  The width and shape of photon burst histograms pose significant limitations
to the identification of single molecules in micro/nano-fluidic channels, and
the nature of these histograms is not fully understood. To reach a deeper
understanding, we performed computer simulations based on a Gaussian beam
intensity profile with various fluidic channel diameters and assuming (i) a
deterministic (noise-free) case, (ii) photon emission/absorption noise, and
(iii) photon noise with diffusion. Photon noise in narrow channels yields a
Gaussian burst distribution while additional strong diffusion produces skewed
histograms. We use the fluctuating residence time picture [Phys. Rev. Lett. 80,
2386-2388 (1998)] and conclude that the skewness of the photon number
distribution is caused by the longitudinal diffusive component of the motion of
the molecules as they traverse the laser beam. In the case of strong diffusion
in narrow channels, this effect leads to a log-normal distribution. We show
that the same effect can transform the separate peaks of the photon burst
histograms of multiple molecule mixtures into a single log-normal shape.
