chapter  15
18 Pages

Fluorescence Lifetime

When a molecule absorbs light, we do not get fluorescence immediately. As we saw in Chapter 3, the electron must lose some energy before it reaches the lowest vibrational level of the S1 state (Figure 15.1). Only from there can fluorescence take place (Kasha’s law). We will therefore see an initial delay at excitation, and then (in the absence of outside events such as resonant energy transfer (FRET) or chemical reactions) decay will follow first-order kinetics — an exponential decay curve — because only one chemical species is involved (Figure 15.2).