rotational relaxation time, ρ

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, $τ c$ , by $ρ = 3 τ c$ . Thus, in the case of a spherically emitting species reorienting itself in a homogeneous fluid, $ρ = 1 / 6 D r$ , with $D r$ the rotational diffusion coefficient.

Source:

PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 416