We explore the kinetic processes that sustain equilibrium in a microscopic, finite system. This is accomplished by monitoring the spontaneous, time-dependent frequency evolution (the frequency autocorrelation) of a single OH oscillator, embedded in a water cluster held in a temperature-controlled ion trap. The measurements are carried out by applying two-color, infrared-infrared photodissociation mass spectrometry to the D₃O⁺·(HDO)(D₂O)₁₉ isotopologue of the “magic number” protonated water cluster, H⁺·(H₂O)₂₁. The OH group can occupy any one of the five spectroscopically distinct sites in the distorted pentagonal dodecahedron cage structure. The OH frequency is observed to evolve over tens of milliseconds in the temperature range (90 to 120 K). Starting at 100 K, large “jumps” are observed between two OH frequencies separated by ∼300 cm⁻¹, indicating migration of the OH group from the bound OH site at 3,350 cm⁻¹ to the free position at 3,686 cm⁻¹. Increasing the temperature to 110 K leads to partial interconversion among many sites. All sites are observed to interconvert at 120 K such that the distribution of the unique OH group among them adopts the form one would expect for a canonical ensemble. The spectral dynamics displayed by the clusters thus offer an unprecedented view into the molecular-level processes that drive spectral diffusion in an extended network of water molecules.

我们探索了在微观有限系统中维持平衡的动力学过程。这是通过监视单个OH振荡器的自发，随时间变化的频率演变（频率自相关）来实现的，该振荡器嵌入埋在温度控制离子阱中的水簇中。通过对“魔术数”质子化水簇H ⁺ ·（H ₂）的D ₃ O ⁺ ·（HDO）（D ₂ O）₁₉同位素分子应用双色红外红外光解质谱法进行测量。O）₂₁。OH基团可以占据扭曲的五边形十二面体笼结构中五个在光谱上不同的位点中的任何一个。在温度范围（90至120 K）内，观察到OH频率会演化数十毫秒。从100 K开始，在两个〜约300 cm ^-1的OH频率之间观察到较大的“跳跃” ，表明OH基团从3,350 cm ^-1的结合OH位置迁移到3,686 cm ^-1的自由位置。将温度提高到110 K会导致许多站点之间发生部分互转换。观察到所有位点都在120 K处相互转换，以使其中唯一的OH基团的分布采用一种规范合奏所期望的形式。因此，由簇显示的光谱动力学为驱动水分子在扩展的网络中的光谱扩散的分子级过程提供了前所未有的视角。