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Energy transfer and quantum effects on rate processes

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One of the holy grails of science is to work out how fast atoms and molecules “change” what they do – react to make new compounds, adsorb/desorb from a surface or diffuse from the proverbial A to B. The theoretical ground work was laid in the 1930’s (transition state theory) and 1940’s (work of Kramer’s on the effect of energy transfer rates) and has since generated a mountain of papers – yet there are very few experiments that can measure rates accurately enough to evaluate these theories as you need to extrapolate the data from normal time scales (seconds) to timescales 12 orders of magnitude smaller. The helium spin echo (HeSE) technique however makes possible measurements on the ideal, fast timescales, and so the data it yields can be used almost uniquely to test these theories and in this talk I will demonstrate that the current almost total reliance on transitions state theory can lead to very significant errors in the prediction or rates, particularly for multistage processes and show HeSE can be used to quantify the rates of energy transfer that have a controlling influence on the motion. The technique also allows processes where quantum mechanical effects dominate “tunnelling” (particles go where classical mechanics would forbid them) and “effective masses” (particles move as though they were much heavier than they are), and, after a crash course in quantum mechanics, I will present a simple scheme for including these effects into rate theory, using the “band structure” of the particle states on a surface. It is widely expected that hydrogen, the lightest atom will show these effects, but I will present data that shows that light molecules such as methane and carbon monoxide can also show the (high) “effective mass” effect that will significantly affect the rate of diffusion.

This talk is part of the Caius MCR/SCR research talks series.

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