Quantum yield is a measure of the efficiency of a photophysical or photochemical process, defined as the ratio of the number of events of a specific type (such as emitted photons, formed molecules, etc.) to the number of photons absorbed. It is a dimensionless quantity often expressed as a decimal or a percentage.

The Racah parameters are a set of coefficients that appear in the theoretical treatment of the interactions among the electrons in a multi-electron atom or ion, particularly when discussing the effects of electron-electron interactions on the energy levels and the spectra of transition metal complexes and rare-earth ions. These parameters are named after the physicist Giovanni Racah.

Radiofrequency Echographic Multi Spectrometry (REMS) is a non-invasive imaging technique used primarily in the medical field for the assessment of tissue characteristics. It combines principles from ultrasound imaging and radiofrequency analysis to provide detailed information about tissue composition, structure, and stiffness. The REMS technique involves transmitting radiofrequency signals into tissues and analyzing the resulting echoes. It can differentiate between various types of tissues based on their physical properties, such as density and elasticity.

Reflectometric Interference Spectroscopy (RIfS) is an optical technique used for measuring thin films and surfaces, particularly in the fields of materials science, photonics, and biology. The method is based on the interference of light waves reflected from different layers of a sample, which can include thin films, coatings, or biological materials.

The Renner-Teller effect is a phenomenon in molecular physics and spectroscopy that occurs in polyatomic molecules with degenerate electronic states. It is a blending of the Renner effect and the Teller effect, which describe how molecular vibrations and electronic states interact, leading to splitting of energy levels. Specifically, the Renner-Teller effect is most notable in systems where the degeneracy of electronic states is lifted due to coupling between electronic and vibrational states.

Rotational spectroscopy is a technique used to study the rotational motions of molecules by measuring the electromagnetic radiation they emit or absorb, particularly in the microwave region of the spectrum. This method provides detailed information about the molecular structure, including bond lengths and angles, as well as the moments of inertia of the molecules. The fundamental principle behind rotational spectroscopy is based on the fact that molecules can rotate about their axes and that these rotational transitions correspond to specific energy levels.

Rotational-vibrational spectroscopy is a technique used to analyze the rotational and vibrational motions of molecules. This type of spectroscopy combines two fundamental aspects of molecular behavior: 1. **Vibrational Spectroscopy**: This focuses on the vibrational transitions of molecules, which occur when the atoms in a molecule oscillate relative to each other. These vibrations belong to specific modes that depend on the molecular structure. Techniques like infrared (IR) spectroscopy and Raman spectroscopy are commonly used to observe vibrational transitions.

The term "repulsive state" can have various meanings depending on the context in which it is used. Here are a few interpretations based on different fields: 1. **Physics**: In the context of physics, particularly in discussions of particle interactions, a "repulsive state" might refer to a condition where particles experience a repulsive force due to their charges (for instance, like charges repel) or due to other forces. This can affect their behavior, energy levels, and scattering processes.

Resonance-enhanced multiphoton ionization (REMPI) is a sophisticated spectroscopic technique used to study the electronic structure of molecules and ions. It involves the absorption of multiple photons by a target molecule to achieve ionization, while also utilizing resonance to enhance the efficiency of the process.

In analytical chemistry and various scientific fields, the term "response factor" (RF) refers to a measure that expresses how the response of an analytical instrument, such as a spectrometer or chromatograph, correlates with the concentration of a compound of interest in a sample. It is often used in methods such as chromatography and mass spectrometry to quantify substances.

The Reststrahlen effect, or Reststrahlen band, refers to a phenomenon observed in the infrared spectrum of certain materials, particularly crystalline substances. It occurs when there are specific vibrational modes of the material's lattice that strongly absorb infrared radiation, leading to distinct spectral features. In essence, when infrared light interacts with a crystal, some wavelengths are absorbed due to the resonant frequency of the lattice vibrations (phonons), while others are reflected.

Rutherford Backscattering Spectrometry (RBS) is a powerful analytical technique used to determine the composition and thickness of thin films and layers of materials. It is based on the principles of nuclear physics and involves bombarding a sample with high-energy ions, typically helium ions (alpha particles), which are directed at the material under investigation.

The Second Solar Spectrum refers to a specific aspect of solar radiation that focuses on the polarization and spectral features of light emitted by the Sun. Unlike the more commonly discussed solar spectrum, which pertains to the intensity of light across different wavelengths, the Second Solar Spectrum emphasizes the presence of subtle polarization signals that can contain important information about the solar atmosphere, particularly the layers of the solar atmosphere above the photosphere, such as the chromosphere and the corona.

Selection rules are criteria or guidelines that dictate the allowed or forbidden transitions between quantum states in quantum mechanics and spectroscopy. These rules are used to determine which transitions can occur during processes such as electronic, vibrational, or rotational transitions in molecules, as well as transitions involving photons, such as in absorption or emission of light. In the context of quantum mechanics, selection rules are derived from the intrinsic symmetries of quantum systems and are often associated with changes in certain quantum numbers.

Olga Taussky-Todd (1906–1995) was an influential mathematician known for her work in linear algebra, matrix theory, and computational mathematics. Born in Austria, she later moved to the United States, where she made significant contributions to the field, particularly in the areas of symmetric and Hermitian matrices, as well as the stability of dynamical systems.

Rydberg ionization spectroscopy is a technique used in atomic and molecular physics to study the properties and behaviors of atoms or molecules at high-energy states, specifically focusing on Rydberg states. Rydberg states are highly excited states of atoms or molecules characterized by principal quantum numbers (n) that are significantly larger than those of ground states. In these states, electrons are further from the nucleus and can exhibit unique properties due to their high angular momentum and reduced electron-nucleus interactions.

The SPC file format, which stands for "SPC File" or "Sound File," is primarily associated with audio data and is commonly used in video game emulation and music production. The SPC format is notably employed by the Super Nintendo Entertainment System (SNES) for storing music and sound effects. Here are some key points regarding the SPC file format: 1. **Audio Storage**: SPC files contain raw audio data formatted specifically for playback on the SNES hardware.

SRAS stands for Short-Run Aggregate Supply. It represents the total quantity of goods and services that firms in an economy are willing and able to produce at various price levels in the short run, while some factors of production, such as wages, are considered to be fixed. In the short run, production can be adjusted by changing the intensity of how existing resources are utilized; for example, by having workers work overtime or by using existing machinery more intensively.

Saturated absorption spectroscopy is a technique used in atomic and molecular physics to study the properties of atoms or molecules, most commonly with laser light. The technique involves the use of a resonant laser beam to probe specific transitions between energy levels in an atom or molecule.

Saturated spectroscopy, also known as saturation spectroscopy, is a spectroscopic technique that relies on the saturation of optical transitions in a medium, typically a gas or a liquid. In this method, a light beam is used to excite a particular transition in atoms or molecules, and the phenomenon of saturation occurs when the intensity of the light is high enough to significantly populate the excited state, leading to a reduction in the absorption of light at the corresponding wavelength.