Fig. 2. Experimental and theoretical signals.

a The real-space difference radial distribution function, $\mathrm{\Delta }\mathrm{RDF}\left(r\right)$, obtained from the experimental data at 25 fs pump–probe delay time. The blue arrows point to the depletion and increase in electron density at short and long electron distances, respectively, as the molecule is excited from the tightly bound ground electronic state to the diffuse excited 3p state. The insert shows the corresponding contour slices of the electron density difference from electronic structure calculations. The left-hand slice shows the difference in a plane through the C=C-C=C atoms, which illustrates the density gains far from the molecule, while the perpendicular right-hand slice, taken through one of the C=C bonds, shows the corresponding loss of density in the HOMO π-orbital. The color intensity is renormalized between −1 and 1 and absolute values <0.01 are not shown. b Fractional difference signals, $\mathrm{\Delta }S\left(q\right)$, shown in percent. The experimental signal at 25 fs delay time is shown in black with 1σ error bars. The corresponding theoretical $\mathrm{\Delta }{S}_{3\mathrm{p}}\left(q,{\mathbf{R}}^{+}\right)$ signal for the electronic 3p state is shown in red with the shaded region accounting for the sampling of geometries in the excited state. For comparison, theoretical signals for the ground electronic state (X) at the 3p geometry, $\mathrm{\Delta }{S}_X\left(q,{\mathbf{R}}^{+}\right)$, and for the excited 3p state at equilibrium geometry, $\mathrm{\Delta }{S}_{3\mathrm{p}}\left(q,{\mathbf{R}}_0\right)$, are included.