Wavelength-dependent measurements of the RNA base uracil, undertaken with nanosecond ultraviolet laser pulses, have previously identified a fragment at m/z = 84 (corresponding to the C₃H₄N₂O⁺ ion) at excitation wavelengths ≤232 nm. This has been interpreted as a possible signature of a theoretically predicted ultrafast ring-opening occurring on a neutral excited state potential energy surface. To further investigate the dynamics of this mechanism, and also the non-adiabatic dynamics operating more generally in uracil, we have used a newly built ultra-high vacuum spectrometer incorporating a laser-based thermal desorption source to perform time-resolved ion-yield measurements at pump wavelengths of 267 nm, 220 nm, and 200 nm. We also report complementary data obtained for the related species 2-thiouracil following 267 nm excitation. Where direct comparisons can be made (267 nm), our findings are in good agreement with the previously reported measurements conducted on these systems using cold molecular beams, demonstrating that the role of initial internal energy on the excited state dynamics is negligible. Our 220 nm and 200 nm data also represent the first reported ultrafast study of uracil at pump wavelengths <250 nm, revealing extremely rapid (<200 fs) relaxation of the bright S₃(¹ππ^*) state. These measurements do not, however, provide any evidence for the appearance of the m/z = 84 fragment within the first few hundred picoseconds following excitation. This key finding indicates that the detection of this specific species in previous nanosecond work is not directly related to an ultrafast ring-opening process. An alternative excited state process, operating on a more extended time scale, remains an open possibility.

中文翻译：

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尿嘧啶中的紫外线弛豫动力学：使用基于激光的热脱附源进行时间分辨的光子产率研究

纳秒级紫外线激光脉冲对RNA碱基尿嘧啶的波长依赖性测量先前已鉴定出m / z = 84的片段（对应于C ₃ H ₄ N ₂ O ⁺激发波长≤232 nm。这已被解释为在中性激发态势能表面上发生的理论预测的超快开环的可能信号。为了进一步研究这种机制的动力学，以及在尿嘧啶中更普遍运行的非绝热动力学，我们使用了新型超高真空光谱仪，并结合了基于激光的热脱附源来进行时间分辨的离子产率测量在267 nm，220 nm和200 nm的泵浦波长下。我们还报告了267 nm激发后相关物种2-硫尿嘧啶获得的补充数据。在可以进行直接比较（267 nm）的地方，我们的发现与先前报道的使用冷分子束在这些系统上进行的测量非常吻合，证明了初始内部能量对激发态动力学的作用可以忽略不计。我们的220 nm和200 nm数据还代表了首次报道的泵浦波长<250 nm的尿嘧啶超快研究，揭示了明亮S的极快（<200 fs）弛豫。₃（¹ ππ ^*）的状态。但是，这些测量结果无法提供激发后头几百皮秒内m / z = 84片段出现的任何证据。这一关键发现表明，在先前的纳秒级工作中对这种特定物种的检测与超快速开环过程没有直接关系。在更长的时间范围内运行的替代激发态过程仍然是一个开放的可能性。