EXCITED STATES OF NUCLEIC ACID BASES

原子核低激发态的位置、特性和寿命 酸碱的研究将用激光光电离和 用喷雾喷射的激光光电子能谱，它允许超音速 非挥发性和热不稳定化合物的膨胀。都是单色的 和双色电离方案，涉及泵浦之间的各种时间延迟 和探测激光，将被用来测量光谱和寿命 激发态。最低层的性质(NPI或PIPI)的问题 激发单线态和低位PIPI态的数目等可以是 对光致电离和光电子进行了分析和比较 光谱，正如我们最近对异喹啉的实验所证明的那样。这个 状态赋值(在裸碱基和未取代碱基中)可通过以下方式进行辅助 研究甲基取代和氢键的光谱后果 (酒精和水)，这是已知的蓝移NPI*相对于 PIPI*状态。氢键对光谱的影响也是 对理解核酸电子结构的重要意义 碱在生物学上更相关的水溶液条件下。 最后，研究了最低激发态单重态的变化 (S1)具有不同基态的能级配置的寿命，以及 将进行甲基取代和氢键，以获得 关于超快内部的可能起源的信息 这些分子中的转化。这些实验的结果可能会导致 为了更好地了解核酸的光谱学和光物理 基地，仍然不完整和支离破碎。

The location, identity, and lifetime of low-lying excited states of nucleic acid bases will be investigated by combining laser photoionization and laser photoelectron spectroscopy with a spray-jet, which allows supersonic expansion of nonvolatile and thermally labile compounds. Both one-color and two-color ionization schemes, involving various time delay between pump and probe lasers, will be used to measure the spectra and lifetimes of the excited states. The question of the nature (nPi or PiPi) of the lowest excited singlet state, and the number of low-lying PiPi states, etc. can be probed by analyzing, and comparing, the photoionization and photoelectron spectra, as demonstrated by our recent experiments on isoquinoline. The state assignments (in bare and unsubstituted bases) can be aided by studying spectral consequences of methyl substitution and hydrogen bonding (with alcohol and water), which are known to blue shift nPI* relative to PiPi* states. The effects of hydrogen bonding on the spectra are also important for the understanding of electronic structure of nucleic acid bases in biologically more relevant conditions of aqueous solutions. Finally, the study of the variation of the lowest-excited-singlet-state (S1) lifetimes with energylevel dispositions in different bases, and with methyl substitution and hydrogen bonding, will be carried out to obtain information concerning the possible origin of the ultrafast internal conversion in these molecules. The results of these experiments can lead to better understanding of spectroscopy and photophysics of nucleic acid bases, which remain incomplete and fragmented.