Nonadiabatic Photodissociation and Dehydrogenation Dynamics of n-Butyl Bromide Following p-Rydberg Excitation

Femtosecond time-resolved mass spectrometry, correlationmapping,and density functional theory calculations are employed to revealthe mechanism of C C and C C formation (and relatedH(2) production) following excitation to the p-Rydberg statesof n-butyl bromide. Ultrafast pump-probe massspectrometry shows that nonadiabatic relaxation operates as a multistepprocess reaching an intermediate state within & SIM;500 fs followedby relaxation to a final state within 10 ps of photoexcitation. Absorptionof three ultraviolet photons accesses the dense p-Rydberg state manifold,which is further excited by the probe beam for C C bond dissociationand dehydrogenation reactions. Rapid internal conversion deactivatesthe dehydrogenation pathways, while activating carbon backbone dissociationpathways. Thus, unsaturated carbon fragments decay with the lifetimeof p-Rydberg (& SIM;500 fs), matching the growth recorded in saturatedhydrocarbon fragments. The saturated hydrocarbon signals subsequentlydecay on the picosecond time scale as the molecule relaxes below theRydberg states and into halogen release channels.

Automated summary

Femtosecond time-resolved mass spectrometry, correlation mapping, and density functional theory calculations were used to investigate the formation of C=C and C=O (and related H2 production) after excitation to the p-Rydberg states of n-butyl bromide. The results showed that nonadiabatic relaxation occurs as a multi-step process, with an intermediate state reached within 500 fs and a final state reached within 10 ps. The absorption of three ultraviolet photons allows access to the dense p-Rydberg state manifold, which is further excited by the probe beam for C-C bond dissociation and dehydrogenation reactions.