Dihydrorhodamine 123

You cannot stop it. Any time you illuminate a dye, including this one, photodegradation of the dye will lead to the production of singlet oxygen and free radicals. This in turn causes many ROS indicators, like this one, to become oxidized. If you are testing ROS using UV illumination, all you can do is use this as a baseline and subtract the signal from the sample with cells. The best you can do is reduce the illumination intensity (though this will also reduce the specific signal emission).

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Rhodamine is a generic term for a wide variety of cationic dyes whose fluorescence emission can range from green, orange to red. The table below lists the excitation and emission maxima (nm), as well as molar extinction coefficients (“EC”; cm^-1 M^-1), for various rhodamine dyes (data derived with dye dissolved in methanol).