which will encourage the type of changes needed to make science and engineering more attractive to women and minority students. “We need to change in order to attract the top minds to science and engineering,” she says.

Murnane is proud of her long-standing collaborations and for trying to help her students attain their dreams. “There are so many things to do with a science or engineering degree. Henry or I would never want to just clone ourselves,” she says. Murnane finds Colorado to be a wonderful place to pursue science. “We ride mountain bikes and ski, but we’re still pretty much nerds,” she says of herself and her husband. Murnane is excited about future work in using fast-laser and x-ray beams to understand and manipulate nature. Generating brighter x-ray beams with shorter wavelengths will open up new applications in understanding molecular function, cellular tomography, nanoimaging, catalysis, and real-time molecular motion, and Murnane expects advances within a year. “Science gets richer as you get older,” she says. “The more you know, the more collaborators you have, the more opportunities there are.”