A Note on Efficiency for Conversion of Light Energy.

A few years ago I gave a method, mainly based on the second law of thermodynamics, for calculating the maximum efficiency with which light energy can be converted into so-called high grade energy such as work or Gibbs free energy (1) [cf. also (2)]. This efficiency approached zero with decreasing light intensity and with increasing temperature of the system for conversion of light energy. An essential condition is that the light is diffuse (so that its intensity cannot be enhanced by means of a lens), or that the light-converting device is as efficient in diffuse as in non-diffuse light. By applying this method to photosynthesis, it was shown, using certain generally accepted experimental results, that the maximum measurable efficiency of algal photosynthesis must be lower than about 70 %. In a recent paper in this journal (3), Dr. Kahn attempted to demonstrate that my arguments and conclusions were wrong. I quote: "This argument rests on the application of the second law of thermodynamics to the photon absorption process. We wish to point out, however, that application of the second law of thermodynamics to the absorption of a photon by an atom or molecule is not valid, owing to the unique nature of such an interaction." Of course thermodynamics is concerned with macroscopic systems, consisting of billions of molecules, not with a single molecule or a single molecular process. I applied, as far as I am aware, thermodynamics only to macroscopic systems. It is clear from the context that my conclusion, that at least four quanta are necessary to reduce one CO. molecule, should be understood macroscopically. I was also criticized for attributing a temperature to a beam of photons of a certain wavelength range. Without discussing whether such an attribution would be correct or not, I only state that I am not aware to have done this. As far as I can see, Dr. Kahn's