10: Electronic Spectroscopy and Photochemistry
- Page ID
- 467671
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- 10.1: Using Atomic Term Symbols to Interpret Atomic Spectra
- The electronic states that result from these excited orbital configurations are characterized by term symbols and are essential in understanding the spectra and energy level structure of atoms, and the orbital electron configurations. The orbital configurations help us understand many of the general or coarse features of spectra and are necessary to produce a physical picture of how the electron density changes because of a spectroscopic transition.
- 10.2: 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.
- 10.3: 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.
- 10.4: 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.
- 10.5: 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.
- 10.6: 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.
- 10.7: 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.
- 10.8: The Beer-Lambert Law
- The Beer-Lambert law relates the attenuation of light to the properties of the material through which the light is traveling. This page takes a brief look at the Beer-Lambert Law and explains the use of the terms absorbance and molar absorptivity relating to UV-visible absorption spectrometry.
- 10.9: Electronic Spectra Contain Electronic, Vibrational, and Rotational Information
- Molecules can also undergo changes in electronic transitions during microwave and infrared absorptions. The energy level differences are usually high enough that it falls into the visible to UV range; in fact, most emissions in this range can be attributed to electronic transitions.
- 10.10: The Franck-Condon Principle
- The Franck-Condon Principle describes the intensities of vibronic transitions, or the absorption or emission of a photon. It states that when a molecule is undergoing an electronic transition, such as ionization, the nuclear configuration of the molecule experiences no significant change. This is due in fact that nuclei are much more massive than electrons and the electronic transition takes place faster than the nuclei can respond. When the nucleus realigns itself with with the new electronic c
- 10.11: 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.
- 10.12: Fluorescence
- Fluorescence, a type of luminescence, occurs in gas, liquid or solid chemical systems. Fluorescence is brought about by absorption of photons in the singlet ground state promoted to a singlet excited state. The spin of the electron is still paired with the ground state electron, unlike phosphorescence. As the excited molecule returns to ground state, it involves the emission of a photon of lower energy, which corresponds to a longer wavelength, than the absorbed photon.