Internal Reflection and ATR Spectroscopy

Milan Milosevic

• Publisher: Wiley
• Year: 2012
• ISBN:
  9781118309766
• Edition: 1
• Topic: Ciencias físicas
• Subtopic: Espectroscopia y análisis de espectro

Attenuated Total Reflection (ATR) Spectroscopy is now the most frequently used sampling technique for infrared spectroscopy. This book fully explains the theory and practice of this method.

• Offers introduction and history of ATR before discussing theoretical aspects
• Includes informative illustrations and theoretical calculations
• Discusses many advanced aspects of ATR, such as depth profiling or orientation studies, and particular features of reflectance

1

Introduction to Spectroscopy

1.1 HISTORY

Spectroscopy got its start with Newton’s observation that the white light from the sun can be separated into different colors using a prism. This observation became not only foundational to the theory of light but also to the understanding of human color vision. Color vision is a crude form of spectroscopy. We can often guess on the nature of a material by its color. We can only perceive three primary colors. That makes the human eye a very crude spectrometer. However, even at this crude level, it was apparently advantageous to us, during evolutionary history, to trade higher image resolution for color vision.

In the times of Newton, the hurdle to overcome was to understand the nature of light. Newton himself proposed the so-called corpuscular theory of light with which he was able to explain all then known properties of light (i.e., the propagation of light in a straight line and the laws of reflection and refraction). Newton proposed that light consists of tiny particles (corpuscles). Huygens, a contemporary of Newton, proposed the so-called wave theory of light, in which light is a wave phenomenon like a wave on the surface of water. Using his theory, he too was able to explain all the known properties of light. To make their respective theories work, Newton and Huygens made opposite assumptions about the speed of light in optically transparent media such as water or glass. Newton needed light to go faster through such media for his theory to work. Huygens needed it to go slower.

However, what settled the dispute was not the measurement of the speed of light in transparent media; it was the observation that light can form interference patterns. Only waves can form interference patterns and that settled the dispute. Huygens won. Later, measurements of the speed of light in denser media such as water confirmed Huygens’s assumption. Light was a wave phenomenon.

Different colors of light correspond to different wavelengths. In studying light dispersion by a glass prism, Herschel noticed that there is an invisible component of solar radiation next to red light. Thus, infrared (IR) light was discovered. Later, it was discovered that there is also an invisible component of solar radiation next to violet that was named ultraviolet (UV).

In the early nineteenth century, Fraunhofer noticed a curious phenomenon. By using a prism to disperse solar radiation, he observed tiny black lines superimposed over a continuous rainbow-colored solar spectrum. Some wavelengths in the continuous spectrum of sunlight were missing. The dark lines, later named Fraunhofer lines, were present whether he used one type of glass prism or another. The appearance of the lines was mysterious, but Fraunhofer could find them not only in the solar spectrum; he was also able to observe them in the spectra of distant stars. Following the work of Thomas Young with interference of light, Fraunhofer developed diffraction grating as a means to disperse light in a more effective way than with glass prisms. The gratings enabled spectroscopy with a much higher resolution than prisms and enabled the direct measurement of the wavelengths of light.

Fraunhofer died without knowing what caused his dark lines. It was Kirchoff, working some 30 years after Fraunhofer’s death, who realized that each element or compound is associated with its own unique set of spectral lines. This was the official birth of spectroscopy as a scientific discipline. The implications of this observation were extremely important. For instance, it told us that distant stars were made from the same elements found here on Earth. We could not know that any other way with any certainty. This is a nontrivial finding about the nature of the universe. There is no a priori reason why the distant worlds, many light-years from Earth, would not be made from an entirely new type of matter, entirely different from what we are familiar with here on Earth.

Intrigued by these mysterious lines and their association with different elements, many researchers started studying spectra of flames and other light sources. It was discovered that, when heated, the atoms emit bright lines. Soon it was realized that these bright lines match some of the dark lines found in the solar spectrum. Associating lines with particular elements became the primary aim of the new science of spectroscopy. Soon people would talk about sodium lines, mercury lines, and so on.

It was also realized that the dark lines were due to absorption of light by the elements that would, when heated, emit those same lines. Soon, it became possible to analyze a spectrum of a mix of elements by sorting out the lines due to each element, that is, to analyze a mixture for its constituents. Furthermore, by observing the relative intensities of lines due to each element, it became possible to estimate the relative abundance of different elements in a mixture. This was now already true spectroscopy.

Early on, spectroscopists realized that they could substitute a photographic plate for their eyes and that they could photograph a spectrum. The spectrograph represented a permanent record of a spectrum and could be subsequently analyzed in great detail. Different spectra could be compared. Employing long exposure times allowed recording spectra from very faint sources otherwise too weak to be observed by the eye. The use of photography also extended the spectral range of spectroscopy from visible to UV and, to a more limited extent, to the IR spectral regions.

By improving the spectroscopic equipment and increasing the resolution of the spectroscopic measurement, spectroscopists soon realized that many single lines seen through the early spectroscopes were not really single and that sometimes, under high spectral resolution, a much finer structure would be revealed. They found single lines resolving into doublets, triplets, quadruplets, and so on. By the dawn of the twentieth century, a great amount of very detailed spectral information was amassed. The experimental precision with which these spectral measurements were pursued seems almost fanatical, but what propelled it was the constant stream of discoveries that accompanied it. For instance, it was discovered that some prominent lines in the sun’s spectrum could not be matched by anything known on Earth. They attributed it to a new element that they aptly named helium after the Greek sun god Helios. Soon thereafter, helium was discovered on Earth.

However, the abundance of information generated by spectroscopy was contrasted with the total lack of understanding of how the spectra themselves are generated. People knew that light is a wave phenomenon similar to sound. The sound generated by a taut string consists of a set of characteristic frequencies. A string with a different tension or of a different length produces a sound of a different frequency. This would make it plausible that different elements would produce different sets of light frequencies. Even the fact that a taut string could be resonantly excited into vibrations by a sound of the same frequency that it would sound if struck was seen analogous to why cold atoms would absorb the same frequencies of light that they would emit when heated.

While not in itself surprising, the existence of these characteristic frequencies associated with different elements was totally stomping the scientists when they tried to understand them based on the available physical theories known collectively as classical physics. Soon, it became obvious that classical physics could not explain the observed spectra. A revolutionary new theory called quantum mechanics had to be developed to provide the explanation. The explanation, however perfect, came with an uneasy requirement to abandon common sense and to proceed into the unintuitive and forbidding world of quantum mechanics following mathematics where intuition fails.

After first providing a spectacular confirmation that the universe is filled with the same atoms and compounds that we find here on Earth, spectroscopy provided another spectacular result. Measuring spectra of distant nebulae in the first half of the twentieth century, Hubble discovered that the spectral lines of elements and compounds from those distant nebulae are shifted from their terrestrial positions toward lower frequencies (referred to as redshift since red light is the visible light with the lowest frequency). This was a puzzling discovery.

The explanation that was eventually accepted is that those distant nebulae recede from us in all directions with high speeds. The recession at high speeds shifts frequencies through what is known as Doppler effect. The effect is commonly observed when a whistling train passes by. The pitch of the whistle is higher while the train is approaching and it suddenly turns lower as the train passes by and starts moving away.

By studying how the redshift correlates with the distance from Earth, Hubble found that the farther away a nebula (today referred to as galaxy) is, the larger the redshift. This finding stood in a distinctly anti-Copernican spirit; that is, that Earth has no special place in the universe, but it was soon realized that the same is true for every point in the universe. The universe is expanding from every point in every direction. The most significant implication ...