Bridging the Pressure Gap Between UHV Surface Chemistry and High Pressure Processes

Surface chemistry under high pressure of gaseous reactants is often different from surface chemistry at low pressure. For example, many surface chemical reactions proceed readily under high pressure conditions typical of a commercial, heterogeneous catalytic reaction but appear not to proceed at low pressures (<10-4 torr), despite favorable thermodynamics. The different chemistry and, in particular, the lack of reactivity at the low pressures where ultrahigh vacuum surface science techniques are operable is known loosely as the pressure gap and casts doubt on the relevance of UHV surface chemistry to high pressure processes such as catalysis, chemical vapor deposition and etching reactions. Much of our research has been and remains directed toward understanding the physical basis for this pressure gap. We have found four previously unrecognized phenomena responsible for or plausibly responsible for differences in surface chemistry at high and low pressure: translational activation, collision induced activation, collision induced desorption and collision induced absorption.