No abstract

Because of the dispute in the literature over the dissociation rate and energy partitioning of the acetone molecule upon photoexcitation to the S 1 state (*←n) and 3s Rydberg state (3s←n), we have remeasured the lifetime of acetone ͑also d 6 -acetone͒ on the S 1 and 3s surfaces by a femtosecond time-resolved multiphoton ionization technique, coupled with a reflectron time-of-flight mass spectrometer. The measured dissociation rate of acetone on the S 1 surface is prompt, and the acetyl radical is long lived. The lifetime of acetone on the 3s surface is measured to be 3.2Ϯ0.4 ps ͑6.0Ϯ0.5 ps for d 6 -acetone͒. The dissociation rate of acetyl is approximately 1.7 ps ͑2.5 ps for d 3 -acetyl͒ from the curve fitting. This agrees well with the Rice-Ramsperger-Kassel-Marcus theory predicted lifetime of 1.0 ps ͑1.9 ps for d 3 -acetyl͒ when the internal excitation energy of the acetyl radical is treated by a statistical-adiabatic-impulsive model.

The study of the interaction of femtosecond laser radiation with matter, especially clusters, has blossomed in recent years due to advances in laser technology. One aspect of this interaction is Coulomb explosion. This effect occurs when the repulsive energy of like charges, known as Coulomb repulsion, overcomes the cluster’s total cohesive energy, causing the cluster to disintegrate into charged fragments. In this study, the interactions of methyl iodide clusters, formed in a supersonic expansion using argon and helium as carrier gases, were investigated at 795 nm using a Ti:Sapphire femtosecond laser. The resulting atomic and cluster ions were analyzed in a reflection time-of-flight mass spectrometer. The focus of these studies was the elucidation of the effects of carrier gas and laser wavelength on the laser-cluster interactions leading to Coulomb explosion. To achieve these goals, the effects of different carrier gases, laser power, cluster distribution, and the resulting Coulomb explosion energies were examined. A secondary consideration was to examine the experimental results with regard to the Coherent Electron Motion and Ionization Ignition models.

The photodissociation dynamics of methyl iodide clusters using λ=270 nm as pump and λ=405 nm as probe are studied using a femtosecond two color pump–probe laser arrangement combined with a reflectron time-of-flight (RTOF) mass spectrometer. This enables the à state and 10s Rydberg state of methyl iodide to be accessed with the pump beam. Of particular interest is a comparison of the femtosecond dynamics of the methyl iodide monomer with the clustered species. Clocking of the monomer dissociation shows a transient which is indicative of a fast C–I bond breakage as is to be expected upon excitation of methyl iodide into the fast dissociating à state, or into the predissociative 10s Rydberg state. Clusters, however, show a very different pump–probe transient composed of a fast decay and a subsequent dip in ion signal followed by a rise for pump–probe delay times greater than 2 ps. The cluster ion signal shows an enhancement for pump probe delay times up to 70 ps. The results are interpreted in terms of the electronic state diagram of the methyl iodide monomer and effects resulting from clustering of these species, shifts of electronic energy levels and caging of excited species in the cluster.

The unimolecular dissociation dynamics of selected alkene cluster ions (ethene, propene, 1-butene, trans-2butene, and cis-2-butene) and their fast fragment ions has been investigated using a reflectron time-of-flight mass spectrometer. These ions were prepared by the supersonic expansion of a premixed sample gas containing 20% alkene in Ar, followed by multiphotoionization (MPI) using a Ti:sapphire femtosecond laser. From the unimolecular decomposition patterns of all the ions investigated, we conclude that after photoionization a radical cation initiated intracluster polymerization takes place in these cluster ions. But, due to steric hindrance, it does not continue indefinitely as the cluster size increases. For ethene, pentamerization can be observed, and for propene, trimerization; while in the case of the three isomeric butenes, 1-butene is partially trimerized and only dimerization is observed in 2-butene.