Cheng Zhi Huang, Jian Ling, Yuan Fang LI 1 Introduction to light scattering
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f = 5 cm
Blue-purple Optical filter Focal length f = 230 cm Diameter D = 18 cm The first lens Yellow-green optical filter Observer Sunlight Rayleigh scattering Figure 1.9: Experiment setup designed for Raman scattering observation by Sir. Raman. Introduction to light scattering 21 Brought to you by | University of Iowa Libraries Authenticated Download Date | 1/19/20 3:36 AM Inspired by Compton e ffect * , Raman named it as modi fied scattering, and wrote a short assay titled “New-type second-level radiation” on 16 February 1928 and pub- lished in Nature on 31 March 1928. The signal was later called Raman scattering. In 1930, an American spectroscopist Robert Williams Wood (1868 –1955) named the modi fied scattering with lower frequency as the Stokes line and modified scattering with higher frequency as the anti-Stokes line. 1.6.2 Raman scattering spectrum Raman ’s discovery proved that, except for some light components of the original incident light (Rayleigh scattering) there are some weak light scattering signals with di fferent wavelength from that of the incident light (Raman scattering). Since the scattering of light is related to the intensity of the incident light, these weak light scattering signals can only be observed with a strong incident light. That is, developing strong light source is an e ffective way to study the weak signal and weak interaction. Raman ’s discovery proved the prediction of the Austrian theoretical physicist Adolf Gustav Stephan Smekal (1895 –1959) in 1923 [7]. Later, the former Soviet Union physicist Grigory Samuilovich Landsberg (1890 –1957) and Leonid Isaakovich Mandelstam (1879 –1944) also observed the scattering in a quartz crystal [8]. Raman ’s discovery provided new evidence for the light quantum theory and quickly won the public recognition, and thus Raman won the Nobel Prize in physics in 1930, only two years after his short note in Nature published. After X-ray Compton e ffect was found in 1920, Heisenberg predicted a similar e ffect in visible light in 1925. Thus, the British Royal Society named that Raman Scattering is “one of the most excellent discoveries of experimental physics in the twenty century. ” Raman’s discovery greatly promoted the field of spectroscopy and also reflected deep influence to scienti fic history of India and even Asia. In 1930 –1934, a Czechoslovakian physicist George Placzek (1905–1955) researched the first Raman spectrograph, greatly promoting Raman Effect research. The spectrograph used the mercury arc as the source of light and a camera for analysis and test. However, the Raman spectrum, owing to weak signal and easily interfered by the strong Rayleigh scattering was limited to the study of chemical molecule vibrational spectra. With the appearance of intense light source laser until 20 st century 60 th year, Raman spectroscopy started its splendid history and now becomes a newly-developed study field [9–10]. As different chemical structures and di fferent physical states, different molecules have their specific Raman spectra. This * The Compton effect was discovered by the American physicist Arthur Holly Compton (1892–1962) in 1920. He found that there is longer wavelength besides the scattering light identical to the original wavelength when X-ray is scattered by a crystal. 22 Cheng Zhi Huang, Jian Ling, Yuan Fang Li Brought to you by | University of Iowa Libraries Authenticated Download Date | 1/19/20 3:36 AM information plays an important role in the study of molecular structure, similar to the fingerprint region of an infrared spectroscopy, which then is named as the Raman fingerprint. The Raman spectrum is produced by the collision of photons and the outer electrons of molecules. During the collision, overlapping of molecular vibration or rotation energy and photon energy occurs, and the electron rises to an imaginary energy level and goes back to a higher or lower energy level, making the frequency of the scattered light changed, and the photon either obtains energy from molecules or passes energy to the molecules. Since the changes of the frequency of scattered light are closely related to the molecular or atomic microstructure, so the Raman spectrum is a useful tool for the investigation of molecular structure. People could understand molecular or atomic structural characteristics through the scattering spectrum. In the Raman scattering spectrometry, on the both sides of the incident light with a frequency v 0 , there are many Raman scattering spectral lines with a frequency of v 0 ±v i (i=1, 2, 3, … ), where v i = ΔE i /h. Lines on the side of the long wavelength is called as Strokes line, which is ascribed to the decrease of the scattering light energy owing to the energy transfer from the photon to a molecule. Anti-Strokes line, on the other hand, is located at the short-wavelength side with an increase in the energy of scattered light owing to the energy acquisition from molecules. Thus, the frequency di fference (v i ) of the Raman scattering spectrum is irrelevant to the frequency of the incident light (v 0 ), but is decided by the structure of the substance. The Raman scattering peak has certain correspondence with its infrared spectrum peak and can be used for qualitative and quantitative analysis of a substance. In the fluorescence spectrum scanning with a fluorospectrophot- ometer, the solvent Raman scattering peak could be detected. By changing the excitation wavelength we can observe the changes of the location of emission peak, which helps to judge whether the peak is Raman scattering peak of the test solvent or not. 1.7 Brillouin scattering 1.7.1 Brillouin scattering discovery Brillouin scattering is one of scattering phenomena, which is caused by the sound speed propagation pressure fluctuation in the substance, and was first put forward by a French physicist Léon Nicolas Brillouin (1889 –1969) in 1922. Brillouin studied physics in Paris in his early years and took part in lattice X-ray di ffraction study. In 1914, he studied scattering light frequency spectrum. Although he joined the army for the First World War in 1914 –1919, he went back to Paris University afterward and continued his study in quantum theory of solids, and became the doctor of science. In Introduction to light scattering 23 Brought to you by | University of Iowa Libraries Authenticated Download Date | 1/19/20 3:36 AM his PhD paper, he mentioned solid-state equation based on atomic vibration (pho- non). In 1922, Brillouin calculated scattering light frequency distribution of density fluctuation of a sound wave in a scatterer, and predicted that there should be symmetrically distributed lines with di fferent wavelengths around the incident light frequency. Brillouin also made great contributions to quantum mechanics, air radio wave propagation, solid-state physics, and information theory. He studied monochromatic light wave transmission and wave band function, and found that scattering light has frequency shifts much smaller than the Raman scattering frequency, which are related to the frequency shift and scattering angle and can be used for acoustic vibration researches in gas, liquid, and solid. 1.7.2 Essence of Brillouin scattering When the light interacts with the areas in a medium (such as air, water, or crystal) with the time-dependent light density to produce the changes of light energy (frequency) and propagation path, Brillouin scattering occurs. The density changes may be caused by the acoustic mode such as phonons, or magnetic modes such as magnetic oscillators, or temperature gradient. As described in classic physics, when a medium is compressed, the refractive index changes, conforming to Doppler frequency shift (refer to Section 1.8.1). Brillouin scattering is an phenomenon that discloses the interaction among light wave, sound wave, magnetic wave, or any other vibration waves to slightly change the photon propagation direction and vibrational frequency. Any change in photon vibrational frequency is comparable to the frequency of the interacting sound wave or magnetic wave, and so the Brillouin scattering spectrum could be applied to analyze and determine the speed of the sound, temperature, and other parameters in the medium. Similar to Raman scattering, Brillouin scattering is an inelastic scattering of elementary excitation in a medium, and the frequency changes show the energy of elementary excitation. However, Brillouin scattering is di fferent from Raman scattering, which only involves in the small-energy elementary excitation, such as phonons and magnetic oscillators. As a practical study method, although Evgenii Fedorovich Gross (1897 –1972), the former Soviet physicist, claimed to observe the Brillouin –Mandelstam light scattering during their observation of the fine structures of Rayleigh scattering in a condensed matter in 1930, the Brillouin scattering signal is weak, and it did not develop quickly until the appearance of intense light source laser. To some degree, Brillouin scattering is in the field of Raman scattering, namely, the inelastic scattering of elementary excitation in a medium. Even though, Brillouin scattering is because of the vibration of sound, magnetic, or other 24 Cheng Zhi Huang, Jian Ling, Yuan Fang Li Brought to you by | University of Iowa Libraries Authenticated Download Date | 1/19/20 3:36 AM vibration waves, while Raman scattering is because of molecule vibration with di fferent sound waves and molecule vibration frequencies. The vibration frequency of Brillouin scattering is below 500 GHz, while Raman scattering could reach THz range. Besides, Brillouin scattering and Raman scattering have obvious di fferences in terms of equipment structure and applications. 1.8 Dynamic light scattering and static light scattering 1.8.1 Dynamic light scattering In terms of time concept, the detection mode of the light scattering signals, there are dynamic light scattering and static light scattering. The concepts are originated from the ignorance of the movements and time-dependance of the scattering particles, namely, the dynamics. In fact, particles in the solution have Brownian movement, which produces the Doppler shift. In 1842, Christian Johann Doppler (1803 –1853), an Austrian physicist and math- ematician, proposed the concept of Doppler shift. He believed that the radiation wavelength of a substance changes with the radiation source and observer ’s relative movements. That is, the wave in front of a moving radiation source is compressed (blue shift), while later the wave is stretched (red shift). The faster the speed of the radiation source, the larger is the shift e ffect. Thus, when the light falls on the moving Download 275.97 Kb. Do'stlaringiz bilan baham: |
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