TOPTION Instrument Co., Ltd. was established in the Hi-tech Industrial Park of Xi'an High-tech Zone. It is a comprehensive company with independent legal personality integrating R & D, production, sales and service. TOPTION company brings together many technologies and talents in electronics, refrigeration, design, manufacturing, etc., and focuses on high-tech fields such as experimental instruments, biochemicals, and medical engineering. The tenet of management is: keeping promise, observing contracts, providing high-quality products with excellent performance.
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The branch of chemistry that studies the permanent chemical effects caused by the interaction of light and matter is called photochemistry, limited to time-related reasons. The wavelength of light involved in photochemistry is generally 100-1000 nanometers, from ultraviolet to near infrared.
Electromagnetic radiation shorter than ultraviolet wavelengths, such as photoionization and related chemical changes caused by X or Î³ rays, belong to the category of radiation chemistry. As for far-infrared or electromagnetic waves with longer wavelengths, it is generally considered that their photon energy is not sufficient to cause photochemical processes, so they do not belong to the research field of photochemistry. In recent years, it has been observed that some chemical reactions can be initiated by high-power infrared lasers, but they are classified as infrared laser chemistry.
The photochemical reaction process is one of the most common and important processes on earth. The photosynthesis of green plants, the vision of animals, the photodenaturation of paints and polymer materials, and the photocatalysis of photography, photolithography, organic chemical reactions, etc. All are related to the photochemical process. In recent years, the photo-separation of isotopes and similar elements, the synthesis and application of photo-control functional systems, etc., which have been widely valued, also show that photochemistry is an extremely active field. However, from the perspective of theory and experimental technology, photochemistry is still immature in various fields of chemistry.
Compared with the general thermochemical reaction, the photochemical reaction has many differences, mainly manifested in: when the molecule is activated by heating, the distribution of molecular energy in the system follows the Boltzmann distribution; and when the molecule is activated by light, it can be done in principle Selective excitation, the distribution of molecular energy in the system is a non-equilibrium distribution. Therefore, the path and product of the photochemical reaction are often different from the ground state thermochemical reaction. As long as the wavelength of the light is appropriate, it can be absorbed by the substance. Even at a very low temperature, the photochemical reaction can still proceed.
The primary process of photochemistry is that molecules absorb photons to excite electrons, and molecules are promoted from the ground state to the excited state. The electronic states, vibration and rotation states in molecules are quantized, that is, the energy changes between adjacent states are discontinuous. Therefore, when the initial state and the end state of the molecular excitation are different, the required photon energy is also different, and the energy values â€‹â€‹of the two are required to match as much as possible.
Because the molecule is in a stable state with lower energy under general conditions, it is called the ground state. After being irradiated with light, if the molecule can absorb electromagnetic radiation, it can be raised to a higher energy state, called the excited state. If molecules can absorb electromagnetic radiation of different wavelengths, they can reach different excited states. According to the level of its energy, it is called the first excited state, the second excited state, etc. in order from the ground state upwards; and all the excited states higher than the first excited state are collectively called the high excited state.
The lifetime of excited molecules is generally shorter, and the higher the excited state, the shorter the lifetime, so that there is no time for chemical reactions to occur, so photochemistry is mainly related to low excited states. There are two main ways to dissipate the electromagnetic radiation energy absorbed by molecules during excitation: one is to merge with the thermal effect of photochemical reactions; the other is to transform into other forms of energy through photophysical processes.
The photophysical process can be divided into radiation relaxation process and non-radiation relaxation process. The radiation relaxation process refers to the process of dissipating all or part of the excess energy in the form of radiant energy, and the molecule returns to the ground state, such as emitting fluorescence or phosphorescence; the non-radiation relaxation process refers to the excess energy in the form of heat The process of dissipating and returning the molecule to the ground state.
Determining the true way of a photochemical reaction often requires the establishment of several hypothetical models corresponding to different mechanisms, finding the kinetic equations between each model system and the concentration, light intensity, and other relevant parameters, and then examining the degree of conformity with the experimental results. The highest to determine which is the most likely reaction path.
In addition to the tracer atom labeling method, the commonly used experimental method for studying the reaction mechanism of photochemistry is still a very effective method. This method is to study the photochemical reaction mechanism through the kinetic measurement of the excited molecules and the quenching of other molecules. It can be used to determine the acidity of molecules when they are in an electronically excited state, the reaction rate of molecular dimerization, and the long-range transfer rate of energy.
Because the absorption of photons of a given wavelength is often the property of a group in the molecule, photochemistry provides the best means to react a specific position in the molecule. For those thermochemical reactions that lack selectivity or the reactants may be destroyed The system is more valuable. Another feature of the photochemical reaction is the use of photons as reagents. Once absorbed by the reactants, no other new impurities will be left in the system, so it can be regarded as the "pureest" reagent. If the reactants are fixed in a solid lattice, photochemical synthesis can occur in the expected configuration (or configuration), which is often difficult to achieve by thermochemical reactions.
The atmospheric phenomena of the earth and planets, such as atmospheric composition, aurora, radiation shielding and climate, are all related to the chemical composition of the atmosphere and its irradiation. The earth's atmosphere is mainly composed of nitrogen and oxygen on the surface. But the composition of atoms and molecules in the atmosphere at high altitudes is very different, mainly related to the photochemical reaction after absorbing solar radiation.
In the process of atmospheric pollution, it contains very rich chemical processes. The comprehensive model used to describe these processes contains many photochemical processes, such as the relationship between the photolysis of fluorocarbons in high-altitude atmosphere and the change of ozone barrier. For example, the high-energy state molecules excited by brown nitrogen dioxide under sunlight are the initiators of the reaction between oxygen and hydrocarbon chains. They are all based on photochemistry.
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