Kenneth Melvin Evenson, NIST fellow and distinguished laser metrologist, died at his home in Sunshine Canyon, Colorado, on 29 January 2002 after a two-year battle with Lou Gehrig’s disease. He was an inventor and experimentalist par excellence in the fields of laser metrology and spectroscopy.
Ken was born on 5 June 1932 in Waukesha, Wisconsin, but grew up in Montana. He received a BS in physics from Montana State College in 1955. That year, he married his classmate Vera Stucky, a microbiology major, and they left for a year of studies on Fulbright scholarships at the University of Tübingen. In 1963, he received a PhD in physics from Oregon State University. His research, done under the guidance of David Burch, was on the gas phase recombination of nitrogen atoms using electron paramagnetic resonance.
After graduation, Ken took a job at NIST in Boulder with a group that was studying electrical discharges. His long career at NIST was characterized by a remarkable series of inventions and developments, more than 300 publications, and highly productive collaborations with scientists worldwide. One of his first projects, done in 1964 with Herb Broida and Fred Fehsenfeld, was to test and develop microwave discharge cavities. Ken’s design for a compact, quarter-wave cavity powered by a 2450-MHz magnetron was an immediate success. Their 1965 paper (published in Review of Scientific Instruments), which included engineering drawings, remains one of the most requested and cited NIST publications. Ken’s device is still used in laboratories to generate emission spectra, atoms, radicals, metastables, and ions, and is commercially marketed as the “Evenson cavity.”
Another collaboration with Broida led to the development of microwave—optical double resonance, a powerful spectroscopic technique. In their experiments, fine structure transitions in cyanogen (CN) radicals were optically detected using modulated microwave pumping. This early 1960s study foreshadowed generations of double-resonance experiments, which have evolved into the myriad of two-photon spectroscopic techniques in use today.
Much of Ken’s research involved measurements in the far-infrared region. He was a pioneer in the development and discovery of many new FIR lasers powered by electrical discharges or IR pumping. In 1968, he led a team that developed laser magnetic resonance (LMR), a sensitive spectroscopic method for detecting paramagnetic species using the Zeeman effect and a fixed frequency FIR or IR laser oscillator. The method has been exploited around the world to study the spectra of transient species, including ions, atoms, and radicals. The excellent sensitivity of LMR has led to its application by chemists as a detector to study the kinetics of atmospheric and combustion chemistry.
During the mid-1980s, another powerful tool developed in Ken’s laboratory was a tunable FIR source. He achieved this source by mixing tunable and fixed-frequency IR laser beams. The spectra of many trace nonparamagnetic species in the FIR region has been revealed using tunable FIR techniques.
One of Ken’s most significant achievements was as the lead researcher on a team of NIST scientists who, in the early 1970s, developed and improved methods for measuring optical frequencies. Using the NIST atomic clock as the standard, they created a ladder of highly accurate frequency standards from the microwave region up to a measurement of the frequency of the methane-stabilized helium—neon laser at 3.37 µm. The result, combined with an accurate measurement by John Hall and Richard Barger at JILA of the wavelength of the laser, produced a 100-fold improvement in the value of the speed of light. Recognizing c as one of the most fundamental of all physical constants, Ken and his coworkers suggested in their 1972 Physical Review Letters article that one could eliminate the problem of combining independent time and length standards to determine c by redefining the length standard as the distance traveled by light in a fixed period of time. A decade later, after their result for c was confirmed and improved upon by measurements in other laboratories, the meter was redefined as the length of a path traveled by light in a vacuum during a time interval of 1/299 792 458 of a second.
Although Ken officially retired from NIST in 1999, he continued his research on electrical discharges until his declining health prevented him from working. His extensive contributions to physics have been acknowledged by many awards and honors: In 1981, the British Chemical Society honored him with the Spiers Memorial Lecture and Prize. Five years later, he won the Humboldt Research Award from the Alexander von Humboldt Foundation in Germany. From the US Department of Commerce, he received a Silver Medal in 1971 and a Gold Medal in 1974. In 1991, he was awarded the Earle K. Plyler Prize by the American Physical Society.
Ken had a wonderful sense of humor and didn’t take himself or the events of life too seriously. He loved brewing and consuming beer, fly-fishing, camping, and hiking. He advocated the preservation of open space and nature. For 30 years, he served as chief of the local volunteer fire department. He supported his wife’s interest in mycology, contributing his expertise in optics and photography in the preparation of her book on mushrooms of Colorado.
A highly motivated and creative researcher, Ken exhibited incredible enthusiasm and optimism every day he spent in the laboratory. He made experiments work because he expected them to work and confronted setbacks or failures with a list of new approaches to try. His attitude was infectious. He accumulated a broad circle of friends and colleagues throughout his career, including many who periodically returned to Boulder to work on a project in Ken’s lab. At a symposium in 2001 honoring Ken, John Brown, a physical chemist at Oxford University and frequent visitor to Ken’s lab, described him as the “most creative and gifted laboratory scientist I have ever had the pleasure to work with.”
