“…The solute molecules are present in sufficiently low concentration that interactions among them are negligible, and we may treat a single solute in its solvent environment. The differential absorption spectrum, ( 2), is defined to be the difference between the absorption coefficient at 2 in the absence of a pump pulse and the absorption coefficient at 2 measured with a probe pulse that follows a pump pulse. The differential absorption at w2 is related to 2( 2), the amplitude of the probe field, and…”