Refer to Fig. 15.11. At what pH does the fluorescence of fluorescein have the greater quantum yield, pH = 9 or pH = 5? How do you know?

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relative absorbance 300 488 nm pH=9 pH = 5 400 wavelength, nm (a) 500 relative fluorescence 510 nm pH=9 pH = 5 500 600 550 wavelength, nm (b) 650 (C) FIGURE 15.11 (a) The UV-visible absorption spectrum of fluorescein at two pH values. (b) The fluorescence emission spectrum of fluorescein at the same two pH values resulting from absorp- tion at the Amax (488 nm). Notice the Stokes shift of fluorescence to greater wavelength. (c) Fluo- rescein sprinkled into tap water emits green fluorescence under a handheld UV light.