I. Basic principles
Infrared light with energy of 4000 ~ 400 cm- 1 is not enough to make the sample produce molecular electronic energy level transition, only vibrational energy level and rotational energy level transition. Because every change of vibration energy level is accompanied by many changes of rotational energy level, infrared spectrum belongs to a band spectrum. In the process of molecular vibration and rotation, when the molecular vibration is accompanied by the change of dipole moment, the change of intramolecular charge distribution will produce an alternating electric field, and only when its frequency is equal to the frequency of incident radiation electromagnetic wave will infrared absorption occur.
The conditions of infrared spectrum generation are as follows: ① radiation should have the energy needed to meet the vibration transition of matter; ② There is mutual coupling between radiation and matter. Symmetric molecules have no dipole moment, radiation will not cause * * * vibration, and there is no infrared activity, such as N2, O2, Cl2, etc. Asymmetric molecules have dipole moment and infrared activity.
(A) the vibration of polyatomic molecules
Due to the increase of the number of atoms, the bonds or groups and spatial structures of polyatomic molecules are different, and the real vibration spectrum of molecules is more complicated than that of diatomic molecules. However, as a good approximation under certain conditions, all possible complex vibration modes of molecules can be regarded as the superposition of finite independent and relatively simple vibration modes, which are called normal vibrations.
(B) the basic form of normal vibration
Normal vibration forms are generally divided into telescopic vibration and bending vibration (deformation vibration).
1. telescopic vibration
It means that the distance between atoms changes periodically along the bond axis, and the vibration with constant bond angle is called telescopic vibration, which is usually divided into symmetric telescopic vibration and asymmetric telescopic vibration. For the same group, the frequency of asymmetric stretching vibration is slightly higher than that of symmetric stretching vibration, while the stretching vibration of functional groups generally appears in the high wave number region.
2. Bending vibration (also called deformation vibration)
Refers to the change of bond angle between two chemical bonds and an atom, or the movement of the whole atomic group relative to other parts of the molecule, which has nothing to do with the mutual movement between atoms in an atomic group. Most of them show that the bond angle changes periodically and the bond length remains unchanged. Deformation vibration is divided into in-plane deformation and out-of-plane deformation vibration. In-plane deformation vibration can be divided into shear vibration and plane swing vibration. Out-of-plane deformation vibration can be divided into non-planar rocking vibration and torsional vibration (see figure 2-2- 12).
Figure 2-2- 12 Basic Forms of Normal Vibration
"+"indicates that the moving direction is perpendicular to the paper surface and inward; "-"indicates that the moving direction is perpendicular to the paper.
(3) division of infrared region
Infrared spectrum is located between visible light and microwave region, that is, electromagnetic wave with wavelength of about 0.78 ~1000μ m. Generally, the whole infrared light is divided into the following three parts:
1. Far infrared region
The wavelength range is 25 ~ 1000μ m, and the wave number range is 400 ~ 10cm- 1. The infrared absorption band in this region is mainly caused by pure rotation transition, vibration-rotation transition in gas molecules, telescopic vibration of heavy atoms in liquids and solids, some angular vibration in crystals, skeleton vibration and lattice vibration. It is rarely used in gemology.
2. Mid-infrared region
The wavelength range is 2.5 ~ 25 microns, and the wave number range is 4000 ~ 400 cm- 1. I.e. the vibrational spectral region. It involves the fundamental frequency vibration of molecules, and the fundamental frequency absorption bands of most gems appear in this area. Fundamental frequency vibration is the most absorbing vibration type in infrared spectrum, which is widely used in gemology. This interval is usually divided into two areas, namely, group frequency area and fingerprint area.
In the fundamental frequency vibration region (also known as functional group region), the characteristic frequency of groups in the region of 4000 ~ 1500 cm ~ 1 is relatively stable, and the infrared absorption band in this region is mainly caused by telescopic vibration. The infrared absorption band in this region can be used to identify the functional groups that may exist in gems.
The fingerprint area is distributed in the area of 1500 ~ 400 cm- 1, and there are infrared absorption bands caused by deformation vibration besides single bond stretching vibration. The vibration of this region is related to the structure of the whole molecule. Molecules with different structures show different infrared absorption bands, so this region is called fingerprint region, and the specific molecular structure can be identified by the map of this region.
3. Near infrared region
The wavelength range is 0.78 ~ 2.5μ m and the wave number range is 12820 ~ 4000 cm- 1. The absorption band in this region is mainly caused by the low-energy electron transition and the frequency doubling absorption of the stretching vibration of hydrogen-containing atomic groups (such as O-H, N-H and C-H). For example, the fundamental frequency of OH in beryl is 3650cm- 1, the combined frequency of tension/bending vibration is 5250cm- 1, and the first harmonic frequency is 72 10cm- 1.
Second, the instrument type and test method
According to the principle of spectroscopy, infrared spectrometers can be divided into two types: dispersive infrared spectrometers (single-beam and double-beam infrared spectrometers) and interferometric infrared spectrometers (Fourier transform infrared spectrometers). The main disadvantages of dispersive infrared spectrometer are its own limitations, slow scanning speed, low sensitivity and resolution. At present, Fourier transform infrared spectrometer is mainly used for the detection and research of gems.
In Fourier transform infrared spectrometer, the light emitted by the light source is first changed into interference light by Michelson interferometer, and then the interference light illuminates the sample. The interferogram is obtained by detector (detector-amplifier-filter), and the spectrum is obtained by Fourier transform of the interferogram by computer (see Figure 2-2- 13 to Figure 2-2- 15). Its characteristics are: fast scanning speed, suitable for instrument combination; No need for light splitting, strong signal and high sensitivity.
Fig. 2-2- 13 Fourier transform infrared spectrometer
Fig. 2-2- 14 internal structure of Fourier transform infrared spectrometer
Fig. 2-2- 15 schematic diagram of working principle of Fourier transform infrared spectrometer
The testing methods of infrared absorption spectrum of gemstones can be divided into two categories, namely transmission method and reflection method.
1 transmission method
Transmission modes can be divided into powder transmission mode and direct transmission mode. Powder transmission method is a nondestructive testing method, and the specific method is to grind the sample into 2? When the particle size is less than m, potassium bromide is mixed with the sample according to the ratio of1:100 ~1:200, and pressed into thin sheets, so that the transmission infrared absorption spectrum of gem minerals can be determined. The direct transmission method is to put the gem sample directly on the sample table. Because the gem sample is thick, it shows total absorption in the wave number range of more than 2000cm- 1, so it is difficult to obtain important information of the gem fingerprint area. Although the direct transmission technology is a nondestructive testing method (see Figure 2-2- 16), the structural information about gemstones obtained from it is very limited, which limits the further application of infrared absorption spectrum. Especially for some opaque gems, stamp stones and gem ornaments embedded in the bottom, it is difficult to implement them in detail.
2. Reflective flaw detection method
Infrared reflection spectrum is an important branch of infrared spectrum testing technology, which has attracted much attention in the testing and research of gemstones. According to the types and accessories of reflected light, it can be divided into specular reflection, diffuse reflection, attenuated total reflection and infrared microscope reflection. Infrared reflection spectrum (specular reflection and diffuse reflection) has a broad application prospect in the field of gem identification and research. According to the infrared reflection spectrum characterization of transparent or opaque gemstones, it is helpful to obtain important information such as the internal and external vibration of hydroxyl and water molecules, the stretching or bending vibration of anions and complex anions, the structural unit of molecular groups and the symmetry of ligands, and especially provides a convenient, accurate and nondestructive detection method for identifying organic polymer fillers in some filled gemstones (see Figure 2-2- 17).
In order to compare and identify gem samples, Nicolet550 Fourier transform infrared spectrometer and specular reflection accessories, TENSOR-27 Fourier transform infrared spectrometer and diffuse reflection accessories can be used respectively. In the specific testing process, according to the specific conditions of the samples, the relevant gem samples are tested in sections (i.e. 4000 ~ 2000 cm- 1, 2000 ~ 400 cm- 1). Considering that the infrared reflection spectrum of gems is distorted (similar to differential spectrum) due to the change of refractive index (abnormal dispersion) in the frequency range of infrared spectrum, the distorted infrared reflection spectrum can be corrected to the normal infrared absorption spectrum familiar to jewelry appraisers, and eliminated by dispersion correction or Kramers Kronig transformation. The specific method is as follows: If the specular reflection accessory of Nicolet550 infrared spectrometer is selected to measure the infrared reflection spectrum of gemstones, select dispersion in the process drop-down menu of OMNIC software to correct it; Similarly, if the diffuse reflectance accessory of TENSOR-27 infrared spectrometer is used to measure the infrared reflectance spectrum of gemstones, Kramers Kronig transform (K-K transform for short) can be selected from the spectrogram processing drop-down menu of OPUS software. Hereinafter, the infrared reflection spectrum after dispersion correction or K-K transformation will be collectively referred to as infrared absorption spectrum.
Fig. 2-2- 16 infrared absorption spectrum of jadeite treated by filling (transmission method)
Fig. 2-2- 17 infrared absorption spectrum of white jade and its imitation (reflection method, K-K conversion)
Third, the application in gemology
Infrared absorption spectrum is a concrete reflection of the molecular structure of gemstones. Usually, molecules or functional groups in gems have their own specific infrared absorption regions in the infrared absorption spectrum. According to the number, wave number and displacement, spectral shape, band intensity and band splitting state of characteristic infrared absorption bands, it is helpful to qualitatively characterize the infrared absorption spectrum of a gem and obtain important information related to gem identification.
1. hydroxyl and water molecules in gemstones
As the strongest vibration type in infrared absorption spectrum, fundamental frequency vibration (mid-infrared region) is widely used in gemology. Generally, mid-infrared is divided into two regions: fundamental frequency region (also called functional group region, 4000 ~ 1500 cm- 1) and fingerprint region (1500 ~ 400 cm- 1).
There are many natural gems containing hydroxyl and H2O in nature, and the corresponding mid-infrared absorption bands caused by stretching vibration are mainly distributed in the range of 3800 ~ 3000 cm- 1 wavenumber in the functional group region. However, the infrared absorption band caused by bending vibration changes greatly, and the infrared absorption band of most gems is in the range of1400 ~17000 cm-1wavenumber. Usually, the specific wave number position of hydroxyl or water molecules is also controlled by the hydrogen bonding force in gems. As for the specific wave number, it mainly depends on the size of hydrogen bonding force in various gems. Compared with crystal water or structural water, the broadband center of infrared absorption caused by symmetric and asymmetric telescopic vibration of adsorbed water is mainly located at 3400cm- 1.
For example, crystal water and adsorbed water generally exist in the crystal structure of natural turquoise, in which the sharp infrared absorption bands caused by hydroxyl stretching vibration are located at 3466cm- 1 and 351cm-1,while the infrared absorption band caused by V (MF ECU-COH) stretching vibration is located at 3293cm- 1. At the same time, the infrared absorption band caused by the expansion and bending vibration of phosphate groups is displayed in the fingerprint region.
On the contrary, in the functional group region, gilson turquoise obviously lacks the unique infrared absorption band caused by the stretching vibration of hydroxyl and water molecules, but presents sharp infrared absorption band (2925cm- 1) and vs(CH2) symmetric stretching vibration (2853cm- 1) caused by asymmetric stretching vibration in the polymer, accompanied by vas(CH3). In the fingerprint region, the characteristic infrared absorption band of carbonate group vibration is displayed. The test results show that the turquoise, commonly known as gilson method, is actually a pressed carbonate imitation turquoise. (see figure 2-2- 18).
Fig. 2-2- 18 infrared absorption spectra of turquoise and imitation turquoise (R% is reflection spectrum and A% is K-K conversion)
Similarly, the infrared absorption spectra of emerald synthesized by flux method and emerald synthesized by hydrothermal method are also different because of the absorption band caused by the stretching vibration of anhydrous molecules. Emerald synthesized by flux method is crystallized at high temperature, so there are generally no water molecules in its structural pores. Emerald synthesized by hydrothermal method is crystallized and grown under hydrothermal conditions, and there are often different amounts of water molecules and a small amount of chlorate ions (mineralizers) in its structural channels.
2. Forms and types of impurity atoms in diamonds.
Diamond is mainly composed of carbon atoms. When the crystal lattice contains a small amount of impurity atoms such as N, B and H, the physical properties of diamond such as color, thermal conductivity and electrical conductivity will change obviously. Characterization based on infrared absorption spectrum is helpful to determine the composition and existing form of impurity atoms, which is one of the main basis for diamond classification (see Table 2-2- 1).
Table 2-2- 1 diamond types and infrared absorption spectrum characteristics
3. Identification of artificial filling gemstones.
Polymer epoxy resin with two or more epoxy groups and aliphatic, alicyclic or aromatic functional groups as the skeleton reacts with curing agent to generate a three-dimensional network structure, which mostly exists in the form of filler and is widely used for artificial filling treatment of jadeite, turquoise and emerald. There are many kinds of epoxy resins, and new varieties are still emerging. Common varieties include epoxidized polyolefin, peracetic acid epoxy resin, epoxidized olefin polymer, epichlorohydrin resin, bisphenol A resin, epichlorohydrin-bisphenol A polycondensate, and bis-epichlorohydrin resin. As can be seen from Figure 2-2- 16, it is obviously different from the infrared absorption spectrum of wax. In the filled jadeite, the weak infrared absorption band caused by benzene ring stretching vibration in epoxy resin is 3028cm- 1 (blue circle in the figure); Accordingly, the infrared absorption band caused by asymmetric stretching vibration of vas(CH2) is 2922cm- 1, while the sharp infrared absorption band caused by symmetric stretching vibration of vs(CH2) is 2850cm- 1 (red circle in the figure).
When testing the infrared reflection spectrum of natural jadeite ornaments with back cover (such as Tielongsheng) with mirror reflection fittings, attention should be paid to eliminating the interference of colloid attached to the precious metal base, because the precious metal base acts as a back mirror, and the reflected infrared light penetrates the colloid and untreated jadeite samples at the same time, sometimes it is easy to show the infrared absorption spectrum characteristics of filled jadeite.
Fig. 2-2- 19 is the infrared absorption spectrum of filled turquoise. In the functional group region, except for the infrared absorption band caused by the stretching vibration of hydroxyl and water molecules in turquoise, the asymmetric and symmetric stretching vibrations of vas(CH2) and vs(CH2) in heterogeneous polymers are both at 2930cm- 1 and 2857cm- 1, and the infrared absorption band caused by the stretching vibration of benzene ring is mostly surrounded by the V (M-OH) absorption band.
4. Identification of similar gemstone species
There are some differences in crystal structure, molecular ligand structure and chemical composition of different kinds of gems, and infrared absorption spectra based on the characteristics of various gems are helpful for identification. In the daily detection process, inspectors often encounter some problems when identifying jadeite and similar jadeite with opaque or poorly polished surface, and infrared reflectance spectroscopy provides a rapid and nondestructive detection method. It is easy to identify jadeite minerals by using the characteristics of wave number position, displacement, spectral shape, intensity and splitting state of infrared absorption band caused by Si-ONB stretching vibration and Si-OBR-Si and O-Si-O bending vibration in the fingerprint area of infrared reflection spectrum (see Figure 2-2-20).
Fig. 2-2- 19 infrared absorption spectra of turquoise and filled turquoise (K-K conversion)
Figure 2-2-20 Infrared absorption spectra (K-K conversion) of natural jadeite and imitation jadeite.
5. Infrared absorption spectrum of ancient jade
In the process of making some antique jade articles, strong acid (such as HF acid) corrosion or high temperature baking are often used for aging treatment. The jade surface treated by the above method is either white (slag), acid etched (mottled) or netted with ox hair, so it is often difficult to correctly identify its jade quality. Using "diffuse reflection infrared accessories" is helpful to identify this kind of ancient jade. Fig. 2-2-2 1 shows that the infrared absorption band generated by the stretching vibration and bending vibration of the fingerprint areas Si-O and Si-O-Si is enough to confirm that the main mineral component of this jade is tremolite (identified as nephrite).
Fig. 2-2-2 1 infrared absorption spectrum of ancient jade products (K-K conversion)