Fig. 3

Vibrational coherence spectroscopy of compounds Z-1 and E-2. The power spectra of the differential absorption ΔA oscillatory residuals revealed in Fig. 2g, h are averaged over the 350–470 nm probing window and displayed in a for Z-1 and b for E-2. The spectra are normalized to 1 at the C=C stretch frequency of 1572 or 1574 cm⁻¹. The vertical scale is linear from 0 to 1 and logarithmic above 1. They reveal the frequencies of the modes in which vibrational wave packets are observed upon resonant excitation at 400 nm (blue spectra). The same analysis is performed on the data acquired upon off-resonance excitation at 800 nm (red spectra). The black spectra correspond to off-resonant excitation of the pure methanol solvent. Stars indicate the vibrational signatures of the solvent or fused silica cuvette. c The same Fourier analysis of the Z-1 oscillatory residuals is performed now on a sliding, 1-ps long time window starting at t₀ = 0.1 ps (red spectrum) or t₀ = 0.3 ps (blue spectrum) i.e., respectively before and after impulsive decay. The dominant 80 cm⁻¹ mode persists. The 185 cm⁻¹ feature instead disappears upon decay to S₀, but it seems too narrow to represent an oscillatory signal, which would last for only 250 fs, which questions its significance and interpretation, especially in the proximity of the very intense 80 cm⁻¹ mode. d Same as c, for E-2, with t₀ starting times ranging from 0.1 to 0.6. The inset displays the semilog plot of the FFT amplitude of the five dominating modes at 232, 456, 651, 736, 1564 cm⁻¹, as a function of t₀, and the corresponding monoexponential decay fits. While the amplitude of all higher-frequency modes decay on the 600 to 1200 fs time scale, the damping of the 232 cm⁻¹ mode is observed to occur on a significantly faster 230 fs time scale, in line with the 300 fs excited state lifetime of E-2