Thermal accommodation coefficients by high speed vibration of solid samples. V. Translational and internal gas–solid energy exchange for alkanes and other molecular gases

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Abstract
Translational γtrans and internal γint energy accommodation coefficients of alkanes and other polyatomics on polycrystalline metal foils are determined using a high speed vibration method described previously. For the normal alkanes CnH2n+2(n=1–8) on Fe foil, γtrans increases with n from 0.55 for methane to 0.95 for n‐octane. γint also increases from 0.52 to 0.91. For the other gases studied on Al or Fe foils—Cl2, CO2, NH3, C3H6, neo‐C5H12, and C6H6—the values of γtrans are 0.94, 0.71, 0.87, 0.75, 0.87, and 0.66; the values of γint are 0.94, 0.54, 0.86, 0.63, 0.75, and 0.54. The Al and Fe foils are covered with layer(s) of oxygen and/or test gas, and presumably appear similar to a given gas. Approximate upper and lower limits for vibrational accommodation coefficients (γvib) can be estimated using the assumption that γint=γrot =γvib, or γtrans=γrot. Comparison is made between vibrational accommodation coefficients estimated in this way and available literature data. Gas–surface energy exchange is examined from the viewpoint of trapping—emphasizing its dominant role in energy accommodation under the conditions of these measurements. It is inferred that γrot is very close to γtrans, but that γvib is less than γtrans, for most of the gases studied. However, γvib approaches γtrans for the heaviest hydrocarbons, C7H14 and C8H16, which have very long surface residence times. For CH4 the value of γint, but not of γtrans, appears exceptional.