Stadlhofer Michael, Thaler Bernhard, Koch Markus
Graz University of Technology, Institute of Experimental Physics, Petersgasse 16, Graz, Austria.
Phys Chem Chem Phys. 2022 Oct 19;24(40):24727-24733. doi: 10.1039/d2cp03571e.
We present femtosecond pump-probe photoionization experiments with indium dimers (In) solvated in helium nanodroplets (He). At short pump-probe time delays, where the excited In* is still located inside the droplet, we surprisingly observe detachment of InHe ions with = 1 to ∼30 from the droplet. These ions indicate that fragmentation of In occurs and that the kinetic energy release enables In to overcome the attractive He potential, which typically prevents ion ejection from the droplet. We find that the transient InHe signal reveals vibrational wave packet motion in neutral In*. By correlating the InHe signal with the corresponding photoelectrons through covariance detection, we unequivocally identify the ionization pathway leading to InHe: pump-excitation from the ground-state In creates a vibrational wave packet in In*, followed by probe-ionization to the cationic ground state In. Subsequently, a further probe photon promotes the molecule to an excited ionic state In+* of nonbonding character, leading to fragmentation and kinetic energy release. This interpretation is additionally supported by probe power- and droplet-size dependencies, as well as energetic considerations. Unambiguous assignment of the ionization path to absorption-ionization-dissociation (fragmentation of the ion) in contrast to absorption-dissociation-ionization (fragmentation of the neutral) is enabled by ion ejection and electron-ion correlation. This complementary observable for ultrafast photochemical processes inside He will be particularly valuable for more complex systems.
我们展示了在氦纳米液滴（He）中溶剂化的铟二聚体（In）的飞秒泵浦 - 探测光电离实验。在短的泵浦 - 探测时间延迟下，此时激发态的In仍位于液滴内部，我们令人惊讶地观察到InHe离子（n = 1至约30）从液滴中脱离。这些离子表明In发生了碎片化，并且动能释放使In能够克服通常会阻止离子从液滴中射出的吸引性He势。我们发现瞬态InHe信号揭示了中性In中的振动波包运动。通过协方差检测将InHe信号与相应的光电子相关联，我们明确地确定了导致InHe的电离途径：从基态In进行泵浦激发在In中产生一个振动波包，随后通过探测电离到阳离子基态In。随后，另一个探测光子将分子提升到具有非键特征的激发离子态In+，导致碎片化和动能释放。这种解释还得到了探测功率和液滴尺寸依赖性以及能量考虑的支持。与吸收 - 解离 - 电离（中性物质的碎片化）相反，通过离子射出和电子 - 离子相关性能够明确地将电离路径确定为吸收 - 电离 - 解离（离子的碎片化）。这种用于He内部超快光化学过程的互补可观测性对于更复杂的系统将特别有价值。
