Departamento de Química Física, Facultad de Ciencias Químicas (Unidad Asociada de I+D+i al Consejo Superior de Investigaciones Científicas), Universidad Complutense de Madrid, 28040 Madrid, Spain.
Departamento de Química and Institute for Advanced Research in Chemical Sciences (IAdChem), Módulo 13, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain.
J Chem Phys. 2020 Jan 7;152(1):014304. doi: 10.1063/1.5134473.
Clocking of electronically and vibrationally state-resolved channels of the fast photodissociation of CHI in the A-band is re-examined in a combined experimental and theoretical study. Experimentally, a femtosecond pump-probe scheme is employed in the modality of resonant probing by resonance enhanced multiphoton ionization (REMPI) of the methyl fragment in different vibrational states and detection through fragment velocity map ion (VMI) imaging as a function of the time delay. We revisit excitation to the center of the A-band at 268 nm and report new results for excitation to the blue of the band center at 243 nm. Theoretically, two approaches have been employed to shed light into the observations: first, a reduced dimensionality 4D nonadiabatic wavepacket calculation using the potential energy surfaces by Xie et al. [J. Phys. Chem. A 104, 1009 (2000)]; and second, a full dimension 9D trajectory surface-hopping calculation on the same potential energy surfaces, including the quantization of vibrational states of the methyl product. In addition, high level ab initio electronic structure calculations have been carried out to describe the CH 3p Rydberg state involved in the (2 + 1) REMPI probing process, as a function of the carbon-iodine (C-I) distance. A general qualitative agreement is obtained between experiment and theory, but the effect of methyl vibrational excitation in the umbrella mode on the clocking times is not well reproduced. The theoretical results reveal that no significant effect on the state-resolved appearance times is exerted by the nonadiabatic crossing through the conical intersection present in the first absorption band. The vibrationally state resolved clocking times observed experimentally can be rationalized when the (2 + 1) REMPI probing process is considered. None of the other probing methods applied thus far, i.e., multiphoton ionization photoelectron spectroscopy, soft X-ray inner-shell photoelectron spectroscopy, VUV single-photon ionization, and XUV core-to-valence transient absorption spectroscopy, have been able to provide quantum state-resolved (vibrational) clocking times. More experiments would be needed to disentangle the fine details in the clocking times and dissociation dynamics arising from the detection of specific quantum-states of the molecular fragments.
在一项结合实验和理论的研究中，我们重新检查了 CHI 在 A 带中快速光解的电子和振动态分辨通道的计时。实验上，我们采用飞秒泵浦-探测方案，通过共振增强多光子电离（REMPI）探测不同振动态下的甲基碎片，并通过碎片速度图离子（VMI）成像作为时间延迟的函数进行检测。我们重新研究了在 268nm 处激发到 A 带中心以及在 243nm 处激发到带中心蓝移处的情况。理论上，我们采用了两种方法来深入了解观测结果：首先，使用 Xie 等人的势能面进行降维 4D 非绝热波包计算[J. Phys. Chem. A 104, 1009 (2000)]；其次，在相同的势能面上进行全维 9D 轨迹表面跳跃计算，包括甲基产物振动态的量子化。此外，我们还进行了高精度的从头算电子结构计算，以描述参与（2+1）REMPI 探测过程的 CH3p 里德堡态，作为碳-碘（C-I）距离的函数。实验和理论之间取得了大致的一致，但甲基的振动激发在伞状模式下对计时时间的影响并没有得到很好的重现。理论结果表明，在第一个吸收带中存在的锥形交叉处的非绝热交叉对状态分辨出现时间没有显著影响。当考虑（2+1）REMPI 探测过程时，可以合理地解释实验观测到的振动状态分辨计时时间。迄今为止应用的其他探测方法，即多光子电离光电子能谱、软 X 射线内壳光电子能谱、VUV 单光子电离和 XUV 芯到价态瞬态吸收光谱，都无法提供量子状态分辨（振动）计时时间。需要更多的实验来解开分子碎片特定量子态检测产生的计时时间和离解动力学的细节。
