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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Spectroscopy | 2/5 | https://en.wikipedia.org/wiki/Spectroscopy | reference | science, encyclopedia | 2026-05-05T13:33:39.825730+00:00 | kb-cron |
where
I
0
{\displaystyle I_{0}}
is the light intensity before passing through the sample,
I
{\displaystyle I}
is the output intensity,
ε
{\displaystyle \varepsilon }
is the extinction coefficient,
ℓ
{\displaystyle \ell }
is the path length through the sample, and
c
{\displaystyle c}
is the concentration of the sample. The extinction coefficient depends on the wavelength selected and the molecule being sampled. Resonances by the frequency were first characterized in mechanical systems such as pendulums, which have a frequency of motion noted famously by Galileo. In quantum mechanical systems, the analogous resonance is a coupling of two quantum mechanical stationary states of a system, such as two atomic orbitals, via an oscillatory source of energy such as a photon. The coupling of the two states is strongest when the source energy matches the energy difference between the two states. That is, a photon at the right energy is more likely to cause an electron to jump between two orbitals, a process called electron excitation. The energy E of a photon is related to its frequency ν by E = hν where h is the Planck constant, and so a spectrum of the system response vs. photon frequency will peak at the resonant frequency or energy. Any part of the electromagnetic spectrum may be used to analyze a sample from the infrared to the ultraviolet telling scientists different properties about the very same sample, a discovery that led to a broadening of the field of spectroscopy. For instance in chemical analysis, the most common types of spectroscopy include atomic spectroscopy, infrared spectroscopy, ultraviolet and visible spectroscopy, Raman spectroscopy and nuclear magnetic resonance. In nuclear magnetic resonance (NMR), the theory behind it is that frequency is analogous to resonance and its corresponding resonant frequency.
== Classification of methods ==
Spectroscopy is a sufficiently broad field that many sub-disciplines exist, each with numerous implementations of specific spectroscopic techniques. The various implementations and techniques can be classified in several ways.
=== Type of radiative energy === The types of spectroscopy are distinguished by the type of radiative energy involved in the interaction. In many applications, the spectrum is determined by measuring changes in the intensity or frequency of this energy. The types of radiative energy studied include:
Electromagnetic radiation was the first source of energy used for spectroscopic studies. Techniques that employ electromagnetic radiation are typically classified by the wavelength region of the spectrum and include microwave, terahertz, infrared, near-infrared, ultraviolet-visible, X-ray, and gamma spectroscopy. Particles, because of their de Broglie waves, can be a source of radiative energy. Both electron and neutron spectroscopy are used. For a particle, its kinetic energy determines its wavelength. Acoustic spectroscopy involves radiated pressure waves. Dynamic mechanical analysis can be employed to impart radiating energy, similar to acoustic waves, to solid materials.
=== Nature of the interaction === The types of spectroscopy can be distinguished by the nature of the interaction between the energy and the material. These interactions include:
Absorption spectroscopy: Absorption occurs when energy from the radiative source is absorbed by the material. Absorption is often determined by measuring the fraction of energy transmitted through the material, with absorption decreasing the transmitted portion. Emission spectroscopy: Emission indicates that radiative energy is released by the material. A material's blackbody spectrum is a spontaneous emission spectrum determined by its temperature. This feature can be measured in the infrared by instruments such as the atmospheric emitted radiance interferometer. Emission can be induced by other sources of energy such as flames, sparks, electric arcs or electromagnetic radiation in the case of fluorescence. Elastic scattering and reflection spectroscopy determine how incident radiation is reflected or scattered by a material. Crystallography employs the scattering of high energy radiation, such as X-rays and electrons, to examine the arrangement of atoms in proteins and solid crystals. Impedance spectroscopy, where impedance is the ability of a medium to impede or slow the transmittance of energy. For optical applications, this is characterized by the index of refraction. Inelastic scattering phenomena involve an exchange of energy between X-ray radiation and the matter that shifts the wavelength of the scattered radiation. These include Raman and Compton scattering. Coherent or resonance spectroscopy are techniques where the radiative energy couples two quantum states of the material in a coherent interaction that is sustained by the radiating field. The coherence can be disrupted by other interactions, such as particle collisions and energy transfer, and so often requires high intensity radiation to be sustained. Nuclear magnetic resonance (NMR) spectroscopy is a widely used resonance method, and ultrafast laser spectroscopy is possible in the infrared and visible spectral regions. Nuclear spectroscopy are methods that use the properties of specific nuclei to probe the local structure in matter, mainly condensed matter, molecules in liquids or frozen liquids and bio-molecules. Quantum logic spectroscopy is a general technique used in ion traps that enables precision spectroscopy of ions with internal structures that preclude laser cooling, state manipulation, and detection. Quantum logic operations enable a controllable ion to exchange information with a co-trapped ion that has a complex or unknown electronic structure.
=== Type of material === Spectroscopic studies are designed so that the radiant energy interacts with specific types of matter. These studies can be divided into three broad categories: electronic spectroscopy, which measures the transition of electrons between different energy states through absorption or emission of visible or ultraviolet energy; vibronic spectroscopy of molecules induced by the absorption of infrared energy; and rotational spectroscopy of molecules caused by microwave energy. The last two can be combined into rotational–vibrational spectroscopy of a gas.
==== Atoms ====