Session 1: The Basic Physics of Spectroscopy - Light-Matter Interaction for Fun and Profit

This article presents the main points of a series of four presentations offered by Oxford Instruments Andor, a spinoff of Queen's University physics department established in 1989, focusing on spectroscopy. As a leader in scientific imaging spectroscopy, Andor aims to provide insight into spectroscopy covering the basics and more details in subsequent presentations, to reach potential customers and individuals with special interest in the topic.

Adam Wise, an expert on all sorts of spectroscopy, optics, and metrology started defining spectroscopy according to the International Union of Pure and Applied Chemists as “The study of physical systems by the electromagnetic radiation with which they interact or that they produce”. However, this definition can lead to several questions regarding the nature of the physical systems mentioned, the kind of electronic radiation, the kind of interaction, and how this is measured. Considering the vagueness of the words used in the definition, electromagnetic radiation can be from x-rays to gamma rays and visible light, the physical system can be a living creature, a gas, or even a solution. Finally, regarding the interaction, this can include absorption, scattering, re-emission, and so on. If all these variables are possible, does that mean that simply observing differences in tomatoes’ colors can be considered spectroscopy? Unfortunately, no.

One key distinction Wise highlighted is the absence of quantitative elements in casual observations. Unlike scientific spectroscopy, where precise measurements and numerical data play a crucial role, casual observations lack the rigor and precision required for scientific inquiry. Therefore, even when the human eye is a powerful instrument, it falls short in providing quantitative information, limiting its ability to discern specific colors outside broad categories and wavelengths which is directly linked to wavelength resolution.

Continuing with the analysis of the given definition, it is critical to define electromagnetic radiation which extends beyond visible light, encompassing a broad spectrum of waves, a fact that was confirmed in the mid-19th century, even when the nature of waves was realized earlier in the 17th century. Therefore, the revelation that colors, such as red, green, and blue, are manifestations of the same wave with varying spatial distances between crests (wavelengths) was particularly surprising. This notion prompts contemplation on the nature of color perception and the underlying connection between the seemingly diverse palette of hues, leading to the conclusion that color is a construct of human perception. Even when we can assign numerical values to wavelengths and intensities, our brains interpret these values as distinct colors, suggesting that color is a phenomenon intricately tied to our subjective experience.

Now that wavelengths have been established as a critical concept for spectrometry, it is relevant to acknowledge their connection to energy, starting with how the visible spectrum is not only relevant to human perception of color but also as a form to illustrate the energy ranges among the spectrum. These energy ranges are the consequence of the molecular vibrations and electronic transitions in atoms and molecules occurring. Here, another relevant definition is introduced, linking wavelength and frequency. This relationship is expressed through a fundamental equation involving the speed of light, which remains constant in a particular medium, Therefore, as the wavelength (meters per cycle) increases, the frequency (cycles per second) decreases, and vice versa.

Considering the initial use of the human eye as a detector in the study of electromagnetic radiation offers insight into the progression in the evolution of spectroscopy from the main limitation of the human eye: the difficulty in distinguishing yellow-orange light from a mix of red and green light. Additive color mixing occurs when thinking of green and red lights overlapping to create a perceived yellow color making the observer question if these lights are physically mixing, however, there are nonlinear spectroscopies that can indeed create such mixtures, confirming the limitation. The basis of how our vision works is the use of three photoreceptors for color detection that respond to blue, greenish, and red light, respectively, unfortunately, the separation between red and green pigments is not pronounced, shaping our perceived color experience and affects its sensitivity and accuracy. This also affects the color space perception, limiting the perception of color saturation or depth. Therefore, these limitations can be considered the biggest challenge faced by early scientists who relied on visual observations leading to the exploration of advanced detection methods in modern spectroscopy.

In contrast to human eye capacities, the mantis shrimp presents a remarkable ability to perceive color with unique features such as having up to 12 photoreceptors in its eyes. However, it is relevant to note that whether shrimp truly possesses superior color vision or if it employs an information-processing trick remains a topic of contention among experts in the field. Nevertheless, the existence of these creatures makes clear that there are alternative strategies to achieve remarkable visual capabilities, triggering the search for a tool that could physically separate colors for easier measurement.

In the mid-17th century, luminaries like Isaac Newton began experimenting with prisms to achieve this separation. By allowing a narrow beam of light to pass through a prism, Newton observed the dispersion of light into its component colors, creating a spectrum on a wall. In this setup, the human eye was still the detector, but the emphasis shifted from color vision to the intensity of spots on the wall. Newton's experiment marked a pivotal moment in the history of spectroscopy, as it allowed researchers to establish a relationship between the position of the separated colors and their respective wavelengths. The introduction of prisms as tools opened new avenues for understanding the nature of color and laid the foundation for the development of spectroscopy as a scientific discipline.

The essence of Newton's contribution lies not in the mere observation that prisms create rainbows, a phenomenon known two millennia ago, but in his ability to integrate this knowledge into a mathematical model of light. A drawing from Newton's records reveals his experimental setup, capturing the components necessary for building a spectrometer, an entrance slit that shapes and limits incoming light, a dispersive element (in this case, a prism) that separates light into its constituent colors, and a detector (a white screen on the wall) that captures the spectrum produced. Therefore, this prototype represents the birth of a machine designed to separate colors and facilitate the recording and measurement of spectral information, in other words, a spectrometer. When a wave encounters an angle on a medium with a different refractive index, the wavefronts get bunched up and pivot into the material due to the change in the speed of light transmission within that medium; this is known as refractive index and is the basis of why prisms can separate color. Different wavelengths or frequencies of light have different refractive indices in materials.

As technology and science advanced, the sophistication of spectrometers increased. In the early 19th century, Kirchhoff, showcased a spectrometer capturing more details than Newton's original setup. This occurred thanks to the precision of metalwork during that period that allowed the presence source of electromagnetic radiation (such as a hot flame) and a precision brass machined aperture serving as the entrance slit. This marks a transition into the contemporary era, emphasizing the refinement of spectrometers as sensitive devices for spatially separating colors. Unfortunately, these devices still presented a critical limitation: the human eye as the detector. By rotating the prism, a specific wavelength or range of wavelengths was directed toward the detector, in this case, the human eye, even when at the time the photography was developed, and could have been a replacement for the human eye.

As researchers continued exploring the aspects of spectrometry, they started to elaborate spectra, tables, or visualizations of numerical wavelengths and intensities to analyze the characteristics of light instead of relying on color perception. This led to a jump from qualitative to quantitative measures. A great example of this jump was the study of the spectrum of the Sun performed by Joseph von Fraunhofer in 1815. Fraunhofer's method involved projecting the Sun's spectra through a spectrometer and capturing the results on a large wall, allowing for detailed observations and measurements. As observed, the precision and innovation involved in creating these early spectrometers, marked a significant leap in scientific instrumentation, introducing the concept of spectral resolution. Therefore, going from subjective color experiences to objective and quantitative assessments of the interaction of different wavelengths with material systems provided a more rigorous and standardized approach to studying and understanding the properties of various substances.

The wavelength spectrum, measured in nanometers, is presented alongside a color rainbow as a visual aid. This connection between numerical values and the corresponding colors reinforces the transition from subjective to objective measurements. Going back to the tomatoes' color perception, we can introduce “Tomato Reflectance Spectroscopy” to differentiate the color of tomatoes. Using actual wavelengths, we find green-yellow at 540-550 nanometers, followed by a shift to deep red in the range of 640 to 660 nanometers. Having these measures, it is possible to acknowledge the potential variations in individuals' ability to perceive near-infrared light, highlighting the limitations of personal experiences in certain wavelength ranges. Therefore, spectroscopy allows for quantitative analysis and standardization.

But this is not the only advantage of spectroscopy, beyond the visible spectrum, additional information can be obtained in the near-infrared region. While unripe tomatoes reflect more green light (as observed in the visible spectrum), the near-infrared absorption can be used to measure sugar content or soluble solids content in tomatoes. This expands the utility of spectroscopy into a realm where human vision is limited providing valuable insights into the composition of materials, paving the way for advancements in quality control, agriculture, and various scientific fields.

In tomatoes’ case, exploring the pigments responsible for their color such as lycopene and chlorophyll, can reveal intricate details about these molecules. Two main pigments are highlighted: lycopene and chlorophyll. Lycopene, known for its potential health benefits, contributes to the red color of tomatoes while chlorophyll, arguably the most important molecule globally, plays a central role in photosynthesis. Interestingly, chlorophyll absorbs in the blue and red regions of the spectrum but not in the green revealing that the green color of plants, including tomatoes, is due to the reflection of green light that is not absorbed by chlorophyll. Now it is possible to observe that, spectroscopy as a tool, can also allow scientists to explore the molecular composition of plants and gain insights into their fundamental processes. Regarding lycopene, its absorption includes the blue and most of the green regions, while allowing yellow and red light to pass through thanks to its conjugated structure that alternates single and double bonds, hinting at the importance of understanding color transitions and absorption phenomena.

At the molecular scale, the wavelength appears significant, and the experience for a molecule is not observing waves but rather the interaction with a large electric field oscillating in one direction and then the other. Atoms or molecules are significantly smaller than the wavelength of the electromagnetic wave interaction with the oscillating electric field. The molecules are polarized rhythmically, experiencing a homogeneous time-varying electric field that alternately pushes their positive and negative charges in different directions. When a system, like a molecule, is pushed rhythmically at its resonant frequency, it efficiently absorbs energy from the incoming waves. This is directly linked to spectroscopy because the resonance of molecular components determines which frequencies of electromagnetic radiation are absorbed, contributing to the unique absorption spectra of different molecules.

To summarize the presentation, spectroscopy, as broadly defined, involves the study of physical systems by examining how they interact with electromagnetic radiation. While the electromagnetic spectrum is vast, practical considerations often limit studies to visible or near-visible wavelengths. Instrumentation, such as spectrometers and detectors, enables the separation and sensitive detection of colors, where "color" encompasses a broad range of wavelengths from gamma rays to radio waves.