FIG. 3.

(a) Kinetics of a PFBT thin film averaged over the wavelength ranges 485–495 nm (black line) and 424–434 nm (red line) showing damped oscillations arising from a coherent longitudinal acoustic phonon. A phase shift of π is observed as for PFPh. (b) Contour map of Fourier amplitudes extracted from the complete spectral dataset (top) and representative averaged spectral slices from the contour map for the wavelength ranges indicated (bottom). The slices are vertically shifted to avoid overlap of the traces. The noise of the slice for 365–375 nm is slightly increased due to pump-beam stray light. The slices show the pronounced contribution of a coherent acoustic phonon at 4.6 GHz particularly in the wavelength ranges 390–450 and 472–525 nm. (c) Wavelength-dependent Fourier amplitude averaged over the frequency interval (4.6 ± 0.9) GHz (black line), steady-state absorption spectrum of PFBT (green line) and absolute value of the first derivative of the steady-state absorption spectrum (red line).