Identification of photofragmentation patterns in trihalide anions by global analysis of vibrational wavepacket dynamics in broadband transient absorption data.
Identification of photofragmentation patterns in trihalide anions by global analysis of vibrational wavepacket dynamics in broadband transient absorption data.
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DOI：
10.1039/c6cp06729h
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发表时间：
2016-12
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通讯作者：
Sebastian Schott;L. Ress;J. Hrǔsak;P. Nuernberger;T. Brixner
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作者：
Sebastian Schott;L. Ress;J. Hrǔsak;P. Nuernberger;T. Brixner
三卤阴离子是可以用紫外（UV）光解离的线性分子。而深紫外激发导致三体解离，近紫外激发只有一个分子键被切断，这在理论上开辟了不同的碎片模式的可能性。在这里，我们探讨是否形成的二卤化物片段作为一个阴离子或中性物种，以及是否heterophthalatrihalides可以导致两个不同的二卤化物。该分析是基于明显的波包动力学引起的飞秒紫外脉冲和相关的初始三卤化物和新生的二卤化物物种。对于二氯甲烷溶液中的三卤阴离子I3-、Br 3-、IBr 2-和ICl 2-（点群D∞h），以及I2 Br-和I2Cl-（点群C∞v），我们通过它们的特征振动波数来识别二卤离子碎片，这是我们通过全局拟合振动波包振荡来实现的，考虑了波长依赖的相位。在激发后没有发现来自中性物种的信号，因此D∞h三卤化物中只有一个双原子产物。对于所研究的C∞v三卤代化合物，可以允许一个homestrant和一个heterestrant产品，只有homestrant酮被观察到。由于二卤阴离子是不稳定的中间体，它们的吸收和三卤阴离子的基态漂白显示出双指数恢复所有样品由于重组片段对。产生中性二卤和原子阴离子的电子转移速率，是复合的先决条件，产生双指数行为;快速复合是由振动过剩能量介导的，而缓慢复合发生在冷却的二卤。这些数据揭示了从时域方法，而不是频域振动光谱的碎裂和重组动力学，并有助于深入理解这些多功能的模型分子。
Trihalide anions are linear molecules that can be photodissociated with ultraviolet (UV) light. Whereas deep-UV excitation leads to three-body dissociation, for near-UV excitation just one molecular bond is cleaved, which notionally opens up the possibility for different fragmentation patterns. Here, we explore whether the dihalide fragment is formed as an anionic or neutral species and whether heteronuclear trihalides can lead to two different dihalides. The analysis is based on pronounced wavepacket dynamics induced by femtosecond UV pulses and associated both with the initial trihalide and the nascent dihalide species. For the trihalide anions I3-, Br3-, IBr2-, and ICl2- (point group D∞h), as well as for I2Br- and I2Cl- (point group C∞v) in dichloromethane solution, we identify dihalide fragments by their characteristic vibrational wavenumbers, which we achieve from globally fitting the vibrational wavepacket oscillations, considering a wavelength-dependent phase. No signature from neutral species is found right after excitation, hence there is only one diatomic product in D∞h trihalides. For the investigated C∞v trihalides, which could allow a homonuclear and a heteronuclear product, only the homonuclear one is observed. Since dihalide anions are unstable intermediates, their absorption and the ground-state bleach of the trihalide anion show a biexponential recovery for all samples due to recombining fragment pairs. The rate of the electron transfer yielding a neutral dihalogen and an atomic anion, a prerequisite for the recombination, gives rise to the biexponential behavior; fast recombination is mediated by vibrational excess energy, while slow recombination occurs for cooled-down dihalogens. These data reveal the fragmentation and recombination dynamics from a time-domain approach rather than frequency-domain vibrational spectroscopy and contribute to the in-depth comprehension of these versatile model molecules.