L. Poth

Arresting intermediate states in a chemical reaction on a femtosecond time scale: proton transfer in model base pairs

Abstract A new concept to study the dynamics of chemical reactions in real time is presented using the phenomenon of Coulomb explosion to arrest and directly interrogate the intermediates in a chemical reaction. This technique is employed to study the tautomerization reaction of the model DNA base pair 7-azaindole, providing direct mass spectral evidence of a stepwise versus a concerted mechanism for the proton transfer upon excitation into the S1 state.

Femtosecond laser interactions with methyl iodide clusters. I. Coulomb explosion at 795 nm

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 w...

Ultrafast dissociation dynamics of acetone: A revisit to the S1 state and 3s Rydberg state

Because of the dispute in the literature over the dissociation rate and energy partitioning of the acetone molecule upon photoexcitation to the S1 state (π*←n) and 3s Rydberg state (3s←n), we have remeasured the lifetime of acetone (also d6-acetone) on the S1 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 S1 surface is prompt, and the acetyl radical is long lived. The lifetime of acetone on the 3s surface is measured to be 3.2±0.4 ps (6.0±0.5 ps for d6-acetone). The dissociation rate of acetyl is approximately 1.7 ps (2.5 ps for d3-acetyl) from the curve fitting. This agrees well with the Rice–Ramsperger–Kassel–Marcus theory predicted lifetime of 1.0 ps (1.9 ps for d3-acetyl) when the internal excitation energy of the acetyl radical is treated by a statistical-adiabatic-impulsive model.

FEMTOSECOND LASER INTERACTIONS WITH METHYL IODIDE CLUSTERS. 2. COULOMB EXPLOSION AT 397 NM

Abstract As a continuing effort to elucidate the effects of the interaction of femtosecond laser radiation with clusters, we have extended our studies of Coulomb explosion to a determination of the role of laser wavelength on the process. In the present study, the interactions of methyl iodide clusters, formed in a supersonic expansion using argon and helium as carrier gases, were investigated at 397 nm using a Ti:sapphire femtosecond laser. These studies are a continuation of the work initiated on methyl iodide clusters using 795 nm ionization. The resulting atomic and cluster ions were analyzed in a reflectron time-of-flight (TOF) mass spectrometer. Based on a comparison of the current results and those presented for 795 nm ionization, several suggested mechanisms to account for the formation of highly charged species and their concomitant Coulomb explosion are examined. The resulting analysis indicates that the Coulomb explosion of methyl iodide clusters has characteristics of several of the proposed models to account for the phenomenon, but cannot be fully explained by any one of them.

Femtosecond dissociation dynamics of methyl iodide clusters

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 A 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 A 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 e...

Study of the reaction of NO clusters with water using a pickup technique

Abstract Mixed NO/H 2 O-clusters are produced by merging a beam of pure NO-clusters, formed by supersonic expansion in a pulsed nozzle, with water vapor in a pickup source. After ionization with 400 nm light from a femtosecond laser system, a mixed NO/H 2 O-cluster series of pure protonated water clusters H + (H 2 O) n (with n up to 20) is observed which shows an obvious intensity maximum for cluster size n = 4. Other experimental findings are reported herein on the metastable decay of ionized mixed NO/H 2 O-clusters. Taken together, the results establish that reactions occurring at degrees of hydration corresponding to n ⩾ 4 lead to the production of the protonated hydrates and the concomitant formation of HNO 2 . In this study special attention is paid to NO + (H 2 O) n ions because these ions are considered to be an important precursor for the production of proton hydrates in the ionosphere.

Charge stripping effects from highly charged iodine ions formed from Coulomb explosion of CH3I clusters

Abstract Iodine ions of high charge states are observed upon irradiation of methyl iodide clusters with an intense femtosecond laser pulse. All signals from multicharged ions exhibit a peak splitting in the time-of-flight mass spectra, indicating their origin from a Coulomb explosion process. These main peaks are accompanied by smaller peaks attributed to field ionization of highly charged species in the ion optics of the TOF mass spectrometer. It is shown that highly charged atomic ions formed from Coulomb explosion, upon interaction with electric field close to the mesh, can lose another electron leading to the formation of even higher charged species. The observation of this charge stripping process is evidence for the formation of highly excited ions in the course of the Coulomb explosion process, providing new insights into the mechanisms of femtosecond ionization involving multi-electron loss.

Dissociation dynamics of the HCl dimer ion

Abstract The dissociation pathways of (HCl) 2 + are studied by a molecular beam-femtosecond multiphoton ionization technique, and via ab initio calculations at the MP2/6-31G(d,p) level and using Gaussian-2 theory. Results indicate, besides the commonly accepted proton-transferred HClH + Cl structure, a hydrogen-peroxide-like structure, (HCl–ClH) + , also exists and appears to be more stable. Their interconversion involves a barrier of 0.24 eV. A new dissociation product, Cl 2 + , is observed and shown to come from an intracluster reaction of the HCl dimer cation. Dissociations of the (HCl) 2 + into H 2 Cl + and HCl + channels are preferred at low internal excitations, whereas dissociations into Cl 2 + and HCl 2 + require higher excitation energies.

The Metastable Decay of Alkene Clusters after Photoionization

Metastable fragmentation studies of alkene clusters, ionized with a femtosecond laser system following a supersonic expansion, have been performed. The investigation shows an interesting switching of decay channels from the loss of a molecular fragment to loss of a monomer unit. Taken together, the results indicate that, after ionization, a fast intracluster oligomerization takes place, which does not propagate further than pentamerization for ethene and trimerization for propene clusters, 1‐butene and 2‐butene exhibit oligomerization only up to the dimer. Larger cluster ions of all investigated alkenes appear to contain an oligomerized core solvated by monomer units.