What is Quantum yield?

Example 7.23 (Quinine and anthracene)

Quinine sulfate in $0.5\,\mathrm{M}$ sulfuric acid absorbs at $349\,\mathrm{nm}$ with $\varepsilon = 5700\,\mathrm{L}\,\mathrm{mol}^{-1}\,\mathrm{cm}^{-1}$ and fluoresces with $\Phi_F = 0.546$, which makes it the classic standard for relative quantum yields. Anthracene in cyclohexane absorbs at $356\,\mathrm{nm}$ and has $\Phi_F = 0.36$; most of the rest of its excited molecules cross to the triplet.