## magic angle

https://doi.org/10.1351/goldbook.MT07419
Upon excitation of an '@I03353@' sample (assuming an ultra short excitation pulse) the relationship between the @F02453@ intensity detected at a time $$t$$ and through a @P04712@ analyser oriented at an @A00346@ $$\beta$$ with respect to the electric @P04712@ of the exciting beam is given by $I(t,\beta ) \propto N(t)\left [ 1 + (3\, \text{cos}^{2}\, \beta - 1)R(t) \right ]$ where $$R(t)$$ is the degree of alignment of the emitting transition dipole in the laboratory frame and $$N(t)$$ is the excited-state population, both at time $$t$$. For $$\beta = 54.7^{\,\unicode{x26ac}}$$ (the magic @A00346@), the dipole-alignment contribution vanishes and $$I(t,\,\beta = 54.7^{\,\unicode{x26ac}} ) \propto N(t)$$.
Notes:
1. This concept also applies for time-resolved absorption measurements in cases in which @PT07461@ occurs because the detected species do not freely rotate fast enough to make the measurement @I03353@ within the time of the experiment.
2. Applies for steady-state measurements on fixed samples. In this case $I(\beta ) \propto N\left [ 1 + (3\, \text{cos}^{2}\, \beta - 1)R \right ]$ with $$I(\beta)$$ the intensity of the effect observed at an analyser @A00346@ $$\beta$$ with respect to the electric @P04712@ of the exciting beam, $$N$$ the excited-state population at steady-state equilibrium, and $$R$$ the degree of alignment of the @T06460@ of the excited molecular entity.
3. The term magic @A00346@ is also used in NMR @S05848@.
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 367 [Terms] [Paper]