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Spectra are presented as a linear combination of the two individual two-photon action cross-section spectra: F2 (λ) = σ2,N (λ) φN ρN + σ2,A (λ) φA ρA, where σ2 (λ) is the wavelength-dependent two-photon excitation cross-section, φ is the fluorescence quantum yield, and ρ is the relative concentration of the chromophore species (neutral, N, or anionic, A). Ca2+-saturated (solid circles) and Ca2+-free (open circles).
