  We probe the dynamics of dissociating CS$_2$ molecules across the entire
reaction pathway upon excitation. Photoelectron spectroscopy measurements using
laboratory-generated femtosecond extreme ultraviolet pulses monitor the
competing dissociation, internal conversion, and intersystem crossing dynamics.
Dissociation occurs either in the initially excited singlet manifold or, via
intersystem crossing, in the triplet manifold. Both product channels are
monitored and show that, despite being more rapid, the singlet dissociation is
the minor product and that triplet state products dominate the final yield. We
explain this by a consideration of accurate potential energy curves for both
the singlet and triplet states. We propose that rapid internal conversion
stabilizes the singlet population dynamically, allowing for singlet-triplet
relaxation via intersystem crossing and the efficient formation of
spin-forbidden dissociation products on longer timescales. The study
demonstrates the importance of measuring the full reaction pathway for defining
accurate reaction mechanisms.
