A Spectroscopy and Spectroscopy
Spectroscopy is a method of studying the properties of matter through its interaction with electromagnetic waves of different frequencies (or wavelengths). It is an important means of studying the microscopic particles (atoms or molecules) that make up matter. However, under the action of light, it is not the "body" of the microscopic particle that is directly observed, but its "soul", that is, the interaction between light and microscopic particles with different degrees of freedom, which reflects the motion state of the microscopic particle. This interaction gives a different "likeness", which varies with the frequency of light and the microscopic particles, which is called the spectrum.
Spectroscopy is a general basic science research method, which can be used to extract the required information such as electron energy levels, molecular vibration and rotation dynamics, particle structure and symmetry, transition probability, etc., which are extremely important for the study of microscopic particles in the fields of physics, chemistry, biomedicine, astronomy and environmental scienceSpectroscopy is also a practical application tool, which can be used in many fields such as environmental monitoring, industrial inspection, clinical medicine, earth observation, etc.
What is Spectrum?How is the spectrum represented?
A spectrum is a pattern of electromagnetic radiation intensity arranged in order of frequency from small to large (or large to small), which reflects the energy level structure of a physical system. Spectra can usually be represented in a one-dimensional curve, the ordinate is radiant intensity (i), absorbance [-lg(i i0)] or transmittance (i i0), and the abscissa can be frequency, wavenumber (w**enumber), wavelength (w**elength) or energy (energy), as shown in the figure below for a typical spectral diagram.
There are usually multiple peaks on the spectral map, and each independent peak is generally formed by the transition of microscopic particles between two energy levels, which is the result of the interaction between electromagnetic waves and matter, where the position of the peak (1), the half-width of the peak ( and the intensity of the peak (i1) are useful characteristics for qualitative and quantitative analysis. However, the actual spectrum is more complicated due to factors such as line broadening and peak overlap.
As shown in the figure above, according to the order of frequency from large to small, electromagnetic waves can be divided into rays, X-rays, ultraviolet light, visible light, infrared light, microwave and radio waves.
In general, Y-rays are caused by transitions between nuclear levels, X-rays are caused by transitions between inner electron levels, ultraviolet and visible light are caused by transitions between outer electron levels, near- and mid-infrared light are caused by transitions between molecular vibrational levels, far-infrared light and microwaves are caused by transitions between molecular rotational levels, and radio waves are mainly caused by transitions between electron spins and NMR levels.
In the expression of the spectrum, the choice of spectral abscissa will vary depending on the spectral region。For example, MHz or CM-1 is commonly used in the radio wave region, M or CM-1 in the infrared region, nm in the ultraviolet and visible regions, and KEV in the X-ray region.
In fact,Different spectral abscissa can be converted to each other, there is a simple conversion relationship between frequency, wavenumber, wavelength and energy as follows:
Among them,
As can be seen from the conversion relationship above:
1) The wavenumber and energy are proportional to the frequency, if the frequency unit is Hz, the wavenumber unit is cm-1, and the energy unit is EV(1J6.).24150 1018ev), then the conversion factors between them are shown in the table below.
Table Conversion factors between different spectral units.
2) The wavelength is inversely proportional to the frequency, and if the frequency is equally spaced, the corresponding wavelength interval will decrease with the increase of frequency, which reminds us that the conversion of the abscissa between wavelength and frequency may cause a change in the shape of the spectrum.
Using the above conversion relationship and combining the relationship between electromagnetic wave energy and spectrum, it can be further realized that the inner electron energy level spacing is about 103 ev magnitude, while the outer electron energy level spacing is about 10 eVMolecular rotational energy levels are spaced between about 10-3 EV (about 10 cm-1) of the order of magnitude, while molecular vibration levels are spaced about 10-1 EV apart. Based on these, a preliminary judgment can be made on the energy level of different spectral regions.
2. Spectral measurements
Spectrum is a manifestation of the interaction between light and matter, and this interaction affects light as well as matter. Spectroscopy typically consists of two processes – excitation and detection. The sample is excited by certain measures (such as electromagnetic radiation, high-temperature combustion, chemical reaction, etc.), and then the characteristic light signal after passing through the sample is detected.
Depending on the form of the measured characteristic light signal, the spectrum can be divided into emission and absorption spectra.
Emission spectroscopy measures the characteristic light signal emitted from a sample due to excitation. The sample molecule or atom is excited to be at a high energy level, and then the light signal is emitted outward in the process of transitioning to a lower energy level, such as fluorescence spectroscopy, phosphorescence spectroscopy, Raman spectroscopy, etc. In general, the direction of the characteristic light in the emission spectrum will deviate from the direction of the original incident light, and the characteristic light will contain a new frequency (or wavelength) in addition to the incident light.
Absorption spectroscopy measures the light signal that has been changed by the sample after the external light passes through the sample, it measures the absorption of light at different frequencies (or wavelengths), and the energy level structure of the sample determines that it can only produce strong absorption of light at a specific frequency (or wavelength). In general, the absorption spectrum does not produce a new frequency (or wavelength) other than the incident light, and its ordinate can be expressed in terms of transmittance i i0 or absorbance-ig (i i0).
In general, a spectral detection system consists of three parts: a light source, a dispersion component, and a detector. The light source is used to excite the sample, the detector is used to record the characteristic light signal, and the dispersion unit is the core component of the spectral detection system, its main function is to decompose the polychromatic light into monochromatic light in order to facilitate the recording of the optical signal in frequency (or wavelength) order. Actual spectral detection systems may also require specialized cells or chambers for sample containment, quantitation, or to prevent the external environment from influencing spectral measurements.
In fact, in the spectral measurement, in addition to the sample itself will be affected by external perturbations, the light source and detector of the spectral detection system will also be affected by external perturbations, so we all measure the spectra in the presence of external perturbations, in other words, the spectrum always contains external perturbation information. However, such perturbations are not always harmful, and the measured spectra in the case of known perturbations contain more complete information about the sample to be measured.
3. Application of spectroscopy technology
What exactly is the use of spectroscopy?
In a nutshell, spectroscopy is used to identify substances, measure quantities, and make precise measurements.
The atoms or molecules that make up matter emit or absorb a large number of characteristic spectral lines, and the characteristics of different atoms or molecules are different, so that different substances can be distinguished by spectroscopy. Furthermore, spectroscopy can study the structure of atoms or molecules in detail, such as the electronic structure in the ground state and the excited state, so spectroscopy is an important means to study the material world. In atomic physics, the determination of the energy level structure of hydrogen atoms relies on spectroscopic technologyIn chemistry, most of the elements of the elemental cycle are discovered using spectroscopy;In biology, spectroscopy can be used to study the configuration of protein macromolecules;In daily life, spectroscopy can be used for gem identification, food harmful substance detection, fruit composition detection, etc.
Spectroscopy can also be used for precise measurement of substance content. Parts per million (ppm) can be easily measured using the emission spectra of substances, and the measurement limits of today's spectroscopy techniques are far superior to parts per million. Using the high sensitivity of the spectrum, it can be used for the detection of trace elements in water, food, fruits and other substances, and its detection accuracy is not inferior to the general chemical method, but the detection speed is much faster than the chemical method.
In addition to the above two applications, there are a number of other applications of spectroscopy, which are generally caused by certain factors that cause spectral changes, and these factors are detected by spectral measurements. For example, in astronomy, the speed of a star relative to the Earth can be detected by using the redshift or blueshift effect of the spectrum, and the surface temperature of a star can be estimated by using the Doppler broadening of the spectral linesChemically, femtosecond spectroscopy can be used to detect chemical reaction processes.
*: Tiny Optics 08
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