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How does ultraviolet radiation produce ozone
In theory, photons with energy higher than the bond energy can break chemical bonds. So ultraviolet rays with wavelengths shorter than 243 nm can excite oxygen in air or water to produce ozone. UVC-LEDs with wavelengths above 243 nm do not produce ozone
Low pressure mercury lamps produce ozone because there is a natural 185nm vacuum ultraviolet in the emission spectrum of low-pressure mercury lamps with vapor pressures ranging from 1.3 to 13P. This type of ultraviolet radiation can stimulate oxygen in the air to undergo chemical reactions and become ozone. This is essentially because ultraviolet light breaks the covalent bond between oxygen and oxygen molecules, and then combines with oxygen molecules to form O3. UVC-LED does not produce ozone because the covalent bond between oxygen and oxygen can reach up to 5.1 eV, requiring ultraviolet light with a wavelength at least shorter than 243 nm to break and ultimately generate ozone
Why don't LEDs produce ozone
The low-pressure mercury lamp increases the energy level transition probability of short wave ultraviolet radiation by controlling the vapor pressure, but it also accompanies other transitions, mainly emitting light at 254 and 185nm. The probability of the first energy level transition is predominant in UVC-LED, resulting in pure luminescence. Common UVC-LED wavelengths are higher than 255 nm
Low pressure mercury lamp is a gas light source, and its excited state energy level position is determined by the substance mercury. So although there are high-pressure, medium pressure, and low-pressure mercury lamps, the difference in vapor pressure mainly adjusts the probability of transitions between different excited states, that is, the relative intensity of different characteristic peaks. So low-pressure mercury lamps mainly emit light at 254 and 185nm. As a quantum well structured AlGaN semiconductor light-emitting diode, UVC-LED is dominated by E0 transition, with only one peak wavelength, such as 275nm. It can be observed that the full width at half maximum of the main peak in the deep ultraviolet LED spectrum is significantly lower than that of blue light, which is also due to the fact that quantum well luminescence has a more single electronic transition compared to multi size quantum dot light-emitting devices. Because the emission wavelength of mass-produced UVC-LED is not lower than 243nm, the deep ultraviolet UVC-LED used for sterilization and disinfection does not produce ozone.