pre-exponential factor, A in luminescence spectroscopy

https://dx.doi.org/10.1351/goldbook.P04811
Coefficient in front of the exponential factor expressing the empirical temperature dependence of the rate coefficient, k, on temperature, T,k = A exp ( − E a / R T ), where E a is the activation energy.
Source:
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 177