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The ionic radius of Sr is 132 to 140pm or 0.14nm or 0.00000000014mm radius and 0.00000000028mm diameter (have I got those zeros right?). Clearly the camera cannot resolve such a small object. What I understand to be happening is that the laser is pushing an electron to a higher energy level and the camera is recording the photons emitted as the electron falls back to its ground state. The photon itself probably has a wavelength of ca 400nm (ie the photon is larger than the object emitting it) but of course the camera cannot resolve such a small disc. I suspect the size of the dot is due to scattering (and possibly diffraction?) within the optics.
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