Laser Spectroscopy and Laser Imaging

An Introduction

Helmut H. Telle, Ángel González Ureña

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781482256932
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

"a very valuable book for graduate students and researchers in the field of Laser Spectroscopy, which I can fully recommend"
—Wolfgang Demtröder, Kaiserslautern University of Technology

How would it be possible to provide a coherent picture of this field given all the techniques available today? The authors have taken on this daunting task in this impressive, groundbreaking text. Readers will benefit from the broad overview of basic concepts, focusing on practical scientific and real-life applications of laser spectroscopic analysis and imaging. Chapters follow a consistent structure, beginning with a succinct summary of key principles and concepts, followed by an overview of applications, advantages and pitfalls, and finally a brief discussion of seminal advances and current developments. The examples used in this text span physics and chemistry to environmental science, biology, and medicine.

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• Focuses on practical use in the laboratory and real-world applications

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• Covers the basic concepts, common experimental setups

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• Highlights advantages and caveats of the techniques

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• Concludes each chapter with a snapshot of cutting-edge advances

This book is appropriate for anyone in the physical sciences, biology, or medicine looking for an introduction to laser spectroscopic and imaging methodologies.

Helmut H. Telle is a full professor at the Instituto Pluridisciplinar, Universidad Complutense de Madrid, Spain.

Ángel González Ureña is head of the Department of Molecular Beams and Lasers, Instituto Pluridisciplinar, Universidad Complutense de Madrid, Spain.

1Introduction

Humans have always been fascinated with color and imaging of nature, and semiscientific or philosophical descriptions of these optical phenomena can be traced back more than two millennia. It is difficult to gauge which of the two was really understood first in a rigorous manner since scientific investigations in the modern sense—merging experimental observations with theoretical models—were not always the stated goal of the early “scientists.” It is probably fair to say that imaging might have been the topic easier to understand from the simple, observational standpoint lending itself to conceptual sketching of the phenomenon. Color would have been more elusive to understanding; while its existence was easy to realize, to explain its origins was largely beyond the understanding of physics and chemistry before the Middle Ages.

Here we provide only a few historical remarks regarding the development of spectroscopy and imaging toward the modern-age revolution that the advent of the laser brought to these two fields of science.

Imaging of “reality” in the form of paintings can be traced back to prehistoric times and is evidenced by numerous cave drawings. Such imaging became more sophisticated with time, and different techniques evolved, including among others mosaic laying—which in a certain way may be seen as being akin to modern, pixilated imaging using digital camera sensors. The use of a real imaging instrument in the form of the so-called camera obscura goes back to “before common era” (BC). The first written reference to the camera obscura can be found in the works of the Chinese philosopher Mozi (about 470–390 BC) who correctly noted that the inverted upside-down image was due to light traveling in straight lines from its source. The camera obscura was used by many natural philosophers who experimented with different aspects of pinhole imaging in ever more sophisticated ways.

Probably the first clear description of its workings is by Leonardo da Vinci in his Codex Atlanticus, published in 1502. And the first known drawing of a camera obscura is that published in the works of Rainer Gemma Frisius (Gemma Frisius 1545); he used a camera obscura in a “scientific” way, namely to observe and monitor the solar eclipse of 1544 (see Figure 1.1).

Figure 1.1First known drawing of the camera obscura; translation of the inscription: “Observing the solar eclipse of 24 January 1544.” (From Gemma Frisius, R. “De radio astronomico et geométrico.” Antwerp: apud. Greg. Brontium, 1545.)

Color and its relation to “spectroscopy” proved to be more elusive to understanding and applications of scientific significance. It is probably the spectrum of the rainbow (see the top panel of Figure 1.2) that inspired not only artists but also many natural philosophers to try to explain the phenomenon behind the generation of a color-dispersed spectrum.

Figure 1.2Origin of spectra and their explanation. (Top) Dispersion of white light in a rainbow. (Middle) Sketch of Newton’s prism experiment, from his notes (see also Newton 1671). (Bottom) Nineteenth century color engraving of Newton’s prism experiment.

The earliest natural philosopher—at least according to written records—to seriously investigate the rainbow phenomenon was the Greek scholar Aristotle (384–322 BC). Although some of his reasoning was flawed albeit relatively consistent, his qualitative explanations remained unrivaled until the Middle Ages. Then, for the first time, the Persian astronomer Qutb al-Din al-Shirazi (1236–1311 AD) and his student Kamāl al-Dīn al-Fārisī (1267–1319 AD) provided a fairly accurate explanation for the rainbow phenomenon; the latter developed a mathematically satisfactory explanation, proposing a model of refraction and linking refraction to the decomposition of light. Incidentally, in his experiments, al-Fārisī made use of a camera obscura.

The understanding of color as part of the (rainbow) spectrum evolved from Newton’s prism experiments in the seventeenth century. He discovered that a prism can “disassemble and reassemble white light,” and in his publication on the topic (see Newton 1671), the word “spectrum” is used for the first time in the context of decomposition of white light into its components. The concept of his experiments is given in the bottom part of Figure 1.2. He observed that when a beam of white (sun) light struck the entrance face of a glass prism at an angle, that (1) some fraction of the light was reflected and that (2) another fraction of the light passed through the glass to emerge from the opposite prism face as a band of light with different (continuously changing) colors. Newton hypothesized light to constitute “particles” of different colors, and that these moved at different speeds in transparent matter, with red light particles moving faster than blue light particles in glass, but with red light being bent (“refracted”) to a lesser degree than blue light.

It took until the early nineteenth century that the concepts of spectra and spectroscopy became more palatable, in particular with the discovery of light outside the visible range. And for the first time, the wavelengths of different colors of light were measured (Young 1802). From then on, different spectroscopic phenomena and techniques were developed, hand in hand with the discovery and evolution of photon and image recording devices. With the evolution of quantum physics in the early part of the twentieth century, spectroscopy became one of the indispensable cornerstones for the measurement and understanding of many physical, chemical, and biological processes.

1.1Lasers and their impact on spectroscopy and imaging

It is probably fair to state that the advent of the laser has by and large revolutionized spectroscopy and spectroscopic imaging. In particular, it is the specific properties of laser light sources that make all the difference in comparison to other light sources. Namely, these properties include (1) spectral intensity and monochromaticity; (2) wavelength tunability, often over broad intervals; (3) coherence, both temporally and spatially; and (4) temporal variation from continuous operation to ultrashort pulses.

1.1.1Laser properties of importance to spectroscopy

It is most certainly the intensity within narrow spectral intervals that sets the laser apart from conventional light sources. This is because, in general, response of a system to a light stimulus can be described by the relation Isignal ∝ N · Sλ · Iλ, where N stands for the number of atoms or molecules taking part in the process, Sλ incorporates the transition probability for the photon interaction, and Iλ is the spectral intensity of the light source, optimally matched to the bandwidth of the transition, if possible. The spectral intensity can be orders of magnitude larger than for ordinary light sources. This means many things, among others that a much better signal-to-noise ratio is achievable. But more importantly, it becomes much easier to probe processes in which only very small numbers of particles participate. For example, in chemistry, one can trace transient species, reaction intermediates, photodissociation fragments, and so on; and in (analytical) measurements, one is able to record signals at extremely low particle densities, ultimately investigating individual particles of a species.

The tunability of the laser, in conjunction with high monochromaticity, lends itself for high-resolution spectral measurements. This means that the excitation by laser light can be molecule-specific—a great advantage in chemical reaction dynamics or compositional analysis—and individual quantum states can be selected, even resolving quantum features like hyperfine structure, or targeting a specific isotope of a species.

The extraordinary coherence properties of laser light allow for numerous applications that require, e.g., tight focusing or time-delayed overlap between two light beams. In this context, high temporal coherence will allow for tailoring of short laser pulses, which can be utilized in well-controlled pump—probe experiments to study ultrafast processes in real time. The property of spatial coherence can be exploited to focus a laser beam to diameters of the order of the light wavelength itself, thus reaching extremely high power densities (to initiate, e.g., nonlinear processes), or to single out individual atoms or molecules.

Finally, the exceptional control over the time duration of the laser radiation opens up many applications that would be inaccessible to traditional light sources. While light pulses down to about 1μs are feasible, by and large, any shorter pulse duration (nano-, pico-, femto-, and attosecond) is the domain of lasers. It is in particular the realm of ultrashort pulses with tp ~ 10−15–10−18 s that has given researchers access to physical and chemical processes that could only be postulated theoretically but eluded experimental proof. For example, the full knowledge of chemical reactivity requires the full understanding of an elementary chemical reaction occurring in just one single, short time-scale event. The intermediate steps in such a reaction can be studied spectroscopically (energy regime) and dynamically (time regime), giving rise to the exciting field of femtochemistry.

One extrao...