Gas-Surface energy transfer

Jackson B 1994 Quantum and semiclassical calculations of gas surface energy transfer and sticking Comput. Rhys. Commun. 80 119... 

Classical and Statistical Theories of Gas-Surface Energy Transfer... 

In order to establish certain general features of gas-surface energy transfer it is helpful to consider a simple driven-oscillator model of the collision [Ref.3.1, Chap.10] and [3.6-8]. Although ultimately too simplistic (for example, the model is restricted to col linear collisions), this model provides a qualitatively useful picture of energy transfer and thermal accommodation. The model is shown schematically in Fig.3.1. A gas atom of mass m and energy E is incident upon a surface... 

In order to understand the dynamics of gas-surface interaction, it is necessary to determine how much energy is exchanged between the gas and surface atoms through the various energy-transfer channels. In addition the kinetic parameters (rate constants, activation energies, and preexponential factors) for each elementary surface step of adsorption, diffusion, and desorption are required in order to obtain a complete description of the gas-surface energy transfer process. 

Rettner C, Michelsen HA, Auerbach DJ (1993) Determination of quantum-state-specific gas-surface energy transfer and adsorption probabilities as a function of kinetic energy. Chem Phys 175 157... 

Cushing GW, Navin JK, Donald SB, et al C-H bond activation oflight alkanes on Pt(lll) dissociative sticking coefficients, Evans-Polanyi relation, and gas-surface energy transfer, JPhys Chem C 114(40) 17222-17232, 2010. 

For example, energy transfer in molecule-surface collisions is best studied in nom-eactive systems, such as the scattering and trapping of rare-gas atoms or simple molecules at metal surfaces. We follow a similar approach below, discussing the dynamics of the different elementary processes separately. The surface must also be simplified compared to technologically relevant systems. To develop a detailed understanding, we must know exactly what the surface looks like and of what it is composed. This requires the use of surface science tools (section B 1.19-26) to prepare very well-characterized, atomically clean and ordered substrates on which reactions can be studied under ultrahigh vacuum conditions. The most accurate and specific experiments also employ molecular beam teclmiques, discussed in section B2.3. 

Harris J 1991 Mechanicai energy transfer in particie-surface coiiisions Dynamics of Gas-Surface Interactions ed C T Rettner and M N R Ashfoid (London Royai Society of Chemistry) p 1... 

Figure 25 Energy transfer between two surfaces via a gas at low pressure (L

As the pressure increases from low values, the pressure-dependent term in the denominator of Eq. (101) becomes significant, and the heat transfer is reduced from what is predicted from the free molecular flow heat transfer equation. Physically, this reduction in heat flow is a result of gas-gas collisions interfering with direct energy transfer between the gas molecules and the surfaces. If we use the heat conductivity parameters for water vapor and assume that the energy accommodation coefficient is unity, (aA0/X)dP — 150 I d cm- Thus, at a typical pressure for freeze drying of 0.1 torr, this term is unity at d 0.7 mm. Thus, gas-gas collisions reduce free molecular flow heat transfer by at least a factor of 2 for surfaces separated by less than 1 mm. Most heat transfer processes in freeze drying involve separation distances of at least a few tenths of a millimeter, so transition flow heat transfer is the most important mode of heat transfer through the gas. 

However, it is not known into which part of the potential energy surface these species couple. The reactions of H2CO+ with OH and CH2OH+ with O-atoms would also access the surface although these are not experimentally very tractable. The surface is also accessed to a limited extent by the gas kinetic proton transfer from HCO+ to H20 yielding H30+. 

One of the main differences between a free and an adsorbed CO molecule is that whereas the vibrational energy of the gas phase molecule can only be transferred into a photon, giving the excitation a lifetime of 30 ms, the adsorbed molecule is able to dissipate the vibrational energy into the substrate, giving the mode a lifetime in the picosecond range. This is extremely important for the energy transfer in most dynamical processes at surfaces, as in... 

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