  The low-energy dynamical optical response of dimerized and undimerized spin
liquid states in a one-dimensional charge transfer Mott insulator is
theoretically studied. An exact analysis is given for the low-energy asymptotic
behavior using conformal field theory for the undimerized state. In the
dimerized state, the infrared absorption due to the bound state of two
solitons, i.e, the breather mode, is predicted with an accurate estimate for
its oscillator strength, offering a way to detect experimentally the excited
singlet state. Effects of external magnetic fields are also discussed.
