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Spectroscopy

A primer

By: Dimitry Zakharov

Dimitry Zakharov is a PhD Candidate (ABD) in History at the University of Saskatchewan supervised by Dr. Erika Dyck. His research interests include the history of health and medicine, the history of biology, and the history and philosophy of science. 

Spectroscopy 101

At its most fundamental, spectroscopy is the study of how light interacts with any type of matter, from complex materials to atoms and sub-atomic particles, like electrons. There are many types of light – the visible light that the human eye can detect, but also invisible forms, such as x-rays, microwaves, and ultra-violet light. Depending on what you want to learn about a given type of matter, spectroscopists will use the different types of light to study how they interact with various materials and molecules in order to learn more about the properties of these substances. “Spectroscopy is the study of matter in general — gases, atoms, molecules, solids,” says Alex Moews, the Canada Research Chair in Materials Science at the University of Saskatchewan.

When different types of light, or radiation, are directed at a substance, the interaction creates what Prof. Moews describes as “a fingerprint” that can be detected by a spectrometer and used to identify that material’s unique characteristics, such as its molecular structure, or the way it bonds to other molecules. “That’s what spectroscopists work with – fingerprints of absorption or emission of light by the molecule,” says Robert McKellar, a retired molecular spectroscopist with the National Research Council of Canada (NRC). “By analyzing the pattern of light that’s absorbed or emitted, you can find out a lot of details about the molecule itself.”

The entire field of spectroscopy is intimately connected to the technological evolution of spectrographs, the devices that can capture and record those fingerprints. “Over time, these detectors became better and better,” observes Eva Hemmer, a professor of materials chemistry at the University of Ottawa. “Of course, this opened up whole new spheres for researchers to explore more details, faster processes, you name it.”

A brief history of spectroscopy

As with so many fields of science, spectroscopy stands on a foundation of previous discoveries by physicists and chemists, many of them enabled by pure experimentation and technological innovation, and even chance. As Herzberg observed in a 1984 speech, “The history of science is full of examples of completely unexpected discoveries and inventions that have changed the course not only of the history of science but of history generally.”

In the introduction to the 1959 English translation of Max Planck’s philosophical and theoretical writings, James Franck, the 1925 Nobel Laureate in Physics and one of the founders of spectroscopy, commented on the iterative relationship between advances in science and advances in technology:

Atomic theory, nuclear physics, and many elements of quantum electrodynamics all trace their origins to the development of simple vacuum glass tubes filled with gases like hydrogen, argon or neon. These tubes, which had two electrodes (cathode and anode) at either end, were the invention of Heinrich Geissler (1815-1879), a glassblower who worked at the University of Bonn. The precursors to contemporary neon lights, “Geissler tubes” produced beautiful and brilliant coloured light when filled with different gasses. They were curiosities and evidence of scientific progress in 19^th century Europe. Many artisans created their own designs and versions, and these Geissler tubes were shown at fairs, festivals, and markets. Consumers purchased them and took them home to show off to friends and family.[ii]

At the same time, physicists became interested in the properties of Geissler tubes. Julius Plücker (1801-1868), a professor of physics at Bonn, collaborated with Geissler on a series of 294 experiments, using various configurations and modifications of the tube’s design. He noted that the light beam inside the tube could be ‘deflected,’ or bent, using a magnet. This observation represented a pivotal moment in the history of spectroscopy, paving the way for a variety of novel questions and experiments across Europe involving the use of light to analyze substances.[iii]

Creating diagrams and drawings depicting the structures of atomic or sub-atomic particles, as well as representing the changes that these particles underwent, gradually became a vital part of physics. These particles could not be seen through any microscope. Rather, they could only be surmised from the mathematical equations devised by physicists to describe their geometry and topology. Historians Lorraine Daston and Peter Galison point out that these drawings, which began to appear in about the second half of the 19^th century, had to account for features like statistical chance inherent in the ways that atomic and subatomic particles move, and also the three-dimensional nature of their observed behavior. Indeed, Daston and Galison argue that these diagrams represented a new form of scientific objectivity.[iv]

One of the best known is Niels Bohr’s atomic model. Later quantum mechanical iterations focused on smaller sub-atomic particles, such as hadrons and quarks. Mid-20^th century physicist and mathematician Richard Feynman’s diagrams representing particle scatter during high energy collisions became a standard means of depicting quantum processes.

By contrast, the physicists working with spectrographs depicted atomic and sub-atomic processes very differently, representing the composition of molecules indirectly, through the distinctive spectra cast off by every molecule exposed to electromagnetic light.

Astronomers were also finding applications for spectrographs to analyze the light emitted from distant stars, planets, and gaseous clouds — techniques first developed in the 19th century by scientists like William Hyde Wollaston (1766-1828), a British chemist, and Joseph von Fraunhofer (1787-1826), a German physicist and lens maker. Fraunhofer identified the distinctive bands — later known as Fraunhofer Lines — on a spectrum that indicated distinctive patterns of the absorption or reflection of light from the sun and other stars.  

A century later, Otto Struve (1897-1963), the director of the Yerkes Laboratory in Chicago, commissioned the construction of an auto-focusing spectrograph that could be attached to the 40-inch telescope housed at the observatory That spectrograph was itself only seven inches in diameter and housed within an aluminum tube that could be attached to the telescope as needed. Other innovations followed, as researchers developed more sophisticated spectrographs that could capture ever more precise information.

Herzberg’s Contribution

Physicists like Max Born, Werner Heisenberg and James Franck were making significant advances in quantum physics in early decades of the 20^th century. During his university years, Herzberg attended lectures given by some of the leading physicists of that time. In fact, he later worked with both Born and Franck in 1929 at the Institute of Theoretical Physics in Göttingen, after completing his doctoral degree.[v]

In 1927, Herzberg published a paper describing the spectrum of the hydrogen atom. The black vertical lines denoted those parts of the electromagnetic spectrum that are not visible when light passes through hydrogen gas. Since the electrons in a molecule can only absorb specific amounts of energy before they either change states or break off (a process known as “ionization”), such diagrams can be understood as the unique fingerprints of the molecules present in the gas. In 1934, shortly before they left for Saskatoon, Herzberg and his wife Luise also worked out the band system of the so-called “PN molecule” – Phosphorus mononitride, or a molecule made up of nitrogen and phosphorous. (In the 1980s, this gas would be identified as present in the atmospheres of planets like Jupiter and Saturn.)

While most such fingerprints are now known, Herzberg and his contemporaries during the 1920s and 1930s were starting from scratch. They were using spectroscopy to investigate the natural world atom by atom and molecule by molecule. “In the very early days,” says Prof. McKellar, “he was looking at atoms, but then fairly quickly shifted to the spectroscopy of molecules, which in a sense was more challenging and at the time very new.”

After Herzberg stepped in as director of the NRC’s division of physics, in 1948, he moved quickly to establish a team of researchers and a laboratory with several different spectrographs. While commercial spectrographs were available, “any laboratory expecting to do forefront research in high-resolution spectroscopy of gases had of necessity to build its own instruments.”[vi] Herzberg and his principal assistant Alex Douglas first constructed a seven-metre “concave grating” spectrograph similar to the one he built during his time at the University of Saskatchewan. (Concave grating involves etching lines onto a bowl-shaped mirrored surface, which defract light, but not necessarily towards the focal point.) Due to the expense and complexity of obtaining a high-quality diffraction grating, Herzberg borrowed the grating from the U of S so his new team could begin using the spectrograph as quickly as possible.[vii]

From the beginning of his time at the NRC, Herzberg and his spectroscopy group set out to research the mysterious and hard to detect “free radicals.” These are highly volatile molecules that ‘detach’ from other molecules and then bond rapidly to new molecules. Herzberg’s work particularly focused on the CH₂ free radical – known as methylene, an odourless gas. The difficulty in studying free radicals was that they only exist independently for a very brief moment in time.

“They’re very important because free radicals are, in a sense, the intermediate chemical reaction when you have a reaction of two molecules to create two other molecules,” explains Prof. McKellar. “Often, we don’t really know what the exact structure of free radicals and doing spectroscopy will tell you the structure.”

The behaviour of a free radical is somewhat analogous to the exotic sub-atomic particles studied at facilities like CERN’s Large Hadron Collider in Switzerland. Just like “hadrons” and “bosons” that only exist for millionths of a second before disappearing, the challenge with free radicals was creating the conditions for their appearance and then designing scientific equipment capable of observing them for those very short periods when they exist.

Herzberg’s spectroscopy group developed a so-called ‘flash discharge’ technique to study free radicals. A long glass tube would be filled with gas — deuterium or heavy hydrogen — and then be exposed to both strong flashes of light synchronized precisely to an electrical charge sent through the gas in the tube. The light created by this process would then be directed through a spectrometer for analysis.

Herzberg’s study of free radicals occurred concurrently with improvements in sensors and early computer technology, and these innovations motivated other research groups to develop similar techniques. According to Prof. McKeller, one of Herzberg’s post-doctoral fellows, Donald Allan Ramsey, was instrumental in developing the flash discharge technique at the NRC. At about the same time, a Cambridge team run by Prof. George Porter (1967 Nobel Prize in Chemistry) developed a similar process called flash photolysis, also used to study free radicals. The development of these techniques led to the better understanding of the CH₂ free radical, which was used to determine the presence of the C₃ free radical given off by a comet.[viii] That discovery was among the achievements that led to Herzberg’s 1971 Nobel Prize.

While his award was for chemistry, Herzberg’s work spanned both physics and chemistry. Indeed, at the level of the problems which occupied Herzberg’s scientific work, chemistry was physics and physics was chemistry. One of his major contributions was the understanding of molecular level problems according to quantum level principles.

Conclusion

Over the past century or so, spectroscopy, as a technique for studying matter, has been used in a remarkably broad range of applications.

The emergence of astrophysics is one of the most critical. “Most of astronomy occurs basically from the vantage point of the earth, either telescopes from the ground or orbiting the Earth,” comments James Di Francesco, an astrophysicist with the Herzberg Institute of Astrophysics. “But the universe is so huge, we cannot travel to visit and sample it ourselves, so we are left with whatever means we can to perceive and understand it as best we can. And the primary way in which astronomy has been able to do that is through light. Stars emit light, the sun emits light, planets reflect light. We use this messenger of light as a way of trying to understand the universe. Spectroscopy has been an extremely powerful technique in actually enabling us to understand the universe.”

Yet there are many more terrestrial applications, from medicine to food safety, agriculture, quantum computing and even forensics. Art historians use infrared spectroscopy to identify forgeries by using these techniques to analyze the chemical structures of paints.

Prof. Moews’ U of S research team uses highly sophisticated light sources, such as synchrotrons, as well analytical approaches such as “emission spectroscopy” – which examines the wavelengths of the light emitted by atoms or molecules as they transition from a high energy state to a more stable form – to develop better lighting materials or less corrosive materials or superconductive materials. “This could be to find a new material that has new properties that haven’t existed before,” he says. “The range of application is basically limitless.”

As with other game-changing scientific discoveries, this extraordinarily versatile analytical tool traces its roots to the way that Herzberg and other scientists tackled the kinds of technical puzzles that they confronted in their work. Thomas S. Kuhn, a physicist and philosopher of science, argues that puzzle-solving occupies a special place within the inner workings of modern science. A scientific field has conceptual and methodological rules, he explains. The act of solving emerging puzzles may not necessarily lead to new discoveries. Rather, this intellectual work may serve to further clarify and validate existing concepts.

Herzberg’s work straddled both the realm of technological innovation and bold scientific inquiry. He eventually published 274 articles on molecular spectra and molecular structure. As Max Planck, one of Herzberg’s mentors, wrote, “[the] goal [of the scientist] is nothing less than the unity and completeness of the system of theoretical physics… not only with respect to all particulars of the system, but also with respect to the physicists of all places, all times, all peoples, all cultures. Yes, the system of theoretical physics demands validity not merely for the inhabitants of this earth, but also for the inhabitants of other planets.”[ix]

[i] James Franck, “Introduction” in The New Science by Max Planck, James Murphy trans., (New York: Meridian Books, 1959): xvii.   

[ii] William H. Brock, William Crookes (1832–1919) and the Commercialization of Science, (New York: Routledge, 2012).

[iii] George Sarton, “The Discovery of X-Rays,” Isis 26 No. 2 (1935): 349-369.

[iv] Lorraine Daston and Peter Galison, Objectivity, (New York: Zone Books, 2007): 253-254.

[v] Boris Stoicheff, Gerhard Herzberg: An Illustrious Life In Science, (Ottawa: National Research Council Press, 2002): 43.

[vi] Stoicheff, 224.

[vii] Stoicheff, 232

[viii] Herzberg, The Spectra and Structure of Simple Free Radicals, (Ithaca, New York: Cornel University Press, 1971): 13.

[ix] Max Planck, Acht Vorlesungen uber Theoretische Physik: Gerhalten an der Columbia University in the City of New York im Fruhjahr 1909 (Leipzig: Hirzel, 1910): 6.