I understand why magic angle conditions are necessary when performing steady-state fluorescence measurements on immobile samples (i.e., films), since I know that the transmission efficiency of the monochromator depends on the polarization of the emitted light, and that such an efficiency will probably be different at different observation wavelengths. However, why are magic angle conditions required in the case of immobile samples when performing time-resolved fluorescence measurements? I mean, the transmission efficiency of the monochromator will be different for the parallel and the perpendicular components of the fluorescence decay, but why should the temporal profile of these components be different from each other? I can easily understand different detection efficiencies for the parallel and perpendicular components of the emission, but it is hard for me to see why would the intensity decay of these components be affected by the polarization of the emitted light. I also understand why the temporal profiles of the parallel and perpendicular components are different in the case of fluid samples, since depolarization affects the perpendicular and parallel components in different manners. However, in the case of immobile samples depolarization should have no effect on the parallel and perpendicular components, since the molecules are not rotating. In light of all of the aforementioned, in the case of immobilized samples should not the parallel and perpendicular components of the fluorescence decay be identical to the true photophysical decay?
I think it depends on the photophysics happening in the sample. If you are exciting and fluorescing from parallel states then the dynamics should be the same, but if you have energy transfer (a source of depolarization) or relaxation between orthogonal states, then the signal will show an enhanced decay. I am thinking more in terms of pump-probe spectroscopy, but it should apply similarly to time-resolved fluorescence.
Thanks for your reply, Chanelle. Yes, I agree with you. I think that if there are depolarizing phenomena such as energy transfer the temporal profiles of the parallel and perpendicular components will be different from each other. In the absence of depolarizing phenomena, however, it seems to me that they should be identical.
The influence of rotational rate compared to the lifetime was considered explicitly by Weber and Spence in their 1970 paper in J Chem Phys (Vol 52, p1654). As they demonstrate - with theory and experiment (including the effect of energy transfer) - the lifetimes of the parallel and perpendicular components of the emission do not differ in the extremes when the ratio of rotational rate to lifetime is either very small or very large, but they do differ at intermediate ratios.
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