15: Lasers, Laser Spectroscopy, and Photochemistry
The word 'laser' is an acronym for "light amplification by stimulated emission of radiation." The use of lasers in science and in society has rapidly expanded since their development in the early 1960s. Lasers provides chemists with a powerful and versatile tool for probing the nature of and dynamics of species and chemical reactions. This chapter will discuss the foundations of lasers and the interaction of their output toward understanding atomic and molecular properties. We will describe the generation of laser light from electronically excited atoms using the rate-equation model developed by Einstein. Modern laser designs and applications will then be discussed.
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- 15.1: Electronically Excited Molecules can Relax by a Number of Processes
- This page covers the mechanisms of laser operation, emphasizing molecular transitions between excited and ground states, influenced by fluorescence and phosphorescence. It explains the differences in lifetime and energetics between these processes, including vibrational relaxation, internal conversion, and intersystem crossing.
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- 15.2: The Dynamics of Transitions can be Modeled by Rate Equations
- This page discusses Einstein's three processes of atomic spectral line formation: spontaneous emission, stimulated emission, and absorption. It explains the behavior of atoms under radiation, focusing on the interplay of these processes, thermal equilibrium, and the influence of temperature on electron distribution. Population inversion is identified as essential for optical amplification and laser function.
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- 15.3: A Two-Level System Cannot Achieve a Population Inversion
- This page explains lasing in two-level atomic systems, highlighting key conditions for laser operation: coherence, monochromatic output, collimation, and efficiency. Coherence relies on stimulated emission and requires a population inversion, which is difficult to achieve in two-level systems due to high energy demands. As a result, these systems are mostly limited to pulsed operation, making three-level systems more favorable for continuous lasing.
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- 15.4: Population Inversion can be Achieved in a Three-Level System
- This page explains the concept of optical pumping in laser systems, focusing on the limitations of two-level systems and the advantages of three-level systems, like ruby lasers, which allow population inversion through a metastable state. It describes the conditions for achieving population inversion via the decay rates between energy levels and introduces four-level lasers, such as He-Ne and Nd:YAG, which offer better efficiency and continuous output.
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- 15.5: What is Inside a Laser?
- This page explains the three main components of lasers: the gain medium for light emission, the pump source for energizing the medium, and the optical cavity for light amplification. Gain mediums can be solid, liquid, or gas, with examples like ruby in solid-state lasers and noble gases in gas lasers. Liquid dye lasers are noted for their tunability. The pump source excites the medium, while the optical cavity enhances amplification.
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- 15.6: The Helium-Neon Laser
- This page discusses the He-Ne laser, the first continuous-wave laser developed by Ali Javan and his team. It emits predominantly red light and is stable in wavelength and intensity, making it ideal for applications like holography and laser pointers. While it was dominant until the 1990s, it has been eclipsed by more affordable semiconductor lasers. However, due to its durability and low production costs, the He-Ne laser continues to maintain popularity.
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- 15.7: Modern Applications of Laser Spectroscopy
- This page discusses the diverse applications of laser light in studying atomic and molecular interactions, highlighting its properties like monochromaticity and coherence. It mentions uses in tracking chemical reactions, biological processes, and art conservation, as well as advancements in chemical kinetics and spectroscopy through pulsed and high-intensity lasers.
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- 15.E: Lasers, Laser Spectroscopy, and Photochemistry (Exercises)
- This page includes summaries of exercises from Chapter 15 of McQuarrie and Simon's "Physical Chemistry," covering topics like Einstein coefficients and atomic transitions. It encompasses calculations related to laser pulses, including radiant power, energy per pulse, and photon output for lasers of various wavelengths.
Thumbnail: Six commercial lasers in operation, showing the range of different colored light beams that can be produced, from red to violet. From the top, the wavelengths of light are: 660 nm, 635 nm, 532 nm, 520 nm, 445 nm, and 405 nm. Manufactured by Q-line. (CC BY-SA 3.0 Unported; Sariling gawa via Wikipedia )