Predicted almost forty years ago, the radiation from thermally populated excited electronic states has recently been recognised as an important cooling mechanism in free molecules and clusters. It has presently been observed from both inorganic clusters and carbon-based molecules in molecular beams and ion storage devices. Experiments have demonstrated that many of these systems radiate at rates approaching microsecond time scales, and often with a distinct dependence on the precise number of atoms in the system. The radiation acts as a strongly stabilising factor against both unimolecular decay and thermal electron emission. In astrophysical context, radiative cooling provides a mechanism to dissipate internal energy in star-forming processes, and stabilises molecules selectively in the circumstellar medium. The consequences of an active radiative cooling channel for nanoparticle production will likewise favour special sizes in non-equilibrium formation processes. In this review, the radiative cooling of clusters is presented and illustrated with examples of experiments performed on small carbon, metal, and semiconductor clusters, and on PAH molecules. The experimental and theoretical techniques used are discussed, together with the consequences of radiative cooling on size-to-size stability patterns of clusters.

中文翻译：

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尺寸选择的气相团簇的辐射冷却

近四十年前预测，来自热填充的激发电子态的辐射最近被认为是自由分子和团簇的重要冷却机制。目前已经从分子束和离子存储装置中的无机簇和碳基分子中观察到它。实验表明，许多这些系统以接近微秒时间尺度的速率辐射，并且通常明显依赖于系统中原子的精确数量。辐射作为抵抗单分子衰变和热电子发射的强稳定因素。在天体物理学中，辐射冷却提供了一种在恒星形成过程中耗散内部能量的机制，并选择性地稳定星周介质中的分子。用于纳米颗粒生产的主动辐射冷却通道的结果同样将有利于非平衡形成过程中的特殊尺寸。在这篇综述中，通过对小碳、金属和半导体簇以及多环芳烃分子进行的实验示例，介绍并说明了簇的辐射冷却。讨论了所使用的实验和理论技术，以及辐射冷却对簇大小稳定性模式的影响。