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 -C 2 F 4 I 2 molecules to the Franck–Condon region (red arrow). Thee excited molecules (denoted with an asterisk) rapidly dissociate into anti -C 2 F 4 I• and I• (blue arrow). Along the PES of C 2 F 4 I• (magenta coordinate), an excess amount of anti -C 2 F 4 I• 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 C 2 F 4 I 2 (violet coordinate), a depleted portion of anti -C 2 F 4 I 2 molecules re-equilibrate by gauche -to- anti rotational isomerization of C 2 F 4 I 2 (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.
