Fig. 4: Photodynamics of C₂F₄I₂ as revealed by time-resolved X-ray liquidography.

a A schematic diagram of primary iodine dissociation and rotational isomerization on the potential energy surface (PES) with respect to r_CI and φ_dihed. UV photon irradiation majorly excites anti-C₂F₄I₂ molecules to the Franck–Condon region (red arrow). Thee excited molecules (denoted with an asterisk) rapidly dissociate into anti-C₂F₄I• and I• (blue arrow). Along the PES of C₂F₄I• (magenta coordinate), an excess amount of anti-C₂F₄I• molecules undergo anti-to-gauche rotational isomerization (magenta arrow), overcoming the energy barrier of 3.4 kcal/mol with a time constant of 1.2 ps. Meanwhile, along the PES of C₂F₄I₂ (violet coordinate), a depleted portion of anti-C₂F₄I₂ molecules re-equilibrate by gauche-to-anti rotational isomerization of C₂F₄I₂ (violet arrow), surpassing the energy barrier of 4.0 kcal/mol with a time constant of 26 ps. We note that, at the initial step, only the anti-to-anti excitation is depicted in the diagram for simplicity. b Time-dependent concentrations of anti-C₂F₄I₂, gauche-C₂F₄I₂, anti-C₂F₄I•, and gauche-C₂F₄I•. Upon photodissociation, 5% of anti (blue) and 2% of gauche (green) C₂F₄I₂ convert to their respective conformers of C₂F₄I•, perturbing the anti-to-gauche ratios of both C₂F₄I₂ (to 45.0 mM:12.4 mM = 78:22) and C₂F₄I• (to 2.3 mM:0.2 mM = 91:9). Subsequently, 0.5 mM of anti-C₂F₄I• converts to gauche-C₂F₄I• with a time constant of 1.2 ps (upper-right panel), converging to a dynamic equilibrium anti-to-gauche ratio of C₂F₄I• (to 1.8 mM:0.7 mM = 71:29). Likewise, 0.3 mM of gauche-C₂F₄I₂ isomerizes to anti-C₂F₄I• with a time constant of 26 ps (lower-right panel), recovering a dynamic equilibrium (to 45.3 mM:12.1 mM = 79:21). Finally, 30% of the equilibrated C₂F₄I• undergoes secondary dissociation into the product (C₂F₄) with a time constant of 292 ps, while the remaining C₂F₄I• persist up to 100 ps (as can be seen in Fig. 3a–c).