When Raman scattering is explained by classical theory, it is considered that molecules vibrate at the natural frequency vi, and the polarizability (see polarizability) also changes periodically with vi as the frequency. Under the action of incident light with frequency v0, the coupling of v0 and vi produces three frequencies: v0, v0+vi and V 0-VI. The light with frequency v0 is Rayleigh scattering light, and the last two frequencies correspond to Raman scattering lines. The perfect explanation of Raman scattering needs quantum mechanics theory, which can not only explain the frequency difference of scattered light, but also solve the problems of intensity and polarization.
Raman scattering provides an important means to study the structure of crystals or molecules, and forms a branch of Raman spectrum in spectroscopy. Raman scattering method can be used to quickly determine the natural frequency of molecular vibration, thus determining the symmetry and internal force of molecules. Since the appearance of laser, the research on laser Raman scattering has developed rapidly, and the nonlinear effect caused by intense laser has led to a new Raman scattering phenomenon.
1923, American physicist Compton discovered a new phenomenon when he was studying the experiment of X-ray scattering through physical substances, that is, the scattered L > not only produced X-rays with the original wavelength of l0, but also produced X-rays with the wavelength of L >; The wavelength increment of X-ray of l0 varies with the scattering angle. This phenomenon is called Compton effect.
It is difficult to explain Compton effect with classical electromagnetic theory. Compton, with the help of Einstein's photon theory, satisfactorily explained this experimental phenomenon from the perspective of photoelectron collision. Chinese physicist Wu also made outstanding contributions to Compton scattering experiments.
The study of Compton scattering phenomenon took a decade or two to get the correct results.
Compton effect is the first experimental proof of Einstein's hypothesis that photons have momentum. This occupies an important position in the history of physics development. When photons interact with material particles in the medium, they may make light propagate in any direction. This phenomenon is called light scattering. 1922, American physicist Compton found that the wavelength of some scattered waves was slightly larger than that of incident waves when studying the scattering of X-rays by electrons in graphite. He believes that when a photon collides with an electron, part of the energy of the photon is transferred to the electron. Compton assumes that photons are the same as physical particles such as electrons and protons. There is not only energy, but also momentum. Energy is conserved during collision, and momentum is also conserved. According to this idea, the wavelength difference before and after scattering is obtained, and the results are completely consistent with the experimental data, thus confirming his hypothesis. This phenomenon is called Compton effect.
find
Compton studied the composition of light after X-rays were scattered by lighter substances (graphite, paraffin, etc.). ) at 1922 ~ 1923, it is found that there are components with the same wavelength and longer wavelength in the scattered spectrum. This scattering phenomenon is called Compton scattering or Compton effect. Compton projected 0.7 1 angstrom of X-rays onto graphite, and then measured the X-ray intensity scattered by graphite molecules at different angles. When θ = 0, there is only single-frequency light equal to the incident frequency. When θ ≠ 0 (such as 45, 90,135), it is found that the scattered light has two frequencies. One has the same frequency as the incident light, and the other has a lower frequency than the incident light. The deviation of the latter increases with the increase of angle.
Experimental results:
In (1) scattered light, besides λ0 with the same wavelength, there are spectral lines with λ > λ0.
(2) The change of wavelength δ λ = λ-λ 0 increases with the increase of scattering angle φ (the included angle between scattering direction and incident direction).
(3) For scattering materials with different elements, the wavelength changes are the same at the same scattering angle. The intensity of scattered light with wavelength λ decreases with the increase of atomic number of scatterer.
Compton successfully explained these experimental results with photon theory. X-ray scattering is the result of elastic collision between a single electron and a single photon. Momentum and energy are conserved before and after collision, which is obtained after simplification.
δλ=λ-λ0=(2h/m0c)sin^2(φ/2)
It is called Compton scattering formula.
λ=h/(m0c)
Called the Compton wavelength of electrons.
Why do scattered light have the same wavelength as incident light? Internal electrons cannot be considered as free electrons. If a photon collides with this electron, it is equivalent to colliding with the whole atom. In the collision, the energy transmitted by photons to atoms is very small, and almost keeps its own energy unchanged. In this way, the original wavelength remains in the scattered light. Because the number of electrons in the inner layer increases with the increase of the atomic number of the scatterer, the intensity with wavelength λ0 increases and the intensity with wavelength λ decreases.
Compton scattering is meaningful only when the wavelength of incident light is close to that of electrons, which is why X-ray is chosen to observe Compton effect. In photoelectric effect, the incident light is visible light or ultraviolet light, so Compton effect is not obvious.
explain
(1) Classical interpretation (electromagnetic wave interpretation)
When monochromatic electromagnetic waves act on charged particles with a wavelength smaller than that, they cause forced vibration and radiate electromagnetic waves with the same frequency in all directions. Classical theory can explain the scattering with constant frequency, but it can't reasonably explain Compton effect!
(2) Photon theory explanation
X-rays are photons with e=hν, which collide with free electrons completely elastically, and the electrons gain some energy, so the scattered photon energy decreases, the frequency decreases and the wavelength becomes longer. This process assumes that the conservation of momentum and energy is still true, so it is composed of
Electron: p = m0vE=m0V2/2 (assuming that the electron starts to rest and the potential energy is ignored).
Photon: P=h/λ
Where (h/m0C) = 2.34×10-12m is called Compton wavelength.
pay attention to
1. The order of magnitude of the scattering wavelength change lD is 10- 12m. For the visible wavelength l~ 10-7m, LD
2. There are rays with the same wavelength as the incident light in the scattered light, because photons collide with atoms, and the atoms are of great mass. After photon collision, the energy and the frequency of scattered light remain unchanged.
The discovery of Compton effect and the consistency of theoretical analysis and experimental results not only strongly confirm the correctness of photon hypothesis, but also confirm that the interaction process of microscopic particles strictly abides by the law of conservation of energy and momentum.
discoverer
Professor arthur holly compton is a famous American physicist and the discoverer of Compton effect. Compton was born in Worcester, Ohio on September 1892 and died in Berkeley, California on March 1962 at the age of 70.
Compton was born in a senior intellectual family, and his father was a philosophy professor and dean of college of wooster. Compton's eldest brother Carl is the head of the physics department of Princeton University, and later became the dean of MIT. He is Compton's closest and best scientific leader.
After graduating from Compton Middle School, he was promoted to college of wooster. Hospital has a long history and tradition, which has a decisive influence on Compton's life and career. Here, his basic education almost completely determines his attitude towards life and science all his life. Outside the college, Compton is familiar with many interesting things, such as the summer camp in Michigan, Carl's early scientific experiments and so on. These are also important for Compton's future scientific career.
19 13 After graduating from college of wooster, Compton entered Princeton University for further study. 19 14 got the master's degree, 19 16 got the doctor's degree. His doctoral thesis was first directed by O W Richardson and then by H L Cook. After receiving his doctorate, Compton taught physics at the University of Minnesota (1916-1917) for one year, and then worked as a research engineer at Westinghouse Electric and Manufacturing Company in East Pittsburgh, Pennsylvania for two years. During this period, Compton did a lot of original work for the army signal corps to develop aviation instruments; And also obtained the design patent of sodium vapor lamp. The latter work is closely related to his later establishment of the fluorescent lamp industry in Nellapak, Cleveland, Ohio, USA. During his stay in Naira Park, he worked closely with Zay Jeffries, the technical director of General Electric Company of the United States, which promoted the development of fluorescent lamp industry and made the development of fluorescent lamp enter the most active era.
Compton's career as a scientist began with the study of X-rays. As early as when he was studying in university, he put forward a new theoretical viewpoint in his graduation thesis, to the effect that the intensity of X-ray diffraction in crystals is related to the electron distribution of atoms contained in crystals. During his tenure at Westinghouse (1917 ——1919); Compton continued his research on X-rays. Starting from 19 18, he made theoretical and experimental research on X-ray scattering. Compton put forward the hypothesis of electron finite linearity (radius1.85×10-10 "cm) according to J·J· Thomson's classical theory, and explained the observation relationship between density and scattering angle. This is a simple beginning, but it leads to the concept of "Compton wavelength" of electrons and other elementary particles. This concept was later fully developed in his own quantum theory of X-ray scattering and quantum electrodynamics.
During this period, his second research was started at 19 17 with Oswrald Rognley of the University of Minnesota, and it was about determining the X-ray reflection density of magnetic crystals by using magnetization effect. This study shows that the electron orbital motion has no effect on the magnetization effect. He believes that ferromagnetism is caused by the inherent characteristics of electrons, which are a basic magnetic charge. The correctness of this view was later proved more strongly by the experimental results of J C Stearns, a student he directed at the University of Chicago.
After the First World War, Compton went to England for further study from 19 19 to 1920, and engaged in research at Cavendish Laboratory in Cambridge. At that time, Cavendish laboratory was in the most prosperous era, and many promising young British scientists moved here from the battlefield to study with Rutherford and J·J· Thomson. Compton thinks this is one of the most inspiring periods, during which he not only established a relationship with Rutherford; I met Thomson. At that time, Thomson spoke highly of his research ability, which greatly encouraged Compton and made him more confident in his point of view. The friendly relationship between Compton and Thomson lasted until the last moment of his life.
During his stay in Cambridge, Compton switched to gamma rays for scattering experiments because the high-voltage X-ray device was not suitable. This experiment not only confirmed the early research results of other scientists at T A Gray, but also laid the foundation for Compton to further study the X-ray scattering experiment.
After that, Compton returned to the United States on 1920, and served as Wayman Crow Professor and Head of Physics Department of Washington University in St. Louis. Here he made the greatest discovery for him. At that time, Compton projected X-rays from molybdenum target onto graphite to observe the scattered X-rays. He found that it contains two components with different frequencies, one with the same frequency (or wavelength) as the original X-ray, and the other with a frequency smaller than the original parent X-ray. This frequency change has a certain relationship with the scattering angle. The first component that does not change the frequency can be explained by the usual wave theory, because according to the wave theory of light, scattering does not change the frequency of incident light. However, the second component, which appears less frequently in the experiment, is puzzling and cannot be explained by classical concepts. Facing the facts observed in this experiment, Compton put forward his own explanation in 1923. He believes that this phenomenon is caused by the collision between light quantum and electron. Optical quantum not only has energy, but also has some momentum similar to mechanical meaning. In the process of collision, photons transfer some energy to electrons, reducing its energy and thus its frequency. In addition, according to the conservation of energy and momentum of colliding particles, the dependence between frequency change and scattering angle can be derived, which can well explain the facts observed by Compton. In this way, people have to admit that light has the nature of particles in addition to the well-known fluctuations. This shows that a beam of light is composed of several particles separated from each other, and these particles show the same properties as those of ordinary matter in many aspects. Compton's scientific research results are published in many journals. 1926, he synthesized his published papers and wrote a book "x-rays and electrons".
1923, Compton accepted the position of professor of physics at the University of Chicago (R·A· Millikan once held this position) and worked with Michelson. Here, he named his first study "Compton Effect". Because of a series of experiments of Compton effect and its theoretical explanation, he shared the 1927 Nobel Prize in Physics with A T R Wilson of Britain. He was only 35 years old at this time. In the same year, he was elected as an academician of the National Academy of Sciences, and 1929 became a professor of C H Swift.
From 65438 to 0930, Compton turned his main interest from studying X-rays to studying cosmic rays. This is because the interaction between high-energy gamma rays and electrons in cosmic rays is an important aspect of Compton effect (today, the anti-Compton effect of the interaction between high-energy electrons and low-energy photons is an important research topic in astrophysics). During the Second World War, many physicists were concerned about the "uranium problem", and Compton was no exception. 1941l65438+1October 6, Compton, as the chairman of the uranium Committee of the National Academy of Sciences, published a report on the military potential of atomic energy, which promoted the development of nuclear reactors and atomic bombs. Lawrence discovered plutonium at the University of California. Soon, the metallurgical laboratory in Manhattan was responsible for plutonium production, and these aspects were mainly led by Compton and Lawrence. Fermi's first nuclear chain reactor was also supported and encouraged by Compton.
At the end of the war, Compton accepted the position of President of Washington University in St. Louis. It was in this school that he made the biggest physical discovery 25 years ago-"Compton effect". 1954, Compton reached the age of retiring from the administrative leadership position of the university. After retirement, he continued to lecture, teach and write books. During this period, he published the book Atomic Exploration. This is a masterpiece, which completely and systematically brings together the research results of all colleagues of Manhattan Project during the war.
Compton is one of the greatest scientists in the world. The Compton effect he discovered is the core of developing quantum physics. This discovery earned him an indisputable position among great scientists.