Acetylacetone Photolysis at 280 nm Studied by Velocity-Map Ion Imaging

The photolysis of acetylacetone (AcAc) has been studiedusing velocity-mapion imaging with pulsed nanosecond lasers. The enolone tautomer ofAcAc (CH3C(O)CH C(OH)CH3) was excitedin the strong UV absorption band by UV pulses at 280 nm, preparingthe S-2(& pi;& pi;*) state, and products were probedafter a short time delay by single-photon VUV ionization at 118.2nm. Two-color UV + VUV time-of-flight mass spectra show enhancementof fragments at m/z = 15, 42, 43,58, and 85 at the lowest UV pulse energies and depletion of the parention at m/z = 100. Ion images ofthe five major fragments are all isotropic, indicating dissociationlifetimes that are long on the timescale of molecular rotation butshorter than the laser pulse duration (<6 ns). The m/z = 15 and 85 fragments have identical momentumdistributions with moderate translational energy release, suggestingthat they are formed as a neutral product pair and likely via a Norrishtype I dissociation of the enolone to form CH3 + C(O)CH C(OH)CH3 over a barrier on a triplet surface. The m/z = 43 fragment may be tentatively assigned tothe alternative Norrish type I pathway that produces CH3CO + CH2C(O)CH3 on S-0 followingphototautomerization to the diketone, although alternative mechanismsinvolving dissociative ionization of a larger primary photoproductcannot be conclusively ruled out. The m/z = 42 and 58 fragments are not momentum-matched and consequentlyare not formed as a neutral pair via a unimolecular dissociation pathwayon S-0. They also likely originate from the dissociativeionization of primary photofragments. RRKM calculations suggest thatunimolecular dissociation pathways that lead to molecular productson S-0 are generally slow, implying an upper-limit lifetimeof <46 ns after excitation at 280 nm. Time-dependent measurementssuggest that the observed photofragments likely do not arise fromdissociative ionization of energized AcAc S-0*.

Automated summary

The photolysis of acetylacetone (AcAc) was studied using velocity-map ion imaging with pulsed nanosecond lasers. The results showed that the dissociation of AcAc occurs with lifetimes longer than the laser pulse duration but shorter than the molecular rotation timescale. It was also found that the excitation of AcAc in the strong UV absorption band leads to the formation of neutral product pairs, possibly via a Norrishtype I mechanism.