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334                           Lasers

                                   cards (to make forgery more difficult), and there are, as some people know,
                                   holograms in every supermarket scanner that reads bar-codes.
                                     A scanner works in the following way. A series of holograms are recorded
                                   on a disc. As the disc rotates and is illuminated by a laser (the original reference
                                   beam) each small part of the disc gives rise to an object beam moving in a
                                   different direction. The total effect is a continuously moving light beam that
                                   scans the bar-code.
                                     It is worth mentioning at this stage a potential application that has been
                                   talked about for several decades. It is holographic data storage based on the
                                   recording of multiple holograms. These will be discussed among memory
                                   elements in Appendix VI, Section A6.7.

                                   12.13.19  Raman scattering
                                   Raman scattering is a process involving the interaction of a photon with a mo-
                                   lecule which leads to a change in the photon frequency. It is different from
                                   conventional Rayleigh scattering, which does not involve a frequency change.
                                   Because it necessarily involves a change in energy, it is sometimes known
                                   as inelastic photon scattering. Raman scattering was predicted theoretically
                                   by Adolf Smekal in 1923 and first observed experimentally almost simultan-
                                   eously, in 1928, by the Indian scientist Sir C.V. Raman and his collaborator
                                   K.S. Krishnan (in liquids) and the Russian physicists, G. Landsberg and L.
                                   Mandelstam (in crystals). Priority has therefore been disputed, but Raman was
                                   awarded the Nobel Prize for discovery of the effect in 1930.
                                     Polyatomic molecules typically support many different vibrational states,
                                   each with a different energy. Raman scattering is a two-step process. The first
                                   involves the promotion of a molecule from one vibrational state to a higher, ex-
                                   cited state, as shown in Fig. 12.29, by absorption of the energy of an incoming
                                   photon (hf, where h is Planck’s constant and f is the photon frequency). The
                                   second involves the re-emission of a photon as the molecule relaxes back to a
                                   de-excited state.
                                     Generally there are three possible outcomes. If the molecule relaxes back
                                   to the initial state, the photon will be re-emitted with its energy (and hence
                                   its frequency) unaltered, as shown in Fig. 12.29(a). This process is known as
                                   Rayleigh scattering. If, on the other hand, it relaxes back to a different state, Ra-
                                   man scattering occurs. The de-excited state may have higher vibrational energy


                                             Excited state



                                            hf      hf      hf     hf ΔE – hf     hf + ΔE
                                                                   De-excited
                                                                     state
                                                                  ΔE

                                                                                  De-excited
                                             Initial state
     Fig. 12.29                                                            ΔE       state
     (a) Rayleigh scattering; (b) and (c)
     Stokes and anti-Stokes scattering.      (a) Rayleigh    (b) Stokes     (c) anti-Strokes
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