1928, Indian physicist Raman CV first discovered Raman effect and won the Nobel Prize in Physics (1930). In the early 1960s, the appearance of laser provided an ideal monochromatic light source for the generation of Raman spectrum. Since 1970s, the development of new technologies such as monochromator, detector, optical microscope and computer has greatly improved the testing performance of laser Raman spectrometer. Raman spectroscopy, as a supplementary technology of micro-area nondestructive analysis and infrared absorption spectroscopy, can quickly judge the natural frequency of molecular vibration in gemstones, judge the symmetry of molecules, the magnitude of molecular internal force and the properties of general molecular dynamics, and provide a fast and effective detection means for gemstone appraisers to study molecular composition, molecular ligand structure, molecular cluster structural units, orderly-disordered occupation of ions in minerals, etc. (see Figure 2-2-22).

Figure 2-2-22 Laser Raman Spectrometer

I. Basic principles

Laser Raman spectroscopy is a kind of molecular combined scattering spectrum that changes the original incident frequency after inelastic collision between laser photons and gem molecules. The scattering spectrum of this inelastic collision is usually called Raman spectrum.

In the process of collision between laser photons and molecules, in addition to being absorbed by molecules, scattering will also occur. Due to different collision modes, there are many forms of scattering between photons and molecules:

1. Elastic collision

There is no energy exchange between photons and molecules, but the direction of photon movement is changed, and its scattering frequency is equal to the incident frequency. This scattering is called Rayleigh scattering in the spectrum.

2. Inelastic collision

The energy exchange between photons and molecules in the collision process changes the direction of photon motion and energy, making the scattering frequency and incident frequency different. This kind of scattering is called Raman scattering in spectrum.

3. The energy difference between two transitions of Raman scattering.

When the frequency of scattered light is lower than that of incident light, molecular energy will be lost. This type of scattering line is called Stokes line. If the frequency of scattered light is higher than the frequency of incident light, the molecular energy increases, and this scattered light is called anti-Stokes line. The former is that molecules absorb energy and jump to a higher energy level, while the latter is that molecules release energy and jump to a lower energy level.

Because molecules are usually in vibration ground state at room temperature, Stokes lines are the main ones in Raman scattering, and the intensity of anti-Stokes lines is very low, which is generally difficult to observe. Stokes lines and anti-Stokes lines are collectively called Raman spectra. Generally speaking, Raman shift is determined by the vibrational energy level in the molecular structure of gemstones, and has nothing to do with its radiation source.

Second, the application in gemology

1. Composition and genetic types of inclusions in gemstones

The composition and properties of inclusions in gems are of great significance to identify the origin, species and source of gems. The traditional identification method of solid mineral inclusions is to polish the mineral inclusions to the surface of the sample, and then analyze and test them with electron probe. On the other hand, the study of fluid inclusions mainly uses micro-cooling heating table to observe the change characteristics of each phase in fluid inclusions during freezing and heating, measure the uniform temperature, low melting point temperature and freezing temperature, and finally infer or calculate the molecular composition, density, formation temperature, pressure and salinity of fluid inclusions through phase equilibrium data. The above methods are destructive tests, which are obviously not suitable for gem identification and research.

Raman spectroscopy has the advantages of high resolution, high sensitivity, rapid and nondestructive, and is especially suitable for 1? Identification and study of single fluid inclusions (see Figure 2-2-23) and various solid mineral inclusions with m size. For example, the Raman spectrum test results of diamond inclusions in Liaoning No.50 rock pipe show that the common mineral inclusions in diamonds in this area are olivine, chromite, chromite-magnesium aluminum garnet, magnesium aluminum garnet, metal sulfide minerals, graphite and fluid inclusions.

For another example, the fluid inclusions in yellow sapphire synthesized by hydrothermal method in Guilin were tested by Raman spectroscopy, and it was determined that there were carbonate (mineralizer) in the liquid phase. For another example, the ruby synthesized by flux and molten ruby are tested by Raman spectroscopy, and the Raman peaks of flux residue (crystal) and secondary glass body (amorphous body) are determined. The former has a group of dense Raman sharp peaks with high relative counting intensity in the range of 800 ~ 1000 cm- 1 (see Figure 2-2-24).

Figure 2-2-23 Raman spectra of fluid inclusions in beryl show calcite daughter minerals.

Figure 2-2-24 Raman spectrum of flux residue in synthetic ruby

2. Identification of artificially treated gemstones

In recent years, the artificial filled gemstones in the jewelry market are mostly artificial resin filled jadeite, emeralds, turquoise and lead glass filled rubies, diamonds and so on. All kinds of filling materials in gem cracks bring certain difficulties to jewelry appraisers. However, Raman spectrum analysis and testing technology is helpful to identify them correctly.

The Raman peak characteristics of epoxy resin in jadeite after filling treatment are as follows: the weak infrared absorption band caused by benzene ring stretching vibration is at 3069cm- 1, the corresponding infrared absorption band caused by vas(CH2) asymmetric stretching vibration is at 2934cm- 1, and the sharp infrared absorption band caused by vs(CH2) symmetric stretching vibration is at 2873 cm-65438+. Raman spectrum analysis and testing technology has also achieved satisfactory results in the identification of dyed black pearls and marine cultured black pearls.

3. Identification of similar gemstone varieties

In nature, the Raman spectra of the most widely distributed silicate gemstones are mainly composed of complex silicon-oxygen tetrahedral groups or vibration spectra of groups. Because the characteristic vibration frequencies of molecular groups in various silicate gems (Si-O stretching vibration, Si-O-Si and O-Si-O bending vibration) are obviously different, their Raman spectra show different characteristics. For example, Raman spectroscopy can effectively identify black jadeite and its similar jade species, such as black amphibole jade, black sodium chrome pyroxene jade, black serpentine jade and black nephrite. Figure 2-2-25 shows the Raman spectrum of Mosixiyu, showing that its mineral composition is albite, hornblende, chromite, sodalite and chromite.

Figure 2-2-25 Raman Spectra of Mosaic Jade

Ab.albite; Eh. Hornblende; Ast。 Chrome jadeite; Kch。 Sodium chrome pyroxene; Chronicle chromite