Molecular dynamics in solution probed by the transient grating method with a nanosecond pulsed laser

Molecular dynamics in organic solution is investigated by the transient grating method with a nanosecond pulsed laser and a fast detection system. First, the dynamics of methyl red (MR) is studied for the probe of the translational diffusion process of solution. The origin of the TG signal which represents the mass diffusion is attributed to the phase grating created by the different refractive indices of the cis and trans forms. The cis form of MR, which has a slightly smaller molecular volume than the trans form is found to diffuse more slowly in alcoholic and aldehyde solutions. The measured diffusion coefficients (D) are compared with theoretically calculated values. Second, the TG method is applied to elucidate the dynamics of transient radicals created by photochemical reactions. The determined D of the radicals are about 3 to approximately 4 times smaller than those of the parent non-radical molecules even though the molecular volumes are similar to each other. The advantages as well as the disadvantages of the TG method for elucidating the solvent dynamics are discussed.