The reaction of atomic hydrogen with germane: Temperature dependence of the rate constant and implications for germane photochemistry in the atmospheres of Jupiter and Saturn

Germane (GeH4) has been suggested to be a chemical tracer of atmospheric dynamics on Jupiter and Saturn. The observed abundance of germanium as GeH4 in the atmosphere of Saturn is dramatically low compared to the predicted GeH4 abundance. In order to provide data to help determine the major chemical form in which germanium exists in these planetary atmospheres, studies of the formation and loss processes for GeH4 are required. One of the important reactions to consider is that of hydrogen atom reaction with germane; H + GeH4. Two previous measurements of the rate constant for this reaction, made only at 298 K, yielded values differing by a factor of 200. We have undertaken a study to determine the absolute rate constant for this reaction for the first time as a function of temperature and pressure. The method employed to measure the rate of this reaction is that of flash photolysis of dilute mixtures of GeH4 in argon, combined with time-resolved detection of H atoms via Lyman α resonance fluorescence. Results of this study show the reaction to be moderately rapid, independent of total pressure, but possessing a positive temperature dependence. Rate constant data over the temperature range 210 ≤ T ≤ 450 K yield the Arrhenius expression k1 = (1.94 ± 0.39) × 10−10 exp [(−1130 ± 50)/T] cm3 s−1. Implications for modeling the germane photochemistry in the atmospheres of Jupiter and Saturn derived from the present temperature dependent kinetic study are (1) low-temperature reaction rate data relevant for these atmospheric conditions are provided, and (2) limits may be placed on the role of GeH4 production and loss processes in the modeler's efforts to elucidate the respective chemical forms and abundances in which the total germanium exists in the atmospheric systems of Jupiter and Saturn.