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>This leaves us with around 3E-13 calories.
>
> Assuming a cube of 1 Forster radius is heated in this time we have a
> diameter of 100nm and a volume of 1E-15 cm cubed.  To heat this 13 degrees
> celsius, we need only 1.3E-14 calories.  In other words, we have more than
> 20 times the energy it takes to heat this volume. Even assuming the number
> of photons absorbed before bleaching is high and that there is some loss
> outside this region in  such a short period of time (and with many other
> fluorophores around doing the same thing), this gives one pause.  Am I
> making a math error somewhere?
>

No math error in the calculations you suggest, as far as I can see, but I
think this calculation is incomplete.

Your calculation would be right if all energy would be cumulatively stored
in the sample, but this is not true as conduction of heat will immediately
take place. For a correct calculation I think you would have to follow the
mathematics as described in the article I already mentioned yesterday:

De Smedt, S. C., A. Lauwers, J. Demeester, Y. Engelborghs, G. Demey, and M.
Du. 1994. Structural information on hyaluronic acid solutions as studied by
probe diffusion experiments. Macromolecules 27:141-146.

But to give you an idea without performing the actual calculations, using
the thermal constants for water, the average distance the heat will have
travelled in 10 ms (the bleaching time you suggested) is about 60 microns
(!) which is way past the focal volume, let alone the Forster volume you
mentioned.

So at first there will be some build up of local thermal energy causing a
local thermal gradient. Consequently, conduction of heat will immediately
kick in and the thermal energy will spread very quickly throughout the
sample, causing no more than a very slight local increase in temperature.
According to the calculations in the article mentioned above, the increase
in temperature is in the order of (only) 0.1 degree Celsius (assuming an
infinite watery medium, an illumination time of 15 ms and an illumination
power of 1 mW).

At least, this is how I see it, other suggestions are very welcome of
course.

Cheers,

Kevin