Scattering

Under dispersion is understood in physics in general, the deflection of an object, by interaction with a local another object ( scatterer ). Examples include the atoms or scattering of light by fine particles of electrons or other electrons from neutrons by nuclei.

The strength of scattering is indicated by the scattering cross-section as above. The name comes from the fact that the scattering cross section for scattering of classical mass points to a hard sphere is just equal to the cross section of the sphere.

We distinguish between elastic and inelastic (or inelastic ) scattering:

• Case of elastic scattering ( see also Elastic Collision ) is the sum of the kinetic energy after the collision the same as before
• In inelastic scattering it changes the other hand, for example, is a part of the available kinetic energy into excitation energy of an atom is above or, for example in Ionisationsvorgängen used to break a tie.

Inelastic scattering in the narrow sense means that the incident particle, is after the impact, albeit with reduced energy still present; in a wider sense also absorption processes ( processes in which the incident particle "disappears" ) are counted among the inelastic scattering processes sometimes.

In the scattering of waves one also distinguishes between coherent and incoherent scattering. In the case of coherent scattering there is a fixed phase relationship between the incoming and the scattered wave, in the case of incoherent scattering is not. Become a coherent beams coherently scattered, the scattered beams can interfere with each other. This one uses in particular in the X-ray diffraction.

The theoretical description of scattering is the task of scattering theory. Scattering experiments provide information about the shape of the interaction potential.

Ernest Rutherford was based on the scattering of alpha particles by atoms having a kinetic consideration, that atoms must contain a heavy nucleus.

The experiments of high energy physics are generally referred to as scattering experiments, even when doing such as new particles are formed.

Scattering angle, forward and back scattering

The scattering angle θ is defined as the angle at which the scattered particles is deflected. As a forward scattering scattering processes are referred to, which are only to a small distraction comes ( small scattering angles). Backscatter or backscatter referred scattering processes with a scattering angle between 90 ° and 180 ° (see also kinematics ( particle impingement ) ).

If both collision partners ( scattering and scattered particles ) have a rest mass (ie no photons are ), is often considered the scattering angle in the center of mass system in scattering experiments in nuclear and particle physics; this is important for the theoretical consideration as the scattering angle in the laboratory system.

In many cases, the forward scattering is much stronger than scattering in other directions, and therefore has a relatively large differential cross -section. A well-known from everyday example is the scattering of light by dust particles in the air: one looks almost in the direction of the light source ( for example, when sunlight falls into a dark room ), the dust particles are clearly visible as bright spots.

The scattering in the backward direction ( θ = 180 °) in the framework of classical physics mostly weaker than in all other directions, but may be due to quantum mechanical effects and interference effects stronger than the scattering in neighboring directions. Coherent backscattering is also responsible for the high brightness of the full moon.

Certain cases of scattering

Electromagnetic wave - Elementary:

• Thomson scattering: elastic scattering of quasi-free electrons ( the limiting case of Compton scattering for small photon energies ).
• Compton scattering: elastic scattering of quasi-free electrons.

Electromagnetic wave of matter:

• Rayleigh scattering: Elastic ( no power transmission ) electromagnetic scattering objects that are smaller than the wavelength, and dipole dispersion
• Raman scattering: inelastic scattering of atoms, molecules and solids
• Mie scattering: electromagnetic scattering from objects in the order of the wavelength, also Lorenz -Mie scattering, named after the German physicist Gustav Mie (1868-1957) and the Danish physicist Ludvig Lorenz (1829-1891)
• Phonon Raman scattering: inelastic scattering by optical phonons ( lattice vibrations in the frequency range of visible light )
• Brillouin scattering: inelastic scattering on acoustic phonons ( lattice vibrations in the frequency range of sound ).

Matter to matter (see kinematics):

• Rutherford Scattering: charged particle in the atomic nucleus, elastic
• Mott scattering: Rutherford scattering, but with consideration of the spin
• Neutron scattering: thermal neutron of crystal, elastic or inelastic; fast neutron in the nucleus, elastic ( see also moderator) or inelastic
• Electron diffraction: electron to the solid state ( crystal lattice )

Interaction between electromagnetic radiation and matter

The following is a schematic representation of the interaction of a photon with an atom. The horizontal lines represent the discrete excited states of the atom that can occupy the electron point- represented. The bottom line corresponds to the energetic ground state.

Thomson scattering

As Thomson scattering is called the coherent interaction with a ( quasi- ) free electron. Here, the energy of the scattered photon, but does not change.

Compton scattering

As Compton scattering of the incoherent process is known in which a photon scattered from a free or only weakly bound electron. In electron scattering on an atom that is ionized by this process and it will be a photo- electron and a photon reduced energy emitted. This scattering is elastic, since the sum of the kinetic energy is identical before and after the collision. For an inelastic process kinetic energy has to be converted into internal energy, said internal degrees of freedom are excited (via one electron but has not ). A change in internal energy is also always linked by the mass-energy equivalence with a change in mass. In fact, the rest masses of the photon and the electron does not change.

Rayleigh scattering

The scattering process is coherent, ie, the coherence -preserving. The energy (h is Planck's constant, the frequency ) of the incident photons is too low to excite the atom. The dispersion takes place of bound electrons, the energy of the scattered photon is not changed. In the classical limit, that is a large wavelength of the photon compared to the Bohr radius of the atom, it is called Rayleigh scattering. A special feature is that the scattering cross section σ is strongly dependent on the frequency and increases in proportion to ν4. Twice the frequency is scattered by 24-fold (= 16 ) times more, this is the cause for the sky blue and the sunset.

Raman scattering

In the inelastic se Raman scattering is observed deviation of the energy of the scattered photon and the energy of the incident photon. The energy difference is just the excitation energy of rotation or vibration of the molecule ( the Raman effect 1st order ). This difference in energy is emitted to the atom or is absorbed by the photon. The energy of the scattered photon is then ( energy transfer to the molecule ) or ( energy absorption from the photon ).

Resonance absorption, spontaneous emission, fluorescence and phosphorescence

Does the energy of an incoming photon exactly the difference between two discrete energy levels, the photon is absorbed by the atom ( one also speaks of resonance absorption ). The atom it is in an excited state, which can decompose through various channels. Follow within a short time the emission of a photon of similar frequency, it is called fluorescence. The energy of the fluorescence photon can be lower due to non-radiative relaxation processes in the atom, as its the radiated energy. The life of ( the ) excited state (states ) is typically a few nanoseconds (see fluorescence lifetime ). If the residence time much longer than a few nanoseconds, it is called phosphorescence ( often Phosphoreszenzübergänge spin- forbidden transitions ). Note that in both cases the emitted and absorbed photon have no fixed phase relationship, so it is an incoherent scattering process.

Stimulated emission

When stimulated emission an existing excited atom is excited by a photon with a matching energy irradiated to the emission of a second photon coherent.

Photoelectric effect

An absorption process in which an electron takes full energy of the photon is referred to as a photoelectric effect. For which a certain bond strength of the electron for the sake of kinematics is necessary; therefore, the cross section for the photoelectric effect is greatest in the innermost shell ( K-shell ) of heavy atoms.

This is not really a scattering process, but rather an absorption process, since then no scattered photon longer exists. In photoelectron spectroscopy considering the photoelectrons, a distinction being made among others the excitation with UV or X-ray radiation ( UPS and XPS).