Ultrafast relaxation and modulation in the oxazine dye nile blue

Time-delayed four-wave mixing, using incoherent laser pulses 5 ns long and 350 Å wide, has been used to measure the optical phase relaxation of a large molecule (nile blue) in solution at 300 and 4.5 K. In the latter case an asymmetric response as well as a quantum beat (signaling a vibronic mode at 583 ${\mathrm{cm}}^{\mathrm{-}1}$) is observed. Its magnitude and shape gives its electron-vibron coupling factor S=0.79 together with the homogeneous relaxation time ${\mathit{T}}_2$=43.7 fs and the inhomogeneous relaxation time ${\mathit{T}}_2^{\mathrm{*}}$=13.3 fs (full width of 940 ${\mathrm{cm}}^{\mathrm{-}1}$). Supporting evidence for the large value of S is presented.