Dynamic-Absorption Spectral Contours:  Vibrational Phase-Dependent Resolution of Low-Frequency Coherent Wave-Packet Motion of IR144 on the Ground-State and Excited-State π → π* Surfaces

We have characterized the femtosecond dynamic-absorption spectrum from the tricarbocyanine dye IR144 with the aid of a new approach that uses time-probe-wavelength contour lines to project the phase relationships between the coherent wave-packet motions on the ground-state and excited-state potential-energy surfaces. The distinct phase of the waveform carried by a dynamic-absorption contour line reports the motion of a single wave packet. The spectrum from IR144 exhibits four regions of alternating contour phase; simulations show that this interference pattern results from the antiphase motion and partial spectral overlap of the ground-state depletion and stimulated-emission spectra and the narrowing of the spectra that occurs when the excited-state and ground-state wave packets reach their turning points. The Fourier-magnitude spectra of contour lines and intensity transients observed in the spectral region, which are assigned to the turning point of the ground-state wave packet, have been compared to those obtained in the long-wavelength-limit spectral region that is associated with the excited-state wave packet. The intensities and frequencies of eleven modes observed over the 15−634 cm^-1 region suggest that the excited-state wave packet rapidly moves from the Franck−Condon geometry to a region of the potential-energy surface that is relatively flat with respect to the global normal coordinates that bend and twist the conjugated polyene backbone of IR144. These coordinates are evidently anharmonically coupled to high-frequency modes that are not impulsively excited by the 12-fs pulses used in the dynamic-absorption experiments.