## rotational relaxation time, $$\rho$$

https://doi.org/10.1351/goldbook.RT07475
Parameter describing the time-dependence of the tumbling of a molecular entity in a medium of viscosity η as originally defined by Debye, and used by Perrin in the original development of the theories of rotational motion of fluorophores.
Note:
Related to the rotational correlation time, $$\tau_{c}$$, by $$\rho = 3\tau_{c}$$. Thus, in the case of a spherically emitting species reorienting itself in a homogeneous fluid, $$\rho = 1/(6\, D_{r})$$, with $$D_{r}$$ the @R05411@.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 416 (https://doi.org/10.1351/pac200779030293)