Chemical Physics

Carl Lineberger has spent many years "shedding light" on negative ions. He and his colleagues were the first investigators to use laser techniques to remove electrons from atomic or molecular anions in the gas phase. Over the years, they learned about thresholds for electron detachment and the binding energies that hold electrons inside chemical structures. Lineberger's work laid the foundation for an entire field of chemistry, in which researchers take advantage of the unique properties of negative ions to probe chemical reaction dynamics, molecular structure, forces that hold molecules together, and photodissociation reactions. His research has also revolutionized the spectroscopic study of gas-phase anions. He and his colleagues have determined precise values for the energy of electron attachment for most atoms and many open-shell molecules. These values are found in chemistry textbooks throughout the world.

For the past few years, Lineberger's work has focused on using ultrafast lasers to (1) investigate the dynamics of anions undergoing photodetachment photoionization, (2) elucidate the structure of transient intermediates in chemical reactions, and (3) investigate reaction dynamics in size-selected clusters. Robert Parson works closely with the Lineberger group to model the theoretical dynamics of the ions, molecules, and clusters under experimental study.

In photodetachment-photoionization studies, Lineberger's group combines ab initio electronic structure calculations with experiments that probe the short-time dynamics of anions such as Cu^-(H₂O). In this method, anions are formed by intersecting a pulsed gas jet with an electron beam. A high-voltage pulse extracts the anions from the gas jet, and the mass-selected ion beam is then crossed by 100-300 femtosecond pump-and-probe laser beams. Pump laser photons detach electrons from the anions, forming neutral ions. The researchers then monitor neutral ion dynamics with a process that further ionizes the molecules, creating positive ions that are also studied. Information about the neutral and ionic Cu(H₂O) complexes are expected to guide future investigations, improve models of the solvation of metal atoms and ions, and further the understanding of the role of copper in biological systems.

Lineberger's group is evaluating a series of ringed molecules, beginning with a ring of five carbons (cyclopentadiene). The researchers are systematically substituting a nitrogen atom for a carbon atom in the ring. By substituting additional nitrogen atoms into the ring in a stepwise fashion, they hope eventually to study a 5-member ring consisting of all nitrogen atoms, pentazole. The latter structure's properties are so difficult to measure that no one has ever succeeded in doing it. So far, Lineberger's group has analyzed pyrrole and imidazole, which contain one and two nitrogen atoms, respectively.

A two-step process is used to investigate the ringed molecules. First, the researchers use hydroxide to remove protons from the nitrogen atoms, leaving the molecules as negative ions. Then, they bombard the negative ions with a laser photon beam, removing the extra electrons from the ions and returning the molecules to a neutral-charge state. The experimenters collect the photodetached electrons and measure their energy distribution. Thus far, it appears that as you add more nitrogen atoms to the ring, it gets easier to remove a proton from one of them in the first step, but harder to remove the extra electron later. Work on the project is continuing with triazole, which contains three nitrogen atoms in its ring.

The second Lineberger project uses photoelectron spectroscopy (PES) to investigate negative ions such as AuO^- and AuS^-. In PES, a laser is used to detach extra electrons from negative ions in a beam. A hemispherical energy analyzer then measures the kinetic energies of the detached electrons, and spectra are created of photoelectron counts versus kinetic energy. The spectra provide measurements of the adiabatic electron affinity. Because Lineberger developed PES, most of the entries in the electron affinity tables in the CRC Handbook of Chemistry and Physics come from measurements made in his laboratory. PES also yields information on vibrations, electronic state excitations, and structural differences between negative ions and their neutrals in many species.

Cluster ions are the focus of Lineberger's third project. Lineberger and his group use a cluster ion machine consisting of a cluster ion source and a tandem time-of-flight spectrometer together with a picosecond mode-locked laser system to probe the energy structures of ion clusters such as ICl^-(CO₂)_n and IBr^- (CO₂)_n. The researchers photo excite the ion clusters at two different wavelengths with a laser and investigate the photoproducts that are produced. These studies not only reveal important information about the electronic transitions during the photodissociation process, but also shed light on the role of the solvent in driving particular electronic processes such as spin-orbit relaxation and charge transfer. The cluster studies also provide information about the environment-induced recombination, or "caging" of photodissociated diatomic molecules. Comparative studies of the photodissociation dynamics of different dihalide solutes in the same gas-phase solvent are helping researchers understand the step-by-step process and dynamics of solvation.