= Solution
For a <fluorescence excitation spectrum>, hold the emission detector at a fixed wavelength within the fluorescence band and scan the excitation wavelength. Correct for the incident photon flux and instrumental response at each excitation wavelength.
\b[For one emitting molecular species with an approximately excitation-independent fluorescence yield, it should follow the absorption spectrum.] In a dilute sample the detected emission is proportional to the photons absorbed, $I_{\rm exc}(1-10^{-A})\phi_F\simeq I_{\rm exc}(\ln10)A\phi_F$. Without the dilute approximation it follows absorptance rather than absorbance exactly. Impurities, photochemistry or multiple emitting states could invalidate that correspondence.
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