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   photochemical reaction process is one of the universal and important processes on the earth. the photosynthesis of green plants, animal vision, the photocatalysis of coatings and polymer materials, as well as the photocatalysis of photography, lithography and organic chemical reactions are all related to photochemical processes. in recent years, photochemistry is a very active field, such as the photoinduced separation of isotopes and similar elements, the synthesis and application of optically controlled functional systems and so on. however, in terms of theory and experimental technology, photochemistry is still immature in various fields of chemistry.

   there are many differences between photochemical reaction and general thermochemical reaction, mainly in: when heating activates molecules, the distribution of molecular energy in the system obeys boltzmann distribution; when molecules are activated by light, they can be selectively excited in principle, and the distribution of molecular energy in the system belongs to non-equilibrium distribution. therefore, the path and products of photochemical reaction are often different from those of ground state thermochemical reaction. as long as the wavelength of light is appropriate and can be absorbed by substances, photochemical reaction can still be carried out even at very low temperature.

   the primary process of photochemistry is that molecules absorb photons to excite electrons, and molecules rise 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 molecular excitation are different, the required photon energy is also different, and the energy values of the two are required to match as much as possible.

   because the molecule is in a stable state with low energy under general conditions, it is called the ground state. after being irradiated by light, if molecules can absorb electromagnetic radiation, they can be promoted to a higher energy state, which is called an 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 excited state, the second excited state and so on from the ground state up; all excited states above the excited state are collectively referred to as highly excited states.

   the lifetime of excited state molecules is generally short, and the higher the excited state, the shorter its lifetime, so that there is no time for chemical reactions, 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 combine with the thermal effect of photochemical reaction; second, it is transformed into other forms of energy through photophysical processes.

   photophysical processes can be divided into radiative relaxation processes and non radiative relaxation processes. radiative relaxation process refers to the process that all or part of the excess energy is dissipated in the form of radiative energy, and the molecules return to the ground state, such as emitting fluorescence or phosphorescence; non radiative relaxation process refers to the process that all excess energy is dissipated in the form of heat and molecules return to the ground state.

   to determine the real path of a photochemical reaction, it is often necessary to establish several hypothetical models corresponding to different mechanisms, find out the kinetic equations between each model system and concentration, light intensity and other relevant parameters, and then investigate the degree of consistency with the experimental results to determine which is the possible reaction path.

   in addition to the tracer atom labeling method, the quenching method used in photochemistry is still a very effective method. this method is to study the mechanism of photochemical reaction by measuring the kinetics of fluorescence emitted by excited molecules and quenching by other molecules. it can be used to measure the acidity of molecules in the electronic excited state, the reaction rate of molecular dimerization and the long-range transfer rate of energy.

   since the absorption of photons at a given wavelength is often the property of a group in the molecule, photochemistry provides a means to make a reaction occur at a specific position in the molecule, which is more valuable for those systems where thermochemical reactions lack selectivity or reactants may be destroyed. another characteristic of photochemical reaction is that photons are used as reagents. once absorbed by reactants, no other new impurities will be left in the system, so they can be regarded as "pure" reagents. if the reactant is fixed in a solid lattice, photochemical synthesis can occur in the expected conformation (or configuration), which is often difficult for thermochemical reactions.

   atmospheric phenomena of the earth and planets, such as atmospheric composition, aurora, radiation shielding and climate, are related to the chemical composition of the atmosphere and its irradiation. the earth's atmosphere is mainly composed of nitrogen and oxygen on the earth's surface. however, the composition of atoms and molecules in the atmosphere at high altitude is very different, which is mainly related to the photochemical reaction after absorbing solar radiation.

   air pollution processes contain extremely rich and complex chemical processes. at present, the comprehensive models used to describe these processes include many photochemical processes. for example, the high-energy molecule excited by brown nitrogen dioxide under sunlight is the initiator of the chain reaction between oxygen and carbohydride. for example, the relationship between the photolysis of fluorocarbons in the upper atmosphere and the change of ozone shielding layer is based on photochemistry.