For fluorescence lifetime measurements in isotropic solutions, it is well known that detection using an emission polarization of 54.7°, the so-called magic angle, eliminates contribution of molecular reorientation to the fluorescence decay function. For macroscopically oriented samples and evanescent wave excitation, the magic angle may not necessarily be the 54.7°. With the use of a typical optical scheme of evanescent wave excitation and detection along the surface normal, a mathematical expression is derived for the magic angle condition. The magic excitation angle is shown to be a function of the angle of incidence of the excitation beam and the refractive indices of the two media involved in total internal reflection. The magic angle relations are valid regardless of the orientational distribution of molecules along the surface normal. The magic angle relations apply to cases where the in-plane orientational distribution is isotropic.
Mary J. Wirth, "Magic Angle Lifetime Measurements in Evanescent Wave Fluorometry," Appl. Spectrosc. 47, 651-653 (1993)