Study and control of Rydberg-molecule interactions with surfaces and adsorbates

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When a molecule absorbs deep ultraviolet radiation it may exist transiently as a Rydberg state in which one of the electrons has been promoted into a high-energy orbit at a long distance from the core of the molecule. In such states, molecules have exotic properties because the excited Rydberg electron is very easily perturbed by external electric or magnetic fields and by interactions with other molecules. This proposal is concerned with the investigation of the novel chemistry and physics that occurs when these Rydberg molecules, created by laser excitation in the gas phase, bump into a solid surface. In broad terms, collisions between molecules in the gas phase and solid surfaces are of importance in a wide range of chemical processes ranging from catalysis, to corrosion, to electronic device technology to the formation of the ozone hole in the stratosphere. One type of gas phase environment, known as a plasma , is made up of a mixture of charged (ionized) species, highly reactive free radicals, and electronically-excited energy-rich species including Rydberg atoms and molecules. The process of plasma deposition is of major importance in the electronics industry and the interaction between the gas-phase plasma and solid surfaces lies at the heart of this process. The role of Rydberg atoms and molecules in plasmas is poorly understood. In the work proposed here, we aim to understand at a fundamental level the interaction between the metastable Rydberg molecules and well-defined surfaces. We will target a beam of molecules, which have been laser excited into Rydberg states, at a well defined and characterised surface and study the processes that occur. These will include: ionization of the impacting molecules by electron transfer to the surface; dissociation of the incoming molecules via chemical bond breaking; deposition of the energy of the Rydberg electron into the surface to break bonds and initiate chemical processes of adsorbates bonded to the surface. We will investigate whether it is possible to control the propensities for these processes using applied electric fields. The orbit of the Rydberg electron can be severely distorted by such fields and this affects the subsequent chemical behaviour and may have a profound effect on its interaction with the surface.