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Updated August 14, 2012


Femtosecond Magnetic Bottle and Velocity Map Imaging Photoelectron Spectroscopy
Dr. Shi-Bo Cheng


Our research utilizes two concurrent experimental techniques of photoelectron spectroscopy (PES) for exploring the electronic structure of gas phase negative ion clusters: Magnetic Bottle and Velocity Map Imaging PES. We can quantitatively determine electron affinities, additional electronic transition energies, vertical detachment energies, and in some cases HOMO-LUMO gaps. Coupled with high-level theory, the full electronic structures and geometries of clusters can be determined. With this information, clusters that we discover to be particularly stable in the gas phase may be used to build cluster-assembled materials in the future. Additionally, we are attempting to establish rules for tuning cluster properties based on atomic composition. Species we are particularly interested in studying include metallocarbohedrenes (Met-Cars, M8C12), semiconductor clusters (III-IV, III-V, IV-V species), and clusters that fit into aromaticity (Al3Bi), jellium model (Al3-), or zintl (Bi3Ga2-) concepts.


Our home-built instrument (above) has been designed to study metal, mixed-metal, nonmetal, and metal-nonmetal clusters with high sensitivity. A schematic of our experimental setup is shown below:

IRIS Diagram

Our supersonic expansion laser vaporization (LaVa) source creates a large distribution of cluster sizes for study. A tightly-focused Nd:YAG laser, pulsed at 10 Hz, vaporizes a metal or mixed-metal target (a rotating and translating rod) while being subjected to a pulse of gas. Anionic clusters are extracted using a high voltage pulse as part of a Wiley-McLaren time-of-flight mass spectrometer (TOFMS). The clusters can be mass-gated and decelerated on their way towards either the magnetic bottle or velocity map imaging assembly, where they are interrogated with the 2nd, 3rd, or 4th harmonic of a Nd:YAG laser. Both magnetic bottle and velocity map imaging techniques provide a photoelectron energy spectrum of the cluster of interest. Additionally, our recently added velocity map imager provides the angular distribution of detached electrons, whose anisotropy is a finger-print to the molecular orbital from which photodetachment occurred. Below, we show an example of raw (left) and reconstructed (right) images where each ring corresponds to a different energy cloud of electrons, representing different electronic transition energies.

Photoelectron Angular Distribution 1 Photoelectron Angular Distribution 1