AI 中文总结
本研究通过实验室脱附实验证明，甲酸甲酯在甲醇冰上的结合能低于水冰，解释了其在低温下提前释放，支持两步热脱附机制。
AI 中文摘要
复杂有机分子在恒星形成区被广泛探测到，是生命前体物种的重要前驱体。由于这些分子被认为主要形成于冰晶粒包层内部，其脱附被认为受周围H$_2$O冰基质升华的调控。然而，观测日益揭示出甲酸甲酯（MF）在远低于水冰升华温度（T~100K）的气相丰度，这对上述观点提出了挑战。为研究冰成分和相态如何影响MF脱附，我们展示了MF在四种天体物理相关基底上的实验室程序升温脱附测量：非晶和晶态H$_2$O及CH$_3$OH。脱附动力学使用基于过渡态理论的前指数因子结合前沿分析和非负最小二乘反演来恢复结合能分布。我们发现MF与CH$_3$OH冰的结合比与富H$_2$O基底更弱，代表性结合能约为非晶H$_2$O的~6247K、晶态H$_2$O的~6213K和非晶CH$_3$OH的~5469K。沉积在晶态CH$_3$OH上的MF表现出与覆盖度无关的前沿，与多层或岛状脱附一致，产生有效结合能5333K。甲醇冰上增强的表面迁移率和降低的捕获效率，加上较弱的MF结合，为恒星形成环境中观测到的气相MF早期出现提供了自然解释，其特征脱附温度在CH$_3$OH上约为~85K，在H$_2$O上约为~95-100K。这些效应共同支持一个两步热脱附路径，其中与富CH$_3$OH冰相关的MF可在主体H$_2$O升华之前释放。
英文摘要
Complex organic molecules are widely detected in star-forming regions and are important precursors to prebiotic species. Because these molecules are thought to form primarily within icy grain mantles, their desorption is assumed to be regulated by the sublimation of the surrounding H$_2$O ice matrix. However, observations increasingly reveal gas-phase abundances of methyl formate (MF) at temperatures well below the sublimation temperature of water ice (T ~ 100K), challenging this view. To investigate how ice composition and phase influence MF desorption, we present laboratory temperature-programmed desorption measurements of MF from four astrophysically relevant substrates: amorphous and crystalline H$_2$O and CH$_3$OH. Desorption kinetics were analyzed using pre-exponential factors derived from transition-state theory with leading-edge analysis and non-negative least-squares inversion to recover binding-energy distributions. We find that MF binds more weakly to CH$_3$OH ice than to H$_2$O-rich substrates, with representative binding energies of ~6247K for amorphous H$_2$O, ~6213K for crystalline H$_2$O, and ~5469K for amorphous CH$_3$OH. MF deposited on crystalline CH$_3$OH exhibits coverage-independent leading edges consistent with multilayer or island desorption, yielding an effective binding energy of 5333K. Enhanced surface mobility and reduced trapping efficiency on methanol ice, combined with weaker MF binding, provide a natural explanation for the early appearance of gas-phase MF observed in star-forming environments, with characteristic desorption temperatures of ~85K on CH$_3$OH and ~95-100K on H$_2$O. Together, these effects support a two-step thermal desorption pathway in which MF associated with CH$_3$OH-rich ice can be released prior to bulk H$_2$O sublimation.
