Consider the experimental evidence you just saw,

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Consider the experimental evidence you just saw, and look at the level diagram shown here. What happens after excitation with a photon energy higher than the S0  S1 energy difference if the excited dye molecule can fluoresce? (A) The molecule does not absorb. No fluorescence is detected. (B) The molecule absorbs on a vibronic transition of the S1 state with higher vibrational quanta can fluoresce back into S0 from that higher-up state. (C) The molecule absorbs into S2 or higher singlet states. From there, it quickly relaxes by other processes than photon emission into the S1(v=0) state, from where it fluoresces. (D) The molecule absorbs into S2 or higher singlet states. From there, it fluoresces. S0 (ground state) S1 S2 S3 S4

Consider the experimental evidence you just saw, and look at the level diagram shown here. What happens after excitation with a photon energy higher than the S0  S1 energy difference if the excited dye molecule can fluoresce? (A) The molecule does not absorb. No fluorescence is detected. Wrong! The experiment shows that green and all higher photon energies are absorbed. (B) The molecule absorbs on a vibronic transition of the S1 state with higher vibrational quanta can fluoresce back into S0 from that higher-up state. Wrong! The experiment shows that the wavelength (i.e. color) of the fluorescence light is the same for all excitation wavelengths. (C) The molecule absorbs into S2 or higher singlet states. From there, it quickly relaxes by other processes than photon emission into the S1(v=0) state, from where it fluoresces. → CORRECT! (Kasha’s Rule) (D) The molecule absorbs into S2 or higher singlet states. From there, it fluoresces. Wrong! The experiment shows that the wavelength (i.e. color) of the fluorescence light is the same for all excitation wavelengths. S0 (ground state) S1 S2 S3 S4