text: Excitation A→A* by  photon, whereby the promoted electron is passed down to the C* molecular orbital. A second photon applied to the system  () causes the excitation of the electron in the C* molecular orbital to the C** molecular orbital -analogous pump probe spectroscopy.
  Above, the energy level diagram illustrating the principle of pump probe spectroscopy –the excitation of an excited state.
  The AND gate is produced by the necessity of both A→A* and the C**→C excitations occurring at the same time -input   and , are simultaneously required. 
  To prevent erroneous emissions of light from a single input to the AND gate, it would be necessary to have an electron transfer series with ability accept any electrons (energy) from C* energy level. The electron transfer series would terminate with a low (non-radiative decay) of the energy 
  The alternatives for producing an AND gate, using molecular photphysics, are two.
  (1) The emission produced by the electron drop from C*→C () is not a valid output frequency. The emission from the C** ( + , ) molecular orbital is a valid output signal;. to be used in subsequent logic gates -arranged to respond to the  emission. 
  The second input of photon(s) to trigger the rapid conversion of a molecule used to complete the electron transfer chain. A very complex molecule like a protein can be engineered to possess high strain energies, so that in the absence of the second light frequency molecule B is inactive (B). The second photon input triggers B→B' where the forward rate constant is much smaller than the reverse. If such a molecule is used as molecule B, the transfer chain can be switched on and off.