Subpicosecond molecular dynamics studied by degenerate four-wave mixing with incoherent light

A time-resolved study of the relaxation times of the third-order susceptibility for the optical Kerr effect in ${\mathrm{CS}}_2$ and nitrobenzene was made by degenerate four-wave mixing using 5-ns pulses from a broadband laser. In the case of ${\mathrm{CS}}_2$ we observed both subpicosecond (∼0.2 ps) and picosecond (∼2 ps) relaxation times for the susceptibility tensor element of ${\chi }_{\mathrm{xyyx}}^{\mathrm{}\left(3\right)\mathrm{}}$ and ${\chi }_{\mathrm{xxxx}}^{\mathrm{}\left(3\right)\mathrm{}}$ both at 553 and 623 nm. The observed relaxation times agree with those obtained utilizing subpicosecond pulses. The relaxation times of nitrobenzene could not be measured because the shorter relaxation time is shorter than the resolution time and the longer one is longer than that of ${\mathrm{CS}}_2$.