Crossed beam scattering with H-Rydberg Tagging technique

Rydberg H-Atom Photofragment Translational Spectroscopy
    The Rydberg H-atom translational spectroscopy technique was developed in the early 1990s by Welge and coworkers.(J.Chem.Phys.1990,92,7027).This technique has provided us with an extremely powerful tool for measuring the state-resolved differential cross-sections for both unimolecular and bimolecular reactions with unprecedented translational energy resolution and extremely high sensitivity. This technique has been applied successfully to studies of the important benchmark reactions H+D2 HD+H, O(1D)+H2 OH+H recently and many important unimolecular dissociation processes in our laboratory. These state-of the-art experimental studies, coupled with the recent advances in theoretical state-to-state dynamics studies, can now provide an in-depth physical understanding of elementary chemical reactions that could not be imagined before.

Advantages of Rydberg Tagging
    Many techniques used for studying photodissociation processes rely on the detection of ions which are produced either directly, or by ionisation of neutral photofragments in the volume where photodissociation occurs (the interaction region). The formation of ions leads to a high detection efficiency, as the use of correctly tuned electric fields (ion optics) may be used to accelerate them toward a detector. However, the formation of many charged species in the small focal volume of the interaction region of the experiment (in this case the focus of a laser beam) can lead to blurring of the resulting data due to Coulomb repulsion forces. In order to circumvent this problem, the photofragments can be initially excited into certain high energy electronic (Rydberg) neutral states that have long (ms) lifetimes and are just below (in energy) an Ionisation Potential. These "Rydberg tagged", but still neutral photofragments can then move with unperturbed velocities to the ion detector where a small electric field is present and (only) ionises the tagged photofragments immediately prior to detection.

Systems we study
    With this powerful technique, we have studied:
         *Dynamical resonances in F+H2;
         *Nonadiabatic effect in F+D2;
         *Nonadiabatic effect in Cl+H2.
    Our experimental results agree very well with the full quantum dynamics calculation based on the highly accurate potential energy surfaces(PES). These studies confirm the fundamental understanding of how elementary steps of chemical reactions occurs.

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