  Conical intersections between electronic states often dictate the chemistry
of photoexcited molecules. Recently developed sources of ultrashort extreme
ultraviolet (XUV) pulses tuned to element-specific transitions in molecules
allow for the unambiguous detection of electronic state-switching at a conical
intersection. Here, the fragmentation of photoexcited iso-propyl iodide and
tert-butyl iodide molecules (i-C$_{3}$H$_{7}$I and t-C$_{4}$H$_{9}$I) through a
conical intersection between $^{3}$Q$_{0}$/$^{1}$Q$_{1}$ spin-orbit states is
revealed by ultrafast XUV transient absorption measuring iodine 4d
core-to-valence transitions. The electronic state-sensitivity of the technique
allows for a complete mapping of molecular dissociation from photoexcitation to
photoproducts. In both molecules, the sub-100 fs transfer of a photoexcited
wave packet from the $^{3}$Q$_{0}$ state into the $^{1}$Q$_{1}$ state at the
conical intersection is captured. The results show how differences in the
electronic state-switching of the wave packet in i-C$_{3}$H$_{7}$I and
t-C$_{4}$H$_{9}$I directly lead to differences in the photoproduct branching
ratio of the two systems.
