Femtosecond time-resolved mass spectrometry, correlation
mapping,
and density functional theory calculations are employed to reveal
the mechanism of CC and CC formation (and related
H2 production) following excitation to the p-Rydberg states
of n-butyl bromide. Ultrafast pump–probe mass
spectrometry shows that nonadiabatic relaxation operates as a multistep
process reaching an intermediate state within ∼500 fs followed
by relaxation to a final state within 10 ps of photoexcitation. Absorption
of three ultraviolet photons accesses the dense p-Rydberg state manifold,
which is further excited by the probe beam for CC bond dissociation
and dehydrogenation reactions. Rapid internal conversion deactivates
the dehydrogenation pathways, while activating carbon backbone dissociation
pathways. Thus, unsaturated carbon fragments decay with the lifetime
of p-Rydberg (∼500 fs), matching the growth recorded in saturated
hydrocarbon fragments. The saturated hydrocarbon signals subsequently
decay on the picosecond time scale as the molecule relaxes below the
Rydberg states and into halogen release channels.