Fluorescent ring illuminator

TEM offers two methods of specimen observation, diffraction mode and image mode. In diffraction mode, an electron diffraction pattern is obtained on the fluorescent screen, originating from the sample area illuminated by the electron beam. The diffraction pattern is entirely equivalent to an X-ray diffraction pattern a single crystal will produce a spot pattern on the screen, a polycrystal will produce a powder or ring pattern (assuming the illuminated area includes a sufficient quantity of crystallites), and a glassy or amorphous material will produce a series of diffuse halos. 

When porphyrins dissolved in strong mineral acids or in organic solvents are illuminated by ultraviolet hght, they emit a strong red fluorescence. This fluorescence is so characteristic that it is often used to detect small amounts of free porphyrins. The double bonds joining the pyrrole rings in the porphyrins are responsible for the characteristic absorption and fluorescence of these compounds these double bonds are absent in the porphyrinogens. 

The XOR gate 61 s action arose as follows. The read laser elicited a low level of fluorescence from the porphyrin unit since the betaine unit is a PET acceptor. In addition, 1,064-nm illumination allowed thermal ring closure of the betaine unit to the dihydroindolizine unit and produced 63. With the betaine unit gone, 63 has a high level of fluorescence. And 532-iun illumination also produced the same result. 

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