  We investigate the distribution of number of photons emitted by a single
molecule undergoing a spectral diffusion process and interacting with a
continuous wave field. Using a generating function formalism an exact
analytical formula for Mandel's $Q$ parameter is obtained. The solution
exhibits transitions between: (i) Quantum sub-Poissonian and Classical
super-Poissonian behaviors, and (ii) fast to slow modulation limits. Our
solution yields the conditions on the magnitude of the spectral diffusion time
scales on which these transitions are observed. We show how to choose the Rabi
frequency in such a way that the Quantum sub-Poissonian nature of the emission
process becomes strongest, we find ${\Omega^2 \over \Gamma} = {\Gamma_{{\rm
SD}} + \Gamma \over 2}$ where $\Gamma_{{\rm SD}}$ ($\Gamma$) is the spectral
diffusion (radiative) contribution to the width of the line shape.
