(e,gamma,2e) Threshold Spectroscopy - A new method to study collisional excitation of atoms using combined laser and electron beams

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Since the foundation of modern physics where Rutherford and collaborators discovered the structure of the atom in Manchester and Neils Bohr developed the first quantum theory, collision experiments between an incident particle and an atomic target have provided physicists with precise details on the nature of matter. Rutherford's experiments probed the nucleus and its size using alpha particles, leading to modern nuclear physics. The work of Franck and Hertz soon after used electron beams to probe the structure of the electrons surrounding the nucleus. This work lead to the development of modern quantum theory and atomic physics. Since that time our understanding of the atom and its structure (which has been learned through developing theories and more sophisticated experiments) has paved the way to the development of almost all technologies that we use today. Key to these successes and to the future development of new and emerging technologies is the close collaboration between experimentalists who measure the interactions to high precision and theoreticians who develop the quantum theories that describe these processes. By testing theory with experiment the models are refined and improved, allowing them to accurately predict what happens in many areas of modern science and industry. Processes where the models have direct application include the development of new lasers, in Tokomaks for the generation of fusion energy, in the earth's upper atmosphere (including the ozone layer and ionosphere) and in areas of astrophysical interest including for models of stellar atmospheres and in the atmosphere of exoplanets. Understanding excitation of atoms by electron impact hence plays an essential role in testing different models of the interaction. Results from experiment and theory have converged in recent years, with the models now agreeing well when compared to existing experimental data. The conventional experiments that are used are however limited due to the low efficiency of the detectors or due to limitations of the techniques that are adopted. They therefore cannot measure the excitation of a wide range of target states, including higher lying states that have long lifetimes and metastable states where the atom effectively 'stores' the energy imparted by the collision for a long time. Understanding excitation of these 'metastable' atoms hence is important in many different areas.The new technique that has been proposed here will allow these types of interactions to be studied for the first time. This will be carried out by using a very precise laser beam to further excite the atoms created by the collision to a highly excited Rydberg state that is very close to ionization. These 'Rydberg atoms' can be huge - we can make neutral atoms in our laboratory that have diameters around 1/10th that of a human hair. These enormous atoms can very easily be ionized by applying a small electric field that releases the electron from the atom, which we can then detect. By measuring the ensuing electron yield and by changing the polarization of the laser beam, we can then extract all information about the initial collision in a unique way and with high efficiency. The new experimental technique proposed here hence complements that of existing methods without suffering from the technological limitations that occur with them. The experiments will therefore provide new data to test the quantum models, allowing them to be further refined, enhanced and improved.