Scattering of photons

Perhaps the best known and most used optical spectroscopy which relies on the use of lasers is Raman spectroscopy. Because Raman spectroscopy is based on the inelastic scattering of photons, the signals are usually weak, and are often masked by fluorescence and/or Rayleigh scattering processes. The interest in usmg Raman for the vibrational characterization of surfaces arises from the fact that the teclmique can be used in situ under non-vacuum enviromnents, and also because it follows selection rules that complement those of IR spectroscopy. 

Raman spectroscopy comprises a family of spectral measurements based on inelastic optical scattering of photons at molecules or crystals. It involves vibrational measurements as well as rotational or electronic studies and nonlinear effects. Following, Raman will be used in the established but slightly inaccurate way as a synonym for the most important and most common technique of the family, linear vibrational Raman scattering. 

Vibrations in molecules or in solid lattices are excited by the absorption of photons (infrared spectroscopy), or by the scattering of photons (Raman spectroscopy), electrons (electron energy loss spectroscopy) or neutrons (inelastic neutron scattering). If the vibration is excited by the interaction of the bond with a wave... 

The second term in Eq. (1.11) is the origin of the scattering in the first Born approximation. It leads to an amplitude for the scattering of photons with propagation vector k0 into photons with vector k equal to... 

The continuous change of the liner absorption along each of the two branches is approximately defined as p = kZ, where Z is the atomic number of the chemical element and A is a constant, specific for each of the two continuous parts of the absorption function. The continuous branches correspond to the absorption occurring due to random scattering of photons by electrons, which is observed in all directions, thus reducing the number of photons in the transmitted beam in the direction of the propagation vector. 

Given the similarities of the scattering of photons and photelectrons by O and N atoms, it is not possible to distinguish bonding to O from bonding to N. 

The multiple scattering of photons in the cloud and between the cloud and the ground enlarges the average photon path length. Therefore any absorption by tropospheric ozone or aerosols is amplified and thus the spectral shape of the spectrum is significantly modified by clouds. 

Figure 3.10. Setting of lower and upper energy windows in the energy spectra of a scanner for a line source in air (open circle) and in a water-filled cylinder ( filled circle). The energy spectrum from water-filled phantom shows scattering of photons. (Reprinted with the permission of The Cleveland Clinic Center for Medical Art Photography 2009. All Rights Reserved)...

Einstein in the theory of the photoelectric effect, and Compton in the theory of die scattering of photons (Ch. 6), showed that photons have not only a discrete energy, but also a discrete momentum (cf. 4.2)... 

Scattering of photons by gas molecules and particles can also reduce the radiant energy transmitted through a layer. In contrast to absorption, the radiant energy scattered remains in the form of radiation, but its direction is altered from that of the incident radiation. If we return to Figure 3.7 and assume that the medium does not absorb but only scatters radiation, then we can write, similarly to (3.10),... 

Raman spectroscopy involves the inelastic scattering of photons. The energy lost or gained by the photons is used to create or annihilate phonons in the crystal. The essential features of Raman spectroscopy can be understood using the following simplified argument. Let be the electronic polarizability and jc the displacement ... 

Most young stars are surroimded by CS disks consisting of gas and dust (Strom et al. 1993). The inner parts of CS disks are ionized and some part of the polarized radiation of the Herbig AeBe stars can arise due to Thomson scattering of photons by free electrons. 

Raman Scattering is inelastic scattering of photons of energy hco. In this case, the laws of conservation of momentum and energy analogous to Eqns. (5.2) and (5.3) also hold ... 

---