Coupling Strength for Resonance Force Transfer of Electronic Energy in Van der Waals Solids

The interpretation of the absorption spectrum of a molecular crystal requires a knowledge of the strength of resonance force electrostatic interaction between molecules in the crystal as compared to an energy parameter characterizing the molecular vibrational level pattern. Depending on the relative magnitudes of these energy terms the spectrum reflects absorption by the crystal as a whole or by independent, though oriented molecules. These two familiar cases are examined from both stationary-state and time-dependent points of view with the object of defining the energy parameters. The resonance force interaction term is a theoretically derived quantity, the crystal electronic band width; the comparable vibrational energy term is shown to be the width of the total electronic band comprising all the vibronic transitions for an isolated molecule. Particular attention is given to the case in which these terms are nearly equal.