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Department
of Chemistry (m/c 111)
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The focus of our research is to understand the quantum mechanical nature of molecular interactions, and to use this knowledge to control the outcome of chemical reactions. We use laser beams to excite molecules to specific quantum states and to probe their reaction products, using the tools of multiphoton ionization, time-of-flight mass spectroscopy, Doppler spectroscopy, laser-induced fluorescence, and photofragment imaging. Emphasis is placed on relating spectroscopy and dynamics. This research program inspires, and is energized by teaching a variety of courses in physical chemistry, quantum mechanics, spectroscopy, and reaction dynamics, as well as group seminars.
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We employ the principle of quantum mechanical interference to control the rates and branching ratios of chemical reactions. In a typical experiment we excite hydrogen iodide molecules with three ultraviolet photons of frequency w3 and one vacuum ultraviolet photon of frequency w3 = 3 w1. These two paths interfere with each other, and by varying the relative phases of the two light sources we can produce constructive or destructive interference between them. A key finding is that it is possible to control the branching ratio of a reaction (in this case ionization vs. dissociation) because the phase dependence of the products varies for different reaction channels. The phase lag between different product channels is a new observable that provides new information about the continuum properties of a molecule. Very recently we used phase lag spectroscopy to measure the phase of an eigenfunction.
A theoretical understanding of the channel phases dedeuced from these experiments has been developed in collaboration with Tamar Seideman.
This work is supported by the National Science Foundation.
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Molecular optics is an emerging field which uses light to manipulate the motion of molecules. Taking advantage of the polarizability of matter, one can use a focused laser beam to align, focus, and steer the motion of molecules in space. A variety of experiments are in progress in our group. In one of these we use a focused laser beam to align a beam of molecules along the electric field direction of the laser. We are currently setting up an experiment to create pendular states with an ultrafast laser. In another experiment we are studying the behavior Rydberg molecules in intense laser fields.
This work is supported by the Department of Energy and a Manufacturing Resources Center grant sponsored by Motorola..
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We have studied the effect of spectroscopic perturbations on dynamical processes. For example, we have shown how rotational perturbations of Rydberg states of HCl alter the spin-orbit population and angular anisotropy of Cl atoms produced by predissociation of the molecule.
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We use pump-and-probe techniques to study nonadiabtic interactions in complex molecules such as vinyl chloride, where competing dissociation pathways occur on multiple potential energy surfaces. Very recently we have observed the effects of dissociative ioinization in iodine, and have discovered an "hourglass" effect in iodobenzene and methyl iodide.
Revised Jan. 30, 2004
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