SCATTERING OF LIGHT IN MOLECULES

1. SCATTERING OF LIGHT IN MOLECULES Dr. Anilkumar R.Koppalkar S.S.Margol College, SHAHABAD

2. Scattering in molecules • Two types of scattering takes place in case of molecules. • Coherent Scattering and • Incoherent Scattering. • In case of coherent, the wavelength of light after scattering does not change but intensity of light changes. • Rayleigh shown that the intensity of light is inversely proportional to fourth power of wave length of light. Iα 1/λ4

3. Rayleigh scattering • Rayleigh scattering causes the blue hue of the daytime sky and the reddening of the sun at sunset • Rayleigh scattering is more dramatic after sunset. This picture was taken about one hour after sunset at 500m altitude, looking at the horizon where the sun had set. • Rayleigh scattering is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. • It can occur when light travels in transparent solids and liquids, but is most prominently seen in gases.

4. Rayleigh scattering • The size of a scattering particle is parametrized by the ratio x of its characteristic dimension r and wavelength λ: • Rayleigh scattering can be defined as scattering in the small size parameter regime x ≪ 1. Scattering from larger spherical particles is explained by the Mie theory for an arbitrary size parameter x. The Mie theory reduces to the Rayleigh approximation. • The amount of Rayleigh scattering that occurs for a beam of light is dependent upon the size of the particles and the wavelength of the light. Specifically, the intensity of the scattered light varies as the sixth power of the particle size and varies inversely with the fourth power of the wavelength. • The intensity I of light scattered by a single small particle from a beam of unpolarized light of wavelength λ and intensity I0 is given by: • where R is the distance to the particle, θ is the scattering angle, n is the refractive index of the particle, and d is the diameter of the particle. • The angular distribution of Rayleigh scattering, governed by the (1 + cos2θ) term, is symmetric about the plane normal to the incident direction of the light (i.e. about θ = 90°), and so the forward scatter equals the backwards scatter. Integrating over the sphere surrounding the particle gives the Rayleigh scattering cross section • The Rayleigh scattering coefficient for a group of scattering particles is the number of particles per unit volume N times the cross-section. As with all wave effects, for incoherent scattering the scattered powers add arithmetically, while for coherent scattering, such as if the particles are very near each other, the fields add arithmetically and the sum must be squared to obtain the total scattered power.

5. Rayleigh scattering • A 5 mW green laser pointer is visible at night due to Rayleigh scattering and airborne dust. • Rayleigh scattering from molecules is also possible. An individual molecule does not have a well-defined refractive index and diameter. Instead, a molecule has a polarizabilityα, which describes how much the electrical charges on the molecule will move in an electric field. In this case, the Rayleigh scattering intensity for a single particle is given by • The amount of Rayleigh scattering from a single particle can also be expressed as a cross sectionσ. For example, the major constituent of the atmosphere, nitrogen, has a Rayleigh cross section of 5.1×10−31 m2 at a wavelength of 532 nm (green light). This means that at atmospheric pressure, about a fraction 10-5 of light will be scattered for every meter of travel. • Rayleigh scattering is responsible for the blue color of the sky during the day. Rayleigh scattering is inversely proportional to the fourth power of wavelength, which means that the shorter wavelength of blue light will scatter more than the longer wavelengths of green and red light. This gives the sky a blue appearance.

6. This picture was taken about one hour after sunset at 500m altitude, looking at the horizon where the sun had set Rayleigh scattering causes the blue hue of the daytime sky and the reddening of the sun at sunset A 5 mW green laser pointer is visible at night due to Rayleigh scattering and airborne dust.

7. Tyndall Scattering • The Tyndall effect is an effect of light scattering by colloidal particles or particles in suspension. • It is named after the 19th century Irish scientist John Tyndall. It is similar to Rayleigh scattering, in that the intensity of the scattered light depends on the fourth power of the frequency, so blue light is scattered more strongly than red light. • An example in everyday life is the blue colour sometimes seen in the smoke emitted by motor bikes. The phenomenon is best explained by Mie theory as the particle size is much greater than the wavelength of light.

8. Tyndall Scattering • The basis for distinguishing between Tyndall and Rayleigh scattering is the intensity of the phenomenon. • Colloidal particles are much larger than atoms or molecules. • It follows from scattering theory that Tyndall scattering (by colloidal particles) is much more intense than Rayleigh scattering (by atoms or molecules). • Tyndall scattering can be used to determine the size of colloidal particles. Tyndall scattering is also often used to describe light scattering by macroscopic particles such as dust in the air. • However, this phenomenon is more like reflection, as the macroscopic particles become visible in the process.

9. Porous glass filled with water, sample about 1 mm thick, made by phase separation in a thermal gradient (high temperature on the right) of a sodium borosilicate glass, followed by acid leaching. The colors are caused by the Tyndall effect. Flour suspended in water appears to be blue because blue light is scattered by the flour particles more strongly than red light.

10. Brillouin scattering:inelastic scattering • The phenomenon of inelastic scattering of light due to acoustic phonons was first described by Leon Brillouin (1889-1969) in 1922 and 4 years later in 1926 independently by Leonid Mandelstam. • In order to credit Mandelstam it is also denoted as Brillouin-Mandelstam scattering (BMS). Other commonly used names are Brillouin light scattering (BLS) and Brillouin-Mandelstam light scattering (BMLS). • Brillouin scattering, named for LéonBrillouin, occurs when light in a medium (such as water or a crystal) interacts with time dependent density variations and changes its energy (frequency) and path. • The density variations may be due to acoustic modes, such as phonons, magnetic modes, such as magnons, or temperature gradients. • As described in classical physics, when the medium is compressed its index of refraction changes and the light's path necessarily bends.

11. Brillouin scattering • From a quantum point of view, Brillouin scattering is an interaction of light photons with acoustic or vibrational quanta (phonons), with magnetic spin waves (magnons), or with other low frequency quasiparticles interacting with light. • The interaction consists of an inelastic scattering process in which a phonon or magnon is either created (Stokes process) or annihilated (anti-Stokes process). • The energy of the scattered light is slightly changed, that is decreased for a Stokes process and increased for an anti-Stokes process. • This shift, known as the Brillouin shift, is equal to the energy of the interacting phonon and magnon and thus Brillouin scattering can be used to measure phonon and magnon energies. • The Brillouin shift is commonly measured by the use of a Brillouinspectrometer based on a Fabry-Pérot interferometer.

12. Raman scattering : inelastic scattering • In 1922, Indian physicist ChandrasekharaVenkata Raman published his work on the "Molecular Diffraction of Light," the first of a series of investigations with his collaborators which ultimately led to his discovery (on 28 February1928) of the radiation effect which bears his name. • The Raman effect was first reported by C. V. Raman and K. S. Krishnan, and independently by GrigoryLandsberg and Leonid Mandelstam, in 1928. Raman received the Nobel Prize in 1930 for his work on the scattering of light. • In 1998 the Raman Effect was designated an ACS National Historical Chemical Landmark in recognition of its significance as a tool for analyzing the composition of liquids, gases, and solids. • Raman scattering or the Raman effect is the inelastic scattering of a photon. • Discovered by C.V. Raman in liquids and by GrigoryLandsberg and Leonid Mandelstam in crystals. • The effect is sometimes called Smekal-Raman effect due to the prediction by Adolf Smekal (de). • When light is scattered from an atom or molecule, most photons are elastically scattered (Rayleigh scattering), such that the scattered photons have the same energy (frequency) and wavelength as the incident photons. • However, a small fraction of the scattered light (approximately 1 in 10 million photons) is scattered by an excitation, with the scattered photons having a frequency different from, and usually lower than, the frequency of the incident photons. • In a gas, Raman scattering can occur with a change in vibrational, rotational or electronic energy of a molecule (see energy level). Chemists are concerned primarily with the vibrational Raman effect.

13. Stokes and anti-Stokes scattering • The different possibilities of visual light scattering: Rayleigh scattering (no Raman effect), Stokes scattering (molecule absorbs energy) and anti-Stokes scattering (molecule loses energy) • There are two types of Raman scattering, Stokes scattering and anti-Stokes scattering. • The interaction of light with matter in a linear regime allows the absorption or simultaneous emission of light precisely matching the difference in energy levels of the interacting electrons. • The Raman effect corresponds, in perturbation theory, to the absorption and subsequent emission of a photon via an intermediate electron state, having a virtual energy level (see also: Feynman diagram). There are three possibilities: • no energy exchange between the incident photons and the molecules (and hence no Raman effect) • energy exchanges occur between the incident photons and the molecules. The energy differences are equal to the differences of the vibrational and rotational energy-levels of the molecule. • In crystals only specific photons are allowed (solutions of the wave equations which do not cancel themselves) by the periodic structure, so Raman scattering can only appear at certain frequencies. • In amorphous materials like glasses, more photons are allowed and thereby the discrete spectral lines become broad.

14. Stokes and anti-Stokes scattering • Molecule absorbs energy: Stokes scattering. The resulting photon of lower energy generates a Stokes line on the red side of the incident spectrum. • Molecule loses energy: anti-Stokes scattering. Incident photons are shifted to the blue side of the spectrum, thus generating an anti-Stokes line. • These differences in energy are measured by subtracting the energy of the mono-energetic laser light from the energy of the scattered photons. • The absolute value, however, doesn't depend on the process (Stokes or anti-Stokes scattering), because only the energy of the different vibrational levels is of importance. • Therefore, the Raman spectrum is symmetric relative to the Rayleigh band. • In addition, the intensities of the Raman bands are only dependent on the number of molecules occupying the different vibrational states, when the process began.

15. Raman Effect: Applications • Raman spectroscopy employs the Raman effect for materials analysis. The frequency of light scattered from a molecule may be changed based on the structural characteristics of the molecular bonds. • A monochromatic light source (laser) is required for illumination, and a spectrogram of the scattered light then shows the deviations caused by state changes in the molecule. • Raman spectroscopy is used as a tool to detect high-frequency phonons, magnons and electronic excitations. • Raman spectroscopy is also used in combustion diagnostics. Being a completely non-intrusive technique, it permits the detection of the major species and temperature distribution inside combustors and in flames without any perturbation of the (mainly fluid dynamic and reactive) processes examined. • Stimulated Raman transitions are also widely used for manipulating a trapped ion's energy levels, and thus basis qubit states. • Raman amplification is used in optical amplifiers.