15: Lasers, Laser Spectroscopy, and Photochemistry
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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.
- 15.1: Electronically Excited Molecules can Relax by a Number of Processes
- A particle in an excited electronic state will eventually relax back to its electronic ground state, but several relaxation pathways are often available. These pathways may involve a combination of radiative decay and nonradiative decay, including a change in spin state.
- 15.2: The Dynamics of Transitions can be Modeled by Rate Equations
- Einstein proposed that electrons may transition between energy levels by means of absorption, spontaneous emission, and stimulated emission. In this section, we will describe the rates of these transitions, introducing the terms of spectral radiant energy density and the proportionality constants called Einstein coefficients.
- 15.3: A Two-Level System Cannot Achieve a Population Inversion
- In this section, we will show that achieving population inversion in a two-level system is not very practical. Such a task would require a very strong pumping transition that would send any decaying atom back into its excited state. This would be similar to reversing the flow of water in a waterfall. It can be done but is very energy costly and inefficient. In a sense, the pumping transition would have to work against the lasing transition.
- 15.4: Population Inversion can be Achieved in a Three-Level System
- The presence of a third energy level in a system allows for a population inversion to be created. Thus, a three-level system can act as a gain medium and can serve as a laser. The two possible lasing mechanisms for a three-level system will be described in this section.
- 15.5: What is Inside a Laser?
- Laser light is produced by a gain medium inside the laser optical cavity. The gain medium is a collection of atoms or molecules in a gaseous, liquid, or solid form. For lasing to take place, the gain medium must be pumped into an excited state by an electric current or an intense light source, such as a flashlamp. To induce stimulated emission, the laser cavity must reflect emitted light into the gain medium, but also must allow a portion of the laser light to leave the optical cavity.
- 15.6: The Helium-Neon Laser
- The He-Ne laser was the first continuous-wave (cw) laser invented. A few months after Maiman announced his invention of the pulsed ruby laser, Ali Javan and his associates W. R. Bennet and D. R. Herriott announced their creation of a cw He-Ne laser. This gas laser is a four-level laser that uses helium atoms to excite neon atoms. The atomic transitions in the neon produce the laser light. The most commonly used neon transition in these lasers produces red light at 632.8 nm.
- 15.7: Modern Applications of Laser Spectroscopy
- Laser light offers valuable tools to researchers who wish to use the interaction of light with matter to interrogate atomic and molecular systems. Most laser light is characterized by its near monochromaticity (relative to light from other sources), directionality, and coherence. Those characteristics are used in modern laser spectroscopy.
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)