Excitation Photoselection of Fluorophores

The probability distribution given by Eq. [10.18] determines the maximum photoselection that can be obtained using one-photon excitation of an isotropic solution. More highly oriented populations can be obtained using multiphoton excitation. Recall that the anisotropy is a simple function of (cos ) (Eq. [ 10.17]), so calculation of (cos 6) allows calculation of the anisotropy. 

For collinear absorption and epaesion dipoles, the maximum value of (cos B) is given by 

Substituticm of Eq. [10.18] into Eq. [10,19] yields co 0)=. Recalling Eq. [10.17], one finds a maximum anis X ropy of 0.4. This is die value which is d saved when die isorption and emission dip 4es are collinear, and lA n diere are no m cesses wdiich result in dep arizati( . Un der these condificms, die excited-state population is vef-erentially oriented along the z-axis (Hgine 10.S), and the value of I is one-third the value of It (It = 3/x). We note that this value (r == 0.4) is considend ly smaller dian that possible for a single fluort hore oriented along the z-axis (r= W). 

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