Penn State Castleman Group

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Updated August 14, 2012


Velocity Map Imaging Photoelectron Spectroscopy


The molecular beam apparatus known as "Lola" in the Castleman group is a time-of-flight (TOF) mass spectrometer coupled to a photoelectron imaging array for studying the electronic properties of molecules and clusters. Clusters are created in a laser vaporization source, where a cylindrical metal rod is ablated by the second harmonic (532 nm) of a Nd3+:YAG pulsed laser forming a highly energetic plasma of electrons and ionized species. Soon after the laser impinges upon the rod, a pulsed jet of reactant gas (CH4, O2, NH3, etc.), seeded in He or Ar, discharges across the plasma region forming clusters of various sizes and charge states. The emergent clusters, after undergoing a supersonic expansion into vacuum, are skimmed through an orifice forming a molecular beam which passes through a three stage electrostatic grid assembly. Clusters anions are detected in the experiment by employing negatively pulsed electrostatic TOF grids.

LOLA Diagram

The clusters anions which are accelerated by the TOF grids are then intersected by a linearly polarized laser pulse (Nd3+:YAG). If the photon energy of the laser pulse is high enough, electrons will be photodetached from the cluster anion. The photoelectron imaging technique based on the principles of Velocity Map Imaging (VMI) is employed to essentially take digitized pictures, via a charged coupled device camera, of expanding electrons from this photodetachment event. These electron snapshots can then be converted to photoelectron spectra from which electron affinities, HOMO-LUMO gaps, and vibrational separations (useful for structural analysis) can be measured. Moreover, VMI can also simultaneously measure the angular distribution of the detached electrons, whereby a quantity named the anisotropy parameter can be calculated. This anisotropy parameter is useful for deducing the nature of the atomic or molecular orbital the ejected electron came from.

Photoelectron Spectrum
Photoelectron Angular Distribution