Fig. 5: Temporal dynamics of neutral H₂ formation dynamics.

a Time-resolved ion yields of C₂H\({}_{4}^{2+}\). An enhancement of ion yield was observed after a time delay of ~700 fs, as indicated by the dots in the inset. The main panel in a shows an enlarged and normalized view of the ion yield enhancement. The experimental data was fitted using an exponentially rising function. The rising time constant was determined to be 567( ± 61) fs, and the initial instant time was found to be 748( ± 24) fs. The oscillations in the ion yield were observed in both the raw data and the residual (lower sub-panel) obtained from the difference between the experimental data and the fitting curve. The errors of residual signals originate from the statistical uncertainties. b The Fourier transformed spectrum of the residual signal. The peaks labeled in red numbers were assigned to the vibrational mode frequencies (in brackets) of the molecular geometry around the CI. c, d The overlap between the time-resolved frequency distribution of the residual signal and velocity autocorrelation function of MD simulation with the characteristic frequency distribution of representative CI geometry (defined in Supplementary Note 2). The peak time for this overlap, corresponding to the characteristic time scale when C₂H\({}_{6}^{+}\) reaches the CI geometry, was determined to be 1234 fs and 1286 fs from the experiment and MD simulation, respectively.