Transient X-ray Absorption Spectral Fingerprints of the S1 Dark State in Uracil
Transient X-ray Absorption Spectral Fingerprints of the S1 Dark State in Uracil
复制标题
尿嘧啶中 S-1 暗态的瞬态 X 射线吸收光谱指纹
DOI：
10.1021/acs.jpclett.9b02692
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发表时间：
2019-11-21
影响因子：
5.7
通讯作者：
Luo, Yi
中科院分区：
文献类型：
--
作者：
Hua, Weijie;Mukamel, Shaul;Luo, Yi
低位暗npi*态在有机生色团的许多光物理和光化学过程中起着重要的作用。瞬时X射线吸收光谱(TxAS)通过激发电子进入高位核心激发态，为探测价态的动力学提供了强有力的技术手段。我们采用多组态自洽场计算方法研究了尿嘧啶沿其非辐射光衰变路径的TxAs。一个悬而未决的问题是，当亮的npi*态S-2被选择性激发时，暗npi*态S-1(n是定域在氧原子上的孤子对)是否可以到达。沿着S-0-S-2三个最低能级的势能面计算了垂直的核激发，这三个态内插在两个极小值和两个最小能量锥形交点之间。从C、N和OK边缘计算的TxAS数据显示了在所有光谱区域中暗状态的明显光谱指纹。在O IS边，npi*态在526-527 eV有很强的吸收，而在C(N)是边，在279-282 eV(397-398 eV)几乎没有(很弱)吸收。所有的K边光谱都可以用来灵敏地探测暗态。我们提出的O1s特征已经在最近的Txas胸腺嘧啶实验中观察到了。用自然跃迁轨道分析解释了沿反应坐标的三个最低价态的所有主要特征，揭示了核心激发的一些重要的价态精细结构信息。
Low-lying dark n pi* states play an important role in many photophysical and photochemical processes of organic chromophores. Transient X-ray absorption spectroscopy (TXAS) provides a powerful technique for probing the dynamics of valence states by exciting the electrons into high-lying core excited states. We employ multiconfigurational self-consistent field calculations to investigate the TXAS of uracil along its nonradiative photodecay pathways. An open issue is whether dark n pi* state S-1 (n is the lone pair localized on an oxygen atom) is accessible when bright n pi* state S-2 is selectively excited. Vertical core excitations were calculated along the potential energy surfaces of the three lowest states, S-0-S-2, interpolated between two minima and two minimum-energy conical intersections. Computed TXAS data from the C, N, and O K edges show distinct spectral fingerprints of the dark state in all spectral regimes. At the O is edge, the n pi* state has a very strong absorption at 526-527 eV, while at the C (N) is edge, by contrast, there is almost zero (very weak) absorption at 279-282 eV (397-398 eV). All K-edge spectra can be used to sensitively detect the dark states. Our proposed O 1s feature has already been observed in a recent TXAS experiment with thymine. Natural transition orbital analysis is used to interpret all dominant features of the three lowest-valence states along the reaction coordinate and reveal some important valence fine-structure information from the core excitation.