Low Temperature Rate Coefficients for Outer Planet Atmospheres

红外空间天文台最近的观测首次测量了土星和海王星大气中甲基自由基CH3的混合比例。CH3是由CH4光解产生的，是在巨大的行星和卫星上形成复杂有机分子的关键光化学中间体。然而，观测到的CH3红外发射比现有的光化学模型预测的要弱得多。一些模型师倾向于在140-190K的温度范围内使用CH3+CH3+M--C2H6+M的速率系数，该系数至少比296K时的值大10倍，296K是实验室测量的最低温度。对于这个反应和其他低温和压力下的反应，需要可靠的实验和理论压力相关的速率常数来模拟碳氢化合物、甲基、丁腈和CO的浓度，并得出大气传输速率。史密斯博士和他的同事们将把理论方法与实验室中选定的低温-低压复合速率测量相结合，以解决这一建模问题。理论计算将提供一致的速率参数来描述数据和势能面，并将覆盖比现有公式更宽的温度范围(30-3000K)，这些公式是为燃烧模拟目的而设计的，对低温不可靠。实验将在200K及更低的温度下进行，使用低压池中的激光光解、共振增强多光子电离(REMPI)技术来提供关键的缺失低温数据。将进行一项为期三年的研究计划，以改进行星大气模型中的低温碳氢化合物动力学参数，重点是(1)通过灵敏度分析检查现有光化学模型中的机制，以确定重要的不确定性，(2)CH3+CH3+M--C2 H6+M的依赖温度的速率的理论参数化和其他关键步骤，以及(3)用于输入参数的这些反应速率的低温和压力测量。必须提供这些速率的可靠数值，以便准确地模拟外行星的大气光化学，并适当解释新旧行星数据，包括得出传输速率。该奖项由行星天文学计划资助。*

AST 0074140SmithRecent observations from the Infrared Space Observatory have provided the firstmeasurements of the mixing ratio of the methyl radical, CH 3 , in the atmospheres of Saturn and Neptune. CH 3 is produced by photolysis of CH 4 and is the key photochemical intermediate leading to complex organic molecules on the giant planets and moons. However, the observed CH 3 infrared emissions are much weaker than predicted by current photochemical models. Several modelers favor using a rate coefficient for CH 3 + CH 3 +M-- C 2 H 6 + M for the temperature range 140-190 K that is at least 10 times larger than the value at 296 K, the lowest temperature at which laboratory measurements are available. Reliable experimental and theoretical pressure-dependent rate constants are required for this reaction and for others at low temperatures and pressures to model hydrocarbon, methyl, nitrile, and CO concentrations and to derive atmospheric transport rates. Dr. Smith and his colleagues will combine a theoretical approach with selectedlow temperature-low pressure recombination rate measurements in the laboratory to solve this modeling problem. The theoretical computations will provide consistent rate parameters to describe the data and potential energy surfaces and will cover a wider temperature range (30-3000K) than the current expressions, which were designed for combustion modeling purposes and extrapolate unreliably to low temperatures. Experiments will be conducted at 200K and lower, using a laser photolysis, resonance-enhanced multiphoton ionization (REMPI) technique in a low-pressure cell to provide key missing low-temperature data. A three-year program of research will be conducted to improve low-temperature hydrocarbon kinetics parameters in models of planetary atmospheres, emphasizing (1) examination of mechanisms in existing photochemical models with sensitivity analysis to identify important uncertainties, (2) theoretical parameterization of the temperature-dependent rates of CH 3 + CH 3 + M -- C 2 H 6 + M and other key steps, and (3) low temperature and pressure measurements of these reaction rates for input to the parameterizations. Reliable values for these rates must be made available to accurately model outer planet atmospheric photochemistry and to properly interpret old and new planetary data, including the derivation of transport rates. This award is funded through the Planetary Astronomy Program.***