Unlike in the liquid phase, the debate of whether and how hydrogen-bonded structures exist in neutral ammonia dimer (NH₃)₂ in the gas phase has been ongoing for several decades. Here, we distinguish the structures of neutral ammonia dimers with and without hydrogen bonds by photoionization because the ions inherit initial structures from the neutral dimers and lead to significantly different Coulomb explosion channels in our pump–probe experiment, i.e., the direct dissociation (NH₃⁺ + NH₃⁺) and indirect dissociation with proton migration (NH₂⁺ + NH₄⁺). With quantum chemical and molecular dynamics simulation, we showcase that these two different Coulomb explosion channels originate from the ammonia dimer cations with different structures. The dimer cations without hydrogen bonds correlate with the direct Coulomb explosion channel. In contrast, dimer cations with hydrogen bonds are likely to undergo ultrafast proton migration in ∼48 fs, which has no potential barrier and correlate with the indirect dissociation channel in the Coulomb explosion. The 48 fs characteristic time is used to exclude the slower indirect dissociation initiated from non-H-bonded cations. Our work demonstrates a highly sensitive approach to probe weakly bonded and fluxional structures of gas-phase molecular clusters by utilizing both channel and time resolutions of Coulomb explosion.

中文翻译：

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飞秒时间分辨库仑爆炸探测的氨二聚体阳离子的构象异构体敏感核动力学

与液相不同，关于气相中性氨二聚体 （NH₃）₂ 中是否存在氢键结构以及如何存在的争论已经持续了几十年。在这里，我们通过光电离区分了有氢键和无氢键的中性氨二聚体的结构，因为离子继承了中性二聚体的初始结构，并导致在我们的泵-探针实验中产生明显不同的库仑爆炸通道，即直接解离 （NH₃⁺ + NH₃⁺） 和间接解离与质子迁移 （NH₂⁺ + NH₄⁺).通过量子化学和分子动力学模拟，我们证明了这两种不同的库仑爆炸通道起源于具有不同结构的氨二聚体阳离子。没有氢键的二聚体阳离子与直接库仑爆炸通道相关。相比之下，带有氢键的二聚体阳离子可能在 ∼48 fs 内发生超快质子迁移，这没有潜在的势垒，并且与库仑爆炸中的间接解离通道相关。48 fs 的特征时间用于排除由非 H 键阳离子引发的较慢的间接解离。我们的工作展示了一种通过利用库仑爆炸的通道和时间分辨率来探测气相分子簇的弱键合和通量结构的高灵敏度方法。