  Due to its element- and site-specificity, inner-shell photoelectron
spectroscopy is a widely used technique to probe the chemical structure of
matter. Here we show that time-resolved inner-shell photoelectron spectroscopy
can be employed to observe ultrafast chemical reactions and the electronic
response to the nuclear motion with high sensitivity. The ultraviolet
dissociation of iodomethane (CH$_3$I) is investigated by ionization above the
iodine 4d edge, using time-resolved inner-shell photoelectron and photoion
spectroscopy. The dynamics observed in the photoelectron spectra appear earlier
and are faster than those seen in the iodine fragments. The experimental
results are interpreted using crystal field and spin-orbit configuration
interaction calculations, and demonstrate that time-resolved inner-shell
photoelectron spectroscopy is a powerful tool to directly track ultrafast
structural and electronic transformations in gas-phase molecules.
