As a Raman scattering (also Raman effect or Smekal Raman-effect ) is the inelastic scattering of light of molecules referred to. It is named after CV Raman , who was able to demonstrate the effect for the first time in 1928.
Due to the inelastic interaction, an energy transfer takes place, i. That is, the scattered light has a higher or lower frequency than the incident light beam and is specific to the scattering atom or molecule. Due to the smaller scattering cross-section, however , the proportion of the frequency-shifted light is a factor of 10 3 to 10 4 less than the light of the elastic scattering, which is referred to as Rayleigh scattering .
The effect was predicted by Adolf Smekal in 1923 - hence the occasional Smekal-Raman effect - and demonstrated on February 28, 1928 by CV Raman and KS Krishnan (on liquids) and independently of that by Grigory Landsberg and Leonid Mandelstam (on crystals) . Raman received the Nobel Prize in Physics for this in 1930.
If there is an interaction between a photon and a molecule or crystal , there is a very low probability of an energy transfer between the stimulating photon and the stimulated matter, in which the rotation and oscillation energy of the molecule involved or the oscillation energy in the Crystal lattice changes. Both directions of energy transfer are possible:
- Stokes-Raman scattering (cf. Stokes shift ): Energy transfer from the photon to the scattering molecule. After the scattering process, this is at a higher energy level than before, the energy and frequency of the scattered photon are lower than those of the exciting photon.
- Anti-Stokes-Raman scattering : transfer of energy to the photon from the scattering molecule. After the excitation process, this is at a lower energy level than before; the scattered photon has higher energy and higher frequency than the exciting photon.
The energy difference between the radiated and scattered photon is linearly linked to the Raman frequency shift via Planck's quantum of action and is characteristic of the scattering molecule:
- ( here stands for the frequency of the oscillation of the molecule )
If the scattering molecule is in the gaseous or liquid phase, molecular oscillations and rotations are considered. If the sample substance is a crystalline solid , lattice vibrations ( phonons ), electron-hole excitations or spin-flip processes are responsible for the Raman effect.
Differences to fluorescence
In fluorescence , the system is excited by the absorption of a photon and, after the life of the excited state, emits a photon whose energy is less than or equal to that of the original. The prerequisite for fluorescence to occur is that the original photon must be resonant to an atomic or molecular electronic transition. Raman scattering, on the other hand, is not a resonance phenomenon . The scattering takes place here - like, for example, Rayleigh scattering - via virtual levels, i.e. it also occurs for photon energies outside of an atomic resonance.
The Raman tensor is used to calculate the interaction of matter and light , which describes the relationship between the scattering intensity and the polarization of the incident light and the polarization of the scattered light:
Since and are freely selectable experimentally, the Raman tensor alone determines the scattering behavior of matter. It is given both by the symmetry of the solid (or molecule) and by the symmetry of the lattice vibration (or molecular vibration). Knowledge of the point groups and the possible symmetry operations is decisive here .
The Raman selection rules can be determined with the aid of the Raman tensor .
Raman scattering in plasmas
While in atomic and molecular physics the Raman effect is usually understood as the inelastic scattering of light by lattice vibrations, in plasma physics it means the scattering of plasma waves . The light amplifies the plasma wave during the scattering process (Raman instability ). The plasma is heated up.
- is the angular frequency of the irradiating laser and
- the plasma frequency in free plasma
In the backward direction, one usually only sees the laser frequency and the Stokes frequency
Resonance Raman Effect
Phonon Raman Scattering
The state space of the phonons in the crystalline solid can be illustrated by the phonon band structure. It is about energy surfaces in the space of wave numbers. A solid body made up of N unit cells with an r atomic basis has 3 r dispersion branches in three dimensions, each with N oscillation states, i.e. a total of 3 No oscillation modes. These 3 r dispersion branches are divided into 3 acoustic branches and 3 r -3 optical branches. For acoustic phonons the frequency disappears linearly in the limit case of long wavelengths, the slope is given by the speed of sound. Optical phonons, on the other hand, have a fixed finite frequency in the limit of long wavelengths.
Since the wavelength of visible light is significantly larger (several orders of magnitude) than the atomic distance in the solid, this means in reciprocal space that the excitation of lattice vibrations by light takes place close to the Γ point, i.e. H. in the small area around the center of the 1st Brillouin zone. The consequence of this is that the momentum transfer is only very small. An excitation of several phonons, the total momentum of which is close to zero, is also possible (multi-phonon process). An example is the excitation of two oppositely running transverse acoustic phonons at the X point (2TAX), whose energies add up. But their total impulse is zero.
Elastic scattering of high-energy radiation
Scattering of high-energy electromagnetic waves (at least X-rays ) on free (or quasi- free ) electrons is called Compton scattering . Since no internal degrees of freedom are excited, the impact is elastic. During the scattering process, energy is transferred to the electron: its momentum increases, so the scattering is inelastic. With lower energies of the incident light, the momentum transfer from the scattering light to the electron is negligible. This scattering is elastic and is called Thomson scattering .
Raman scattering forms the basis for Raman spectroscopy , which is used to investigate material properties such as crystallinity , crystal orientation , composition , strain, temperature , doping , etc. Furthermore, the Raman scattering and its temperature dependency in glass fibers is used for the spatially resolved fiber-optic temperature measurement ( distributed temperature sensing , DTS).
In multi-axis or satellite-based differential optical absorption spectroscopy in air , the recorded spectra have to be corrected according to the Raman scattering in order to be able to draw conclusions about absorbers with lower optical density . The Raman scattering causes the Fraunhofer lines and absorption lines of atmospheric absorbers to be “filled in” depending on the length of the light path and the scattering angle in the atmosphere. The optical thicknesses explained in this way are up to 0.1. Both rotation and vibration ram scatter and the combination of the two effects contribute to this.
Surface enhanced Raman scattering
This effect was first demonstrated by Martin Fleischmann et al. Observed in 1974 when studying the adsorption of pyridine on a rough silver electrode surface. They explained the found intensities of the Raman signals with the fact that the correspondingly larger surface due to the roughness enables an increased absorption of pyridine molecules and thus causes higher signal intensities, which is why they did not attach adequate importance to their discovery. Thus, the actual discovery of the SERS effect goes back to Jeanmaire and van Duyne as well as Albrecht and Creighton.
Raman scattering of molecules has a very small scattering cross-section (approx. 10 −30 cm²), so that a relatively high concentration of molecules is required to obtain a detectable signal; Raman spectra of individual molecules are not possible in this way. However, if the molecule is near a metallic surface (especially silver and gold ), the Raman signal can be extremely amplified. This is the so-called surface-enhanced Raman scattering ( surface enhanced Raman scattering , SERS ). Two mechanisms are discussed here:
- In chemical amplification, the molecule forms a complex which has new energy levels compared to the molecule. Excited electrons can jump from the metal to the molecule and back, leaving the molecule in an excited vibrational state . One also speaks of a temporary transfer of cargo. Reinforcements up to 10 2 are given. In order for a complex to form, a chemical bond between the metal and the molecule is required; i.e. the molecule must be chemisorbed on the surface .
- The electromagnetic amplification is based on the excitation of surface plasmons in the metal, which can generate very high fields locally at tips on the surface or in particles. This field together with the incident light excite the molecule and thus lead to increased Raman scattering. Reinforcements on the order of 10 6 to 10 10 are discussed. This effect drops off rapidly above the surface (roughly with the ninth power of the distance r , i.e. r 9 ), but the molecule does not need to be bound to the surface.
If both effects work together with the resonance Raman effect, it is possible to detect Raman spectra of individual molecules.
The ability to detect different compositions of substances in the nanogram range makes surface-enhanced Raman spectroscopy a versatile analytical method in the fields of pharmacy, materials science, forensics and security sciences. Drug and explosive detectors, among other things, are possible areas of application in this area.
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