spectrograph 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, • 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. •
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 Atomic spectroscopy was the first application of spectroscopy.
Atomic absorption spectroscopy and
atomic emission spectroscopy involve visible and ultraviolet light. These absorptions and emissions, often referred to as atomic spectral lines, are due to
electronic transitions of outer shell electrons as they rise and fall from one electron orbit to another. Atoms have distinct X-ray spectra that are attributable to the excitation of inner shell electrons to excited states. Atoms of different elements have distinct spectra and therefore atomic spectroscopy allows for the identification and quantitation of a sample's elemental composition. After
Robert Bunsen and
Gustav Kirchhoff invented the spectroscope, Bunsen discovered cesium and rubidium by observing their emission spectra. Atomic absorption lines are observed in the solar spectrum and referred to as
Fraunhofer lines after their discoverer. A comprehensive explanation of the
hydrogen spectrum was an early success of quantum mechanics and explained the
Lamb shift observed in the hydrogen spectrum, which further led to the development of
quantum electrodynamics. Modern implementations of atomic spectroscopy for studying visible and ultraviolet transitions include
flame emission spectroscopy,
inductively coupled plasma atomic emission spectroscopy,
glow discharge spectroscopy,
microwave induced plasma spectroscopy, and spark or arc emission spectroscopy. Techniques for studying X-ray spectra include
X-ray spectroscopy Molecules The combination of atoms into molecules leads to the creation of unique types of energetic states and therefore unique spectra of the transitions between these states. Molecular spectra can be obtained due to electron spin states (
electron paramagnetic resonance),
molecular rotations,
molecular vibration, and electronic states. Rotations are collective motions of the atomic nuclei and typically lead to spectra in the microwave and millimetre-wave spectral regions. Rotational spectroscopy and microwave spectroscopy are synonymous. Vibrations are relative motions of the atomic nuclei and are studied by both infrared and
Raman spectroscopy. Electronic excitations are studied using visible and ultraviolet spectroscopy as well as
fluorescence spectroscopy. Studies in molecular spectroscopy led to the development of the first
maser and contributed to the subsequent development of the
laser.
Crystals and extended materials The combination of atoms or molecules into crystals or other extended forms leads to the creation of additional energetic states. These states are numerous and therefore have a high density of states. This high density often makes the spectra weaker and less distinct, i.e., broader. For instance, blackbody radiation is due to the thermal motions of atoms and molecules within a material. Acoustic and mechanical responses are due to collective motions as well. Pure crystals, though, can have distinct spectral transitions, and the crystal arrangement has an effect on the observed molecular spectra. The regular
lattice structure of crystals scatters X-rays, electrons, or neutrons, allowing for crystallographic studies.
Nuclei Nuclei have distinct energy states that are widely separated and lead to
gamma ray spectra. Distinct nuclear spin states can have their energy separated by a magnetic field, and this allows for
nuclear magnetic resonance spectroscopy. == Other types ==