J.W. Gadzuk

Objective: To provide useful theoretical models with predictive power that describe the quantum dynamics of driven atomic systems within the natural vibrational timescale of laser-cooled and trapped atoms and/or chemically bound adsorbed molecules.

Problem: Real time observation and control of transient quantum motion in chemical systems is the holy grail whose pursuit drives a large component of the chemical physics world in all sectors (academic and government) of the research community. Theoretical modeling of the fundamental atomic and molecular processes occuring in a controlled chemical reaction, particularly at solid surfaces, is the area of concern here.

Approach: This work has been inspired by a number of ideas involving some gedanken experiments which were sequentially conceived in a progression moving from femtochemistry to breathing-mode-excitation in resonant neutron-capture-processes to a special realization of electron stimulated desorption, all in confluence with some beautiful real-world experiments from the Phillips’ group at NIST involving ultra-cold atoms trapped in optical lattices. These examples have as a common theme the excitation of quantum mechanically localized atoms by some form of transient wave packet squeezing and spreading. The observable consequences of such compression include bound vibrational oscillations or free translational motion associated with bond breaking, as manifest by delocalization/desorption. Analytic wave packet theory for these (up-to-now-unrelated) processes has been devised which constructively exploits the temporal scaleability of the near-harmonic systems under investigation, thus rendering irrelevant the fact that on an absolute time scale the vibrational period of the quantum oscillator in the optical lattice and in the adsorbed state differ by ten or more orders of magnitude. The motivation for focusing on this parallelism is to use recently obtained ultra-cold atom experimental results as a confirmation of the basic theory which, now verified, can be temporally scaled down to the domain of surface femtochemistry and used there with confidence.
 

Results and Future Plans: Striking experimental observations of oscillatory wave packet spreading and compression (i.e., breathing mode excitation) were made using time-resolved Bragg scattering from Cs atoms in the partially occuppied optical lattice, upon suddenly switching the oscillator force constant, hence frequency. (A similar change in frequency would occur in the gedanken neutron capture process due to the sudden switch of oscillator mass!) Our theoretical wave packet results such as the mean-square wave packet width as a function of time, shown in Fig. 1, reproduce well, without adjustable parameters, most aspects of the experimental results such as the observed oscillatory amplitude, the observed anharmonic dephasing decay, and the correct revival period. Gaining this independent confirmation of the model, the theory can now be applied to the original chemical physics problems which initiated this inquiry and that is what is planned for the near future. This work is described in full detail in a commissioned Topical Review article entitled Breathing Mode Excitation in Near Harmonic Systems: Simple Analytic Theory for Resonant Mass Capture, Desorption, and Atoms in Optical Lattices, published in J. Phys. B 31, 4061 (1998)

Figure 1. Normalized width of breathing mode wave packet (=> excited Cs atom trapped at optical lattice site) as a function of time after excitation (multiplied by the lattice atom oscillator frequency).

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