  The nonlinear interaction of x-rays with matter is at the heart of
understanding and controlling ultrafast molecular dynamics from an
atom-specific viewpoint, providing new scientific and analytical opportunities
to explore the structure and dynamics of small quantum systems. At increasingly
high x-ray intensity, the sensitivity of ultrashort x-ray pulses to specific
electronic states and emerging short-lived transient intermediates is of
particular relevance for our understanding of fundamental multi-photon
absorption processes. In this work, intense x-ray free-electron laser (XFEL)
pulses at the European XFEL (EuXFEL) are combined with a gas cell and grating
spectrometer for a high-intensity transmission spectroscopy study of
multiphoton-induced ultrafast molecular fragmentation dynamics in CF$_4$. This
approach unlocks the direct intra-pulse observation of transient fragments,
including neutral atoms, by their characteristic absorption lines in the
transmitted broad-band x-ray spectrum. The dynamics with and without initially
producing fluorine K-shell holes are studied by tuning the central photon
energy. The absorption spectra are measured at different FEL intensities to
observe nonlinear effects. Transient isolated fluorine atoms and ions are
spectroscopically recorded within the ultrashort pulse duration of few tens of
femtoseconds. An isosbestic point that signifies the correlated transition
between intact neutral CF$_4$ molecules and charged atomic fragments is
observed near the fluorine K-edge. The dissociation dynamics and the
multiphoton absorption-induced dynamics encoded in the spectra are
theoretically interpreted. Overall, this study demonstrates the potential of
high-intensity x-ray transmission spectroscopy to study ultrafast molecular
dynamics with sensitivity to specific intermediate species and their electronic
structure.
