Molecular motion includes the rotation of the whole molecule, the vibration of constituent atoms and the motion of electrons in the molecule. Every motion state of a molecule has a certain amount of energy. In molecules, each atom is maintained in an equilibrium position through the interaction of bond forces, and slightly vibrates near the equilibrium position, which constitutes the vibration mode of molecules. The vibration of molecules is generally complex, so under certain conditions, the vibration of molecules can be regarded as the superposition of several independent simple vibration modes. These independent and simple vibration modes are converted into normal vibration modes. Each normal vibration mode has its characteristic frequency (V), and various normal vibration frequencies are determined by the geometric configuration of molecules, the bond force field between atoms and the mass of atoms.
When a molecule vibrates in a normal mode with a frequency of V, its vibration energy is: En=( 1/2+n)hv, where n is the vibrational quantum number of vibration energy level, integer 0, 1, 2, …, and h is Planck constant.
The vibration ground state E0 is called zero vibration energy, which exists even at absolute zero. When the energy hv of incident photon is exactly equal to the energy level difference of vibration, molecules may absorb photon energy and have dynamic transition of vibration.
It can be seen that hv light =E 1-E0=hv0. When the frequency of incident light is equal to the normal vibration frequency of molecules (V light =v0), molecules can absorb the energy of light and transition from the ground state to the first excited state. According to the classical theory, it is precisely because the frequency of incident light is equal to the natural frequency of vibration that molecules absorb light energy (figure 13-5- 1).
Figure 13-5- 1 infrared spectrum vibration ground state
In addition to the above transition rule, the condition of infrared absorption must also have the change of dipole moment, which is called infrared activity. On the contrary, the vibration mode with constant dipole moment is non-infrared activity. Although it vibrates, it cannot absorb infrared radiation. A polyatomic molecule can have 3N-6 kinds of simple harmonic vibrations (n is the number of atoms that make up the molecule) (linear molecules only have 3N-5 kinds), and all kinds of simple harmonic vibrations have certain energy, which should be absorbed at a unique wave number position, that is, each simple harmonic vibration has a corresponding vibration frequency. Among all kinds of simple harmonic vibrations, some vibrations belong to non-infrared activities, while others have the same frequency (but opposite directions) and produce vibration degeneracy. Therefore, the number of infrared vibration frequencies is always less than the number of vibration forms 3N-6 (or 3N-5). The higher the molecular symmetry type, the more degenerate it is, and the smaller the vibration frequency is.
The instrument for measuring and recording infrared absorption spectrum is called infrared spectrophotometer. According to the different principles of spectroscopy, infrared spectrophotometer can be divided into two types: dispersion type and interference type. According to the refraction and diffraction of light, a dispersive infrared spectrophotometer uses a dispersive element (prism or grating) to split light. Interferometric infrared spectrophotometer is based on the principle of optical coherence and uses interferometer to achieve the purpose of light splitting. Then, according to the characteristics of mathematical Fourier transform function, the interferometer is improved, and the interferogram of its light source is converted into the spectrogram of the light source by computer, so it is also called Fourier infrared spectrophotometer (fTIR).
Because the Fourier transform infrared spectrophotometer abandons the slit device, it can obtain all the information of all frequencies of the radiation source in any measurement time, and at the same time, it eliminates the limitation of slit on spectral energy, greatly improving the utilization ratio of light energy, that is, the so-called large energy output, so it has many advantages in practical use. The sensitivity, resolution and accuracy are improved (0.0 1cm- 1), and stray light is reduced.
Second, the infrared spectrum analysis
Division of infrared region
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(1) Near infrared region: Its absorption band is mainly produced by the low-energy electron transition of hydrogen-containing atomic groups and the frequency doubling absorption of telescopic vibration. The spectrum of this region can be used to study the compounds of rare earth and other transition metal ions, and to analyze water and hydrogen-containing compounds (such as organic dyes in glue, wax and precious stones).
(2) Mid-infrared region: The absorption band in this region is mainly the fundamental frequency absorption band. Because the fundamental frequency vibration is the most absorbing vibration in the infrared spectrum, this region is most suitable for qualitative and quantitative analysis of the infrared spectrum of gemstones. ① The range of 4000 ~1250 cm-1is called the characteristic frequency region, and the absorption peaks in this region are sparse, mainly including: the fundamental frequency peaks of single bond, various triple bonds and double bonds containing hydrogen atoms; ② The frequency range of1250 ~ 400 cm-1is the fingerprint area of gem mineral identification. The energy band is equivalent to the stretching vibration of various single bonds and the bending vibration of most groups. ③ Correlation frequency: the characteristic frequency can prove the existence of functional groups, but in most cases, a functional group has several vibration forms, each infrared active vibration has a corresponding absorption peak, and sometimes a frequency doubling peak can be observed, so the existence of functional groups cannot be confirmed by a single characteristic peak. The characteristic frequency is an absorption peak that depends on the correlation frequency, and its number is determined by the molecular structure and the wavelength range of the spectrogram. In the mid-infrared spectrum, most groups have a set of related peaks.
(3) Far-infrared light region: The absorption band in this region is mainly related to the pure rotational transition and vibrational transition in gas molecules, and gem analysis is generally not carried out in this region.
Thirdly, the preparation of samples.
Modern Fourier infrared spectrometer is equipped with microscopic transmission and reflection infrared spectroscopy devices, which can be directly detected without destroying the sample. Opaque gemstones are detected by reflection infrared spectrum, and transparent gemstones are detected by transmission infrared spectrum. For gem mineral raw materials, powder sampling method is used. There are two main methods for powder sampling: tabletting and pasting.
(1) tabletting method: generally, take out 1 ~ 3 mg gem sample, put it into agate mortar to make powder, add 100 ~ 300 mg KBr, mix and grind evenly, and then put it into a mold to make a transparent sheet with a certain diameter or thickness. And then measure it.
(2) Paste method: If the existing forms of hydrogen in precious stones are studied, the samples are ground into powder and mixed with paraffin oil to make paste, so as to reduce the scattering in the samples.
Generally speaking, the following points should be paid attention to when preparing samples: ① The samples are preferably single-component substances; (2) The concentration or test thickness of the sample should be properly selected so that the transmittance of most absorption peaks in the spectrum is in the range of 15% ~ 70%; (3) The sample should not contain free water.
Fourthly, the application of infrared spectroscopy in gemology.
Infrared spectrum is vibration spectrum, which is a sensitive detector of microstructure and combination in matter. According to the position, symmetry and relative intensity of the observed absorption peaks, very useful information about the structure and composition can be provided. Based on the frequency of characteristic absorption band, it is inferred that there are groups bonding in the molecule. In addition, the characteristics of adjacent groups can be inferred by the shift of characteristic absorption band frequency, and the mixture and compound can be quantitatively analyzed by the change of molecular characteristic absorption band intensity.
Representation of infrared spectrogram: the ordinate represents transmittance (or absorption rate), and the abscissa represents wavelength (nm) or frequency (cm- 1). Infrared spectrum is widely used in gemology.
(1) Identification of gemstone phases: Colorless gemstones similar to diamonds, such as colorless cubic zirconia, yttrium aluminum garnet and cassiterite, are very similar to diamonds, but their infrared spectra are obviously different.
(2) Determination of diamond type: Figure 13-5-2 is a good method to determine diamond type by FTIR.
Figure 13-5-2 Determination of diamond types by FTIR.
Figure 13-5-3 Infrared Spectrum of Diamonds
(3) Detection of disseminated gemstones: for example, detection of jadeite A, B and C, and identification of jadeite at coating.
(4) Near infrared region is a characteristic region for studying the existing forms of carbon, hydrogen and oxygen in gemstones. If water molecules exist in minerals, their binding frequency and frequency doubling are in the near infrared region (such as beryl and tourmaline). Infrared spectrum (figure 13-5-3) shows that there are H2 molecules in the diamond structure of IIb, and its vibration spectrum peak is located at 4 106cm- 1.