What is Optical Spectroscopy?
Spectroscopy is an important interdisciplinary field that mainly involves physics and chemistry. It studies the interaction between electromagnetic waves and matter through spectroscopy. Light is an electromagnetic radiation superimposed by electromagnetic waves of various wavelengths (or frequencies). A spectrum is a type of graph that analyzes a certain property of a beam of electromagnetic radiation into its contribution to the property by means of spectroscopic means such as gratings, prisms, and Fourier transforms. For example, an absorption spectrum can list the absorption degree of a substance to the corresponding wavelength in a certain band in order of wavelength from low to high. With the development of science and technology, the spectrum of electromagnetic waves involved in spectroscopy is getting wider and wider, from gamma rays with a wavelength in the picometer range, to X-rays, ultraviolet rays, visible light regions, infrared rays, microwaves, and radios with wavelengths up to several kilometers. Waves have their characteristic forms that interact with matter. According to the action form of light and matter, the spectrum can be generally divided into absorption spectrum, emission spectrum, scattering spectrum and so on. Through spectroscopy, people can analyze the micro and macro properties of the energy levels and geometry of atoms and molecules, the reaction rate of specific chemical processes, and the concentration distribution of a substance in a specific area in space.
- Spectroscopy is an important interdisciplinary discipline mainly involving physics and chemistry.
- Spectroscopy has been studied for more than 300 years. In 1666,
- According to the different methods of studying spectroscopy, it is customary to distinguish spectroscopy into
- Beam foil spectroscopy is an emerging discipline developed internationally in the 21st century. The main content is to study basic atomic physics and measurement by the method of accelerated ions impacting thin foils of different elements.
- A new type of spectral analysis and detection technology based on photoacoustic effect. A monochromatic light with a modifiable intensity is irradiated onto the sample sealed in the photoacoustic cell, and the sample absorbs
- According to the mode of action of matter and light, it can be divided into the following three categories :
- emission spectroscopy
- Use atomic or molecular emission spectra for research. Each kind of original
- Spectroscopy
- Absorption spectroscopy
- Molecules or atomic groups have characteristic absorption in each band, which is mainly manifested as a band-like absorption spectrum (see Spectrum), which is unique to the molecular spectrum. The widely used infrared absorption spectrum is generated by transitions between different vibrational and rotational energy levels within the same electronic state of a molecule. Infrared absorption spectroscopy is mainly used to study the energy level structure and molecular structure of molecules, or to conduct qualitative and quantitative analysis of molecules. Studies on absorption and emission spectra are often complementary.
- Raman spectroscopy
- In Raman scattering, the Raman spectrum originates from the vibration and rotation of the molecules of the scattering substance, and reflects the internal structure and movement of the molecule. The Raman spectrum can be used to conduct qualitative and quantitative analysis of compounds, determine the vibration and rotation frequency of molecules, About constants, understanding the forces inside or between molecules, inferring the symmetry and geometry of the molecular structure, etc. The application of Raman spectroscopy covers many fields of physics, chemistry, and biology. The application of the new light source laser has strongly promoted the development of Raman spectroscopy.
- According to the different light sources, it can be divided into the following two categories :
- Laser spectroscopy
- A branch of spectroscopy with a laser as a light source. The unique advantages of the narrow spectral line width, high intensity, and good directivity of the laser have brought a new face to spectroscopy.It not only has extremely high spectral resolution and detection sensitivity, but also develops nonlinear effects and coherent pull. Many new fields such as Mann spectroscopy.
- Non-laser spectroscopy
Review of Spectroscopy
- Study the branch of spectroscopy that eliminates Doppler broadening of spectral lines. Only the non-linear domain is involved here. Improving the resolution of spectroscopy has always been one of the issues that spectroscopy scientists strive to solve. Early research on atomic spectroscopy was limited by the resolving power of spectrometers. After using sensitive instruments such as Michelson interferometer and Fabry-Perot interferometer, the accuracy of measuring the wavelength of light waves was effectively improved. However, it is still indistinguishable for close spectral lines, such as the components of the Barr terminal line in the hydrogen atom spectrum. This is not because the performance of the interferometer is not perfect, but because the spectral lines are not sharp enough. The width of the spectral line masks its delicate structure. Part of the reason for the broadening of the spectral lines is the increase in the natural width. Even under the best observation conditions, the spectral lines are not absolutely monochromatic. The reason is that the steady state of the atom is not the real steady state. After the atom is excited, it will radiate energy within a certain period of time, that is, the atom in the excited state always decays. This finite lifetime of the excited state increases the natural width of the spectral line.
- The natural width of the spectral lines sets a limit on the resolution of the spectroscopy. However, until the laser was applied in the spectrum research, this limit could hardly be reached. The reason is that in the gas sample, the spectral line is broadened by the Doppler effect to a greater extent, and ordinary spectroscopy techniques cannot be effective. The Doppler broadening of the spectral lines is eliminated, making it difficult to improve the resolution of the spectroscopy.
- Since 1970, laser spectroscopy technology has developed rapidly. One of the effects of this technology has been a significant increase in the resolution of spectroscopy, which has increased by several orders of magnitude. Laser spectroscopy can effectively eliminate Doppler broadening of spectral lines. These methods are mainly saturation spectroscopy, polarization spectroscopy, and two-photon spectroscopy.
Saturation spectroscopy
- Based on two main properties of lasers: narrow spectral line width and high intensity. The lasers used are mostly continuous wave frequency modulated lasers, especially dye lasers. In a strong laser beam, gas atoms absorb photons at a rate that exceeds the rate at which the atoms return to their original energy level, thereby reducing the number of atoms that can absorb photons of a given frequency. This means that the laser beam "sweep" the atoms that absorb this frequency on the road. When another light beam of the same frequency passes the gas sample along the same path, it is found that the absorption of light of this frequency by the gas atoms is reduced. In fact, when using this effect for high-resolution spectroscopy, the beam of the FM laser is split into a strong saturated beam and a weak test beam. The so-called saturated beam, roughly speaking, is able to excite a large number of atoms, so that the excitation of the atoms appears to be saturated. These two beams pass through the gas atom sample along the same path, but they travel in opposite directions. When the output frequency of the laser is scanned and adjusted to the frequency of the atomic energy level transition, a strong saturated beam is absorbed by a specific group of atoms whose velocity component is zero in the beam direction. Atoms with velocity components in the beam direction will not absorb photons in the saturated beam due to the Doppler effect. The saturated beam reduces the number of atoms in the selected state, so that when the test beam passes through the atomic sample, it undergoes a small absorption accordingly. The frequency range of this absorption is narrow due to the absence of the Doppler effect. If the frequency of the laser beam slightly deviates from the frequency of the atomic transitions, the two beams interact with different atoms separately, instead of the two beams interacting with the same atom when the frequency of the beams is exactly the atomic transition Like that. Therefore, the saturated beam has no effect on the absorption of the test beam. It can be seen that the width of the test beam signal given by the saturation spectrum technology is very narrow, almost close to the natural width of the spectral line.
- Saturation spectroscopy is one of the effective methods to eliminate Doppler broadening of spectral lines. It has a wide range of uses. One example is the fine structure of the Balmer alpha line used to study the hydrogen atom spectrum. The results of the study are much more accurate than before. In addition, the Lam shift of 2S and 2P energy levels was observed for the first time in the absorption spectrum. The precise data of the fine structure of the hydrogen atom spectrum improves the accuracy of the Rydberg constant. Based on this research, the Rydberg constant R = (109737.311 ± 0.012) cm, which is nearly 10 times higher than the previous accuracy.
Polarization spectroscopy
- Another method to eliminate the Doppler effect is polarization spectroscopy. The characteristic of this technique is that it is much easier to measure small changes in the polarization of light than to change the intensity, so the sensitivity of the measurement can be significantly improved. As in saturation spectroscopy, the beam from the laser is also split into two beams, one of which is much stronger than the other and also passes through the sample in question in the opposite direction. However, in polarization spectroscopy, a weak test beam is linearly polarized and passes through a gas sample placed between crossed polarizers. If the test beam does not change its polarization while passing through the sample, it will not reach the detector. But saturated beams can cause this change. Because when it first passes through a quarter-wave plate, it becomes circularly polarized. The direction of the electric field of circularly polarized light is rotating, either clockwise or counterclockwise. The probability that an atom absorbs circularly polarized light depends on the orientation of the atom's angular momentum. The orientation of the initial atoms is random, but when the orientation of some atoms can absorb a circularly polarized light, the saturated beam makes the atomic energy levels of these atoms empty, and the atoms with opposite angular momentum orientation relatively change too much. When a linearly polarized test beam passes through the same region of the gas, the oriented atoms change the test beam's propagation. The reason is easy to understand. Linearly polarized light can be viewed as the superposition of two kinds of circularly polarized light of equal intensity. The electric field of one kind of circularly polarized light rotates in a clockwise direction, and the electric field of another kind of circularly polarized light rotates in a counterclockwise direction. When a test beam passes through a gas, the atoms it encounters absorb too much a kind of circularly polarized light because the relative number of these atoms is large. As a result, the intensity of one type of circularly polarized light is reduced, while the other is relatively stronger. Therefore, the test beam from the gas sample is no longer linearly polarized, but instead becomes elliptically polarized. In this way, the test beam has a component that can pass through the crossed polarizers. However, all of these situations must occur when the saturated beam and the test beam act on the same atom, that is, the atom without the Doppler shift. In this respect, polarization spectroscopy is the same as saturation spectroscopy. In fact, polarization spectroscopy is derived from saturation spectroscopy. The biggest feature of this spectroscopy technique is that it is basically free of noise. Using this technique, you can obtain more precise knowledge of the structure of the energy level. For example, the measurement results have tripled the accuracy of the Rydberg constant value to make it The most accurate known basic constant.
Two-photon spectroscopy
- It is also a good way to eliminate Doppler broadening of spectral lines. This technique was first reported in 1974. In this technique, a beam of light is reflected back by the mirror along the original route, so that they propagate along the same optical axis in opposite directions and become a standing wave after superposition. The gas sample is placed in the standing wave field. If the frequency of the laser beam is adjusted to half of the selected atomic transition frequency, under certain conditions, each atom that interacts with the beam will simultaneously absorb one photon from two beams traveling in opposite directions. .
- Imagine that an atom moving along the optical axis in a standing wave field absorbs two photons from opposite directions, and the Doppler shift of one of the photons is toward the direction of purple light, that is, it has a higher And the Doppler shift of the other photon is in the direction of the red light, and the magnitude of the shift is the same as that of the previous photon. Therefore, the total energy of the two absorbed photons is constant, regardless of the speed of the atom's motion. Therefore, the two-photon absorption cancels the Doppler effect of atomic motion, and the sum of the frequency of light absorbed by the atom is exactly the frequency of the atomic transition. If the output frequency of the laser deviates slightly from half the atomic transition frequency, the atoms will not absorb two photons in opposite directions. Therefore, the effect of eliminating spectral Doppler widening is not good. That is, the Doppler widening can be effectively eliminated only when the laser frequency matches the atomic transition frequency.
- In two-photon absorption spectroscopy, all atoms that interact with the laser beam can contribute to signals without the Doppler effect, not just atoms moving in the direction of the vertical optical axis. Therefore, the signal without the Doppler effect is Very strong. This is different from saturation spectroscopy and polarization spectroscopy. In these two spectra, atoms without the Doppler effect are selective and the motion of the atoms is perpendicular to the optical axis. In two-photon absorption spectroscopy, all atoms that interact with the beam can eliminate their Pooler effect. [2]
Spectroscopy Applications
- High-resolution spectroscopy is widely used in the study of the energy level structure of atoms and molecules. Until now, the data obtained belong to the predictions of the verification theory. But some of the major changes in physics in the 20th century were caused by the discovery of small differences between theory and observation, and high-resolution spectroscopy may contribute in this regard.