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University of Virginia
Department of Chemistry
409 McCormick Road
P.O. Box 400319
Charlottesville, VA 22904-4319
Phone: (434)924-3639/3654
Fax: (434)924-3966

email : Prof. Ian Harrison

 

  Nanosecond Photochemistry Lab.
Graduate Students: Indraneel Samanta
 

 

 

 

 

Research Overview

A long standing interest at the Harrison Research Group, has been to exploit adsorbate photochemistry and investigate the dynamics of catalytic reactions on metal single crystals. Ideally, photofragmentation induced within an ordered submonolayer of coadsorbates can lead to reactive attack of the photofragments on neighboring coadsorbed molecules. The range of impact parameters and relative orientations of attack are constrained by the initial bonding of the coadsorbates to the surface. The dynamics of such "surface aligned photo-reactions can be followed by Time of Flight (TOF) techniques if the photo-products leave the surface promptly and the reactions are initiated with pulsed lasers. Low temperature and UHV conditions,helps us to characterize the initial state of the adsorbates using surface analytical techniques. Photoreaction is initiated using a tunable laser.

The translational and internal energy distributions of the photofragments involved in the reactive collisions can be varied, while the collisional stereochemistry may be modulated by varying the coverage of the coadsorbed precursors. The rapid quenching of electronically excited adsorbate states on metals ( t < 10 fs) implies that bimolecular reactions involving photofragments are likely to proceed. For the most part, along the ground state potentials relevant to catalysis. In consequence, studies of photo-chemical model systems may provide insight to the mechanisms and dynamics of related catalytic reactions that are normally thermally driven.

On metals, most adsorbate photochemistry is believed to proceed indirectly through an electron transfer mechanism in which a photoexcited substrate electron is transferred to the adsorbate. The resulting transient negative ion may go on to dissociate (e.g. O2 & CH3Br) or desorb (e.g. NO, N2). The image potential produced by polarization of the substrate may accelerate such transient ions towards the metal such that a high energy collision with the surface may ensue involving either the ionic or neutral species, dependent on the reneutralization rate. Current experiments are exploring whether photoinduced electron harpooning of an adsorbate followed by Å-scale molecular acceleration towards the metal can lead to a surface collision that is sufficiently energetic to induce dissociation.

 

Experimental Equipment :

Mass Spectrometer (TPD, RGA, & TOF)

10 ^ -11 Torr Ambient Pressure

Auger Electron Spectroscopy (AES)

X-ray Photoelectron Spectroscopy (XPS)

Low Energy Electron Diffraction (LEED)

Alkali Metal Dosing

20 1250 K Temperature Range

Reflection-Absorption Infrared Spectroscopy (RAIRS)

Side or Top Laser Irradiation Geometry

ArF (193 nm) Excimer Laser

Nd-YAG Laser (1064, 532, 355, & 266 nm)

Tunable Dye Laser

 

This material is based upon work supported by the National Science Foundation, the Department of Energy, and the American Chemical Society's Petroleum Research Foundation.
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF, DOE, or ACS PRF.
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