  The UV photochemistry of small heteroaromatic molecules serves as a testbed
for understanding fundamental photoinduced transformations in moderately
complex compounds, including isomerization, ring-opening, and molecular
dissociation. Here, a combined experimental-theoretical study of 268 nm UV
light-induced dynamics in 2-iodothiophene (C$_4$H$_3$IS) is performed. The
dynamics are experimentally monitored with a femtosecond XUV probe pulse that
measures iodine N-edge 4d core-to-valence transitions. Experiments are
complemented by density functional theory calculations of both the pump-pulse
induced valence excitations as well as the XUV probe-induced core-to-valence
transitions. Possible intramolecular relaxation dynamics are investigated by ab
initio molecular dynamics simulations. Gradual absorption changes up to ~0.5-1
ps after excitation are observed for both the parent molecular species and
emerging iodine fragments, with the latter appearing with a characteristic rise
time of 160$\pm$30 fs. Comparison of spectral intensities and energies with the
calculations identify an iodine dissociation pathway initiated by a predominant
$\pi\to\pi^*$ excitation. In contrast, initial excitation to a nearby
n$_\perp\to\sigma^*$ excited state appears unlikely based on a significantly
smaller oscillator strength and the absence of any corresponding XUV absorption
signatures. Excitation to the $\pi\to\pi^*$ state is followed by contraction of
the C-I bond, enabling a nonadiabatic transition to a dissociative
$\pi\to\sigma_\textrm{C-I}^*$ state. For the subsequent fragmentation, a narrow
bond-length region along the C-I stretch coordinate between 230 and 280 pm is
identified, where the transition between the parent molecule and the thienyl
radical + iodine atom products becomes prominent in the XUV spectrum due to
rapid localization of two singly-occupied molecular orbitals on the two
fragments.
