氨雪线和铵盐解吸
Ammonia snow-lines and ammonium salts desorption
摘要：
上下文行星系中的氮气储存是一个长期存在的问题。部分含氮分子可能在恒星演化的冷阶段被结合到冰体中，当冰被加热时，如在活跃的彗星中，可能逐渐释放到气相中。氮储库的化学性质应该会极大地影响氮如何、何时以及以何种形式返回气相，或者被纳入形成行星体的耐火材料中。目的。本文研究了两种铵盐：甲酸铵和乙酸铵从金表面和水冰基底上的热脱附。方法.采用程序升温脱附实验和傅里叶变换红外反射光谱研究了铵盐的脱附行为。结果铵盐是半挥发性物质，在高于水冰的热解吸温度的温度下，在解吸时释放中性物质作为主要组分，即氨和相应的有机酸（HCOOH和CH 3COOH）。它们的脱附遵循一阶Wigner-Polanyi定律。对甲酸铵和乙酸铵的一级动力学参数分别为A = 7.7 $\pm$ 0.6 $\times$ 10 $^{15}$ s $^{-1}$和E $_{bind}$ = 68.9 $\pm$ 0.1 kJ~mol $^{-1}$和A = 3.0 $\pm$ 0.4 $\times$ 10 $^{20}$ s $^{-1}$和E $_{bind}$ = 83.0 $\pm$ 0.2 kJ~mol $^{-1}$。水冰基质的存在不影响解吸动力学。锁定在盐中的氨分子在比以前预期的高得多的温度下作为中性分子解吸，这通常归因于耐火材料。结论.氨雪线的半径小于水雪线的半径。因此，太阳系天体中的NH3/H2O比率含量可以作为它们形成和随后迁移的暗示。
英文摘要：
Context. The nitrogen reservoir in planetary systems is a long standing problem. Part of the N-bearing molecules is probably incorporated into the ice bulk during the cold phases of the stellar evolution, and may be gradually released into the gas phase when the ice is heated, such as in active comets. The chemical nature of the N-reservoir should greatly influence how, when and in what form N returns to the gas phase, or is incorporated into the refractory material forming planetary bodies. Aims. We present the study the thermal desorption of two ammonium salts: ammonium formate and ammonium acetate from a gold surface and from a water ice substrate. Methods. Temperature-programmed desorption experiments and Fourier transform infrared reflection spectroscopy were conducted to investigate the desorption behavior of ammonium salts. Results. Ammonium salts are semi-volatile species releasing neutral species as major components upon desorption, that is ammonia and the corresponding organic acid (HCOOH and CH3COOH), at temperatures higher than the temperature of thermal desorption of water ice. Their desorption follows a first-order Wigner-Polanyi law. We find the first order kinetic parameters A = 7.7 $\pm$ 0.6 $\times$ 10$^{15}$ s$^{-1}$ and E$_{bind}$ = 68.9 $\pm$ 0.1 kJ~mol$^{-1}$ for ammonium formate and A = 3.0 $\pm$ 0.4 $\times$ 10$^{20}$ s$^{-1}$ and E$_{bind}$ = 83.0 $\pm$ 0.2 kJ~mol$^{-1}$ for ammonium acetate. The presence of a water ice substrate does not influence the desorption kinetics. Ammonia molecules locked in salts desorb as neutral molecules at temperatures much higher than previously expected that are usually attributed to refractory materials. Conclusions. Ammonia snow-line has a smaller radius than the water snow-line. As a result, the NH3/H2O ratio content in solar system bodies can be a hint as to where they formed and subsequently migrated.
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