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1. How do ultraviolet rays produce ozone?

Theoretically, a chemical bond can be broken if the photon energy is higher than the bond energy. Therefore, ultraviolet light with wavelength shorter than 243 nm can stimulate oxygen in air or water to produce ozone. Uvc-leds with wavelengths higher than 243 nm do not produce ozone.

Low pressure mercury lamp will produce ozone, because the vapor pressure is between 1.3 and 13P, and there is naturally 185nm vacuum ultraviolet in the light spectrum of low pressure mercury lamp. This ultraviolet light will stimulate oxygen in the air to produce a combination reaction to ozone. In essence, ultraviolet rays break the oxygen oxygen covalent bond in oxygen molecules, and then combine with oxygen molecules to form O3. Uvc-led will not produce ozone, because the covalent bond energy of oxygen and oxygen is as high as 5.1ev, which requires ultraviolet light with wavelength at least shorter than 243nm to interrupt and finally generate ozone.

2. Why doesn't led produce ozone?

By controlling the vapor pressure, the low pressure mercury lamp can increase the energy level transition probability of short wave ultraviolet, but it will also be accompanied by other transitions, mainly at 254 nm and 185 nm. The first level transition probability of uvc-led is dominant, so the luminescence is pure. The wavelength of uvc-led is higher than 255 nm.  

Low pressure mercury lamp is a kind of gas light source, and its excited state energy level position has been determined by mercury. Therefore, although there are high pressure, medium pressure and low pressure mercury lamp, the difference of vapor pressure is only to adjust the probability of transition between different excited states, that is, the relative intensity of different characteristic peaks. Therefore, the low-pressure mercury lamp mainly emits light at 254 nm and 185 nm. As an AlGaN semiconductor light-emitting diode with quantum well structure, the E0 transition is dominant, and the peak wavelength is only one, for example, 275nm. It can be found that the FWHM of the main peak of the deep ultraviolet LED is significantly lower than that of the blue light, which is also because the quantum well luminescence is more unitary than the electronic transition of multi size quantum dot light emitting devices. Because the emission wavelength of uvc-led is not lower than 243nm, the uvc-led for sterilization does not produce ozone.