Abstract

The relaxation dynamics of protonated keto uracil has been investigated through cryogenic UV photodissociation spectroscopy. Steady-state spectroscopy and time-resolved photochemistry, including pump-probe photodissociation and kinetics of appearance of photofragments, are monitored over 10 orders of magnitude as a function of excess energy imparted in the bright \(^{\mathrm {1}}\uppi \uppi \)* state. Although photofragments are produced in the ground electronic state after internal conversion, the non-radiative decay mechanism abruptly changes with a slight increase of excess energy in the \(^{\mathrm {1}}\uppi \uppi \)* state. At the band origin, a three-step decay involving electronic couplings to the charge transfer \(^{\mathrm {1}}\hbox {n}_{\text {o}}\uppi \)* state and the triplet \(^{\mathrm {3}}\uppi \uppi \)* state with lifetimes in the range of \(10\,{\upmu }\hbox {s}\) and 2 ms, respectively, is proposed. However, the pathway through the charge transfer state closes a few hundreds of wavenumbers above the band origin. From this excess energy, the excited state population is transferred through a low energy barrier towards a region of the \(^{\mathrm {1}}\uppi \uppi \)* potential energy surface where a triple crossing with the \(^{\mathrm {3}}\uppi \uppi \)* state and the ground state is located. The experimental results are assigned with the help of ab initio calculations at the spin-component scaled coupled-cluster level.

Graphic Abstract

中文翻译：

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质子化酮尿嘧啶的兴奋态动力学：竞争中的系统间交叉途径

摘要

通过低温紫外光解离光谱研究了质子化酮尿嘧啶的弛豫动力学。稳态光谱和时间分辨的光化学，包括泵浦探针光解离和光碎片出现的动力学，在明亮的\（^ {\ mathrm {1}} \ uppi \ uppi \） *状态。尽管内部转换后在基态电子状态下会产生光碎片，但非辐射衰减机制会随着\（^ {\ mathrm {1}} \ uppi \ uppi \） *状态下多余能量的轻微增加而突然改变。在频带起点，三步衰减涉及到电荷转移\（^ {\ mathrm {1}} \ hbox {n} _ {\ text {o}} \ uppi \）的电子耦合*状态和三元组\（^ {\ mathrm {3}} \ uppi \ uppi \） *状态的生存期分别为\（10 \，{\ upmu} \ hbox {s} \）和2 ms ，建议。但是，通过电荷转移状态的路径会在谱带原点上方关闭几百个波数。从这个多余的能量中，激发态种群通过一个低能垒转移到\（^ {\ mathrm {1}} \ uppi \ uppi \） *势能表面与\（^ {\ mathrm {3}} \ uppi \ uppi \） *状态和基态所在。实验结果是在自旋分量缩放的耦合簇级别上从头算的帮助下分配的。

图形摘要