Linear and nonlinear optical responses in bacteriochlorophyll <i>a</i>

Nonlinear optical responses of bacteriochlorophyll &lt;i&gt;a&lt;/i&gt; (BChl &lt;i&gt;a&lt;/i&gt;) were investigated by means of the three-pulse four-wave mixing (FWM) technique under the resonant excitation into the &lt;i&gt;Q&lt;/i&gt; &lt;i&gt;y&lt;/i&gt; band. The experimental results are explained by a theoretical model calculation including the Brownian oscillation mode of the solvent. We have determined the spectral density, which is the most important function with which to calculate optical signals. The linear absorption spectrum can be reproduced fairly well when the vibronic oscillation modes of the solvent together with those of BChl &lt;i&gt;a&lt;/i&gt; are properly taken into consideration. The FWM signal was also calculated using the spectral density. It was found that a simple two-level model could not explain the experimental result. The effect of the higher-order interactions is discussed