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The UV photochemistry of small heteroaromatic molecules serves as a testbedfor understanding fundamental photoinduced transformations in moderatelycomplex compounds, including isomerization, ring-opening, and moleculardissociation. Here, a combined experimental-theoretical study of 268 nm UVlight-induced dynamics in 2-iodothiophene (C$_4$H$_3$IS) is performed. Thedynamics are experimentally monitored with a femtosecond XUV probe pulse thatmeasures iodine N-edge 4d core-to-valence transitions. Experiments arecomplemented by density functional theory calculations of both the pump-pulseinduced valence excitations as well as the XUV probe-induced core-to-valencetransitions. Possible intramolecular relaxation dynamics are investigated by abinitio molecular dynamics simulations. Gradual absorption changes up to ~0.5-1ps after excitation are observed for both the parent molecular species andemerging iodine fragments, with the latter appearing with a characteristic risetime of 160$\pm$30 fs. Comparison of spectral intensities and energies with thecalculations identify an iodine dissociation pathway initiated by a predominant$\pi\to\pi^*$ excitation. In contrast, initial excitation to a nearbyn$_\perp\to\sigma^*$ excited state appears unlikely based on a significantlysmaller oscillator strength and the absence of any corresponding XUV absorptionsignatures. Excitation to the $\pi\to\pi^*$ state is followed by contraction ofthe C-I bond, enabling a nonadiabatic transition to a dissociative$\pi\to\sigma_\textrm{C-I}^*$ state. For the subsequent fragmentation, a narrowbond-length region along the C-I stretch coordinate between 230 and 280 pm isidentified, where the transition between the parent molecule and the thienylradical + iodine atom products becomes prominent in the XUV spectrum due torapid localization of two singly-occupied molecular orbitals on the twofragments.

Probing C-I Bond Fission in the UV Photochemistry of 2-Iodothiophene with Core-to-Valence Transient Absorption Spectroscopy