Crystal Spectroscopy

the branch of spectroscopy devoted to the study of quantum transitions in the system of energy levels of crystalline solids and to the study of the accompanying physical phenomena. Crystal spectroscopy is an important source of information about the properties and structure of crystals. The theoretical basis of the field is the quantum theory of the solid state. Crystal spectroscopy makes extensive use of group theory, which permits consideration of the symmetry properties of the crystal in establishing the symmetry of the wave functions of the energy levels and determining selection rules for allowed transitions between the energy levels. A variety of experimental methods are employed in crystal spectroscopy; use is made, for example, of low temperatures, lasers (as sources of excitation), photoelectric photon counting, modulation methods of spectral recording, and synchrotron radiation.

The great variety of particles and quasiparticles in a crystal that have markedly different characteristic energies is responsible for the absorption and emission of quanta of electromagnetic energy over a broad range of frequencies, from radio waves to gamma radiation. Small energy quanta are mainly associated with the magnetic interactions of particles and are studied by the methods of radio-frequency spectroscopy. X-ray spectroscopy studies the transitions of electrons in the inner electron shells of the atoms and ions that make up a crystal. Gamma radiation results from transitions between nuclear levels. The term “crystal spectroscopy,” however, is generally used with regard to optical spectroscopy, which embraces the range of electromagnetic waves from the far infrared region to the far ultraviolet region.

Crystal spectroscopy investigates absorption, reflection, luminescence, and scattered-light spectra of crystals and the effect on such spectra of various external influences, such as an electric field (the Stark effect), a magnetic field (the Zeeman effect), hydrostatic pressure on a crystal, and directional deformations (the piezospectroscopic effect). Also studied is the dependence of crystal spectra on the polarization of light and on temperature; temperature effects include changes in structure, shifts and broadening of bands, and changes in intensity. When light is absorbed in a crystal, processes of relaxation and of transfer of excitation energy develop. Time measurements of spectral characteristics are important for the study of these processes; such measurements permit, for example, relaxation times and the lifetimes of certain states to be found. If several particles interact with each other while participating in an interaction with radiation, cooperative phenomena arise.

Crystal spectroscopy studies the effect of crystal defects on the spectra of crystals—both existing defects in real crystals and defects purposely created (by, for example, the introduction of impurities) to give a crystal certain properties. The spectra of thin crystalline films and small crystals may have special characteristics owing to the effect of the surface. Besides single-photon processes, multiphoton processes can be observed when a crystal is excited by laser radiation; in multiphoton processes, several photons are produced or disappear in a single event. Various nonlinear effects in crystals are also studied.

Crystal spectroscopy permits the obtaining of information on the system of energy levels in a crystal, on the mechanisms of the interaction of light with a substance, on the transfer and transformation of the energy absorbed in a crystal and its modifications (phase transitions), on photochemical reactions, and on photoconductivity. Through crystal spectroscopy, information can also be obtained on, for example, the structure of a crystal lattice and on the structure and orientation of various defects and impurity centers in crystals. The data of crystal spectroscopy underlie the use of crystals in quantum electronics, and the use of crystals as phosphors, scintillators, light-energy converters, optical materiais, and information-recording cells. The methods of crystal spectroscopy are employed in spectral analysis.