Acetonitrile (CH₃CN) and water (H₂O) ice particles were generated within a collisional cooling cell coupled to the Australian Synchrotron light source. The evolution of the aerosols was tracked by infrared spectroscopy compiled over the 4000–50 cm^–1 region. Gas pressure and temperature conditions were varied to replicate the lower altitudes of the Titan atmosphere allowing for comparison to far-infrared features detected by the Cassini–Huygens spacecraft. The experimental spectra show that CH₃CN and H₂O particles are microheterogeneous in composition and spherical in shape. CH₃CN lattice bands display temperature-dependent shifts in frequency, implying that pure β-phase is present in the mixed particles. In addition, a red shift identified for the C≡N fundamental stretching mode indicates dipole–dipole and π-electron side-directed hydrogen bond coupling between segregated CH₃CN and H₂O phases exclusively at the grain interface. Discrete dipole approximation theory was implemented to evaluate various cluster architectures where segregated domains of pure CH₃CN and H₂O ices provided the best fit to experiment; confirming the infrared findings. Otherwise, simulations of competing architectures, such as core–shell and cubic shaped particles, did not provide convincing comparison to the aerosol spectra. We conclude that the far-infrared profiles for mixed CH₃CN–H₂O systems do not present as likely carriers for the unassigned 220 cm^–1 “haystack” feature that has been identified in Titan’s lower atmosphere.

中文翻译：

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二元乙腈和水气溶胶：泰坦大气条件下的红外研究和理论模拟

乙腈（CH ₃ CN）和水（H ₂ O）冰粒是在与澳大利亚同步加速器光源耦合的碰撞冷却池内产生的。气溶胶的演变通过在4000–50 cm ^–1区域内汇编的红外光谱进行跟踪。改变气压和温度条件可以复制泰坦大气的较低高度，从而可以与卡西尼－惠更斯号航天器探测到的远红外特征进行比较。实验光谱表明，CH ₃ CN和H ₂ O颗粒在组成上微异质，在形状上呈球形。通道₃CN晶格带显示随温度变化的频率变化，这意味着在混合颗粒中存在纯β相。此外，对于C≡N基本拉伸模式识别出的红移表明，仅在晶粒界面处，在分离的CH ₃ CN和H ₂ O相之间偶极子-偶极子和π电子侧向氢键耦合。实施离散偶极近似理论来评估各种簇结构，其中纯CH ₃ CN和H _2的隔离域冰提供了最适合实验的条件；确认红外线的发现。否则，对竞争性体系结构（如核-壳和立方状颗粒）的仿真无法提供与气溶胶光谱的令人信服的比较。我们得出的结论是，混合的CH ₃ CN–H ₂ O系统的远红外轮廓未显示为尚未分配的220 cm ^–1 “干草堆”特征的可能载体，该特征已在土卫六的低层大气中确定。