Photochemical and thermal efficiencies

3 POLYMERS IN THE RECORDING LAYER 9.3.1 Photochemical and thermal efficiencies 

The pulse of energy from a laser provides a source of photons that can cause photochemical effects. More usually, however, the energy is merely a source of heat within the recording layer. To see why this is so, it is instructive to perform some simple calculations on the number of moles of photons available for photochemistry and the temperature reached within a recording layer during a write pulse. 

The number of moles of photons in a pulse t seconds long of W watts of light of wavelength X is given by 

Assuming all of the input energy could be used for photochemistry, it is apparent that an efficiency or quantum yield (defined as the number of molecules per photon undergoing change) of better than 0.3 is required in order to write information. Such quantum yields for photochemical reactions would be considered unusually high in the solid state. 

On the assumption of total thermal absorption, the same pulse would heat the same volume of material (with a specific heat H = 250 J kg K ) through approximately 

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