Department of Chemistry, University of Durham, Durham DH1 3LE, United Kingdom.
J Chem Phys. 2013 Aug 28;139(8):084302. doi: 10.1063/1.4818597.
A photoelectron imaging study of the gas-phase dianion of pyrromethene-556 is presented. The photoelectron spectra and angular distributions following resonant excitation of the S1 excited state with nanosecond and femtosecond laser pulses are compared, and the influence of the repulsive Coulomb barrier (RCB) in both cases evaluated. Photoelectron angular distributions show the effect of molecular alignment due to an allowed electronic excitation and can be understood qualitatively based on the calculated RCB surface using the Local Static Approximation. Comparison between femtosecond and nanosecond excitation reveals marked differences in the photoelectron spectra. While femtosecond experiments confirm that tunneling through the RCB is adiabatic, nanosecond experiments show a broad photoelectron feature peaking near the RCB maximum. This is explained in terms of the lifetime of internal conversion, which has been determined by time-resolved photoelectron spectroscopy to be ~120 ps: as this is faster than the nanosecond laser pulses, multiple photons can be absorbed through the S1 ← S0 transition which leads to large amounts of internal energy and enables electron detachment directly above the RCB. Fragmentation and detachment from the monoanion are also inferred by the presence of photoelectrons emitted at very low kinetic energy. Our results highlight the difficulty in interpreting photoelectron spectra of polyanions in which a resonant state is excited.
本文呈现了对吡咯并[5,6]苯并吡咯-556 气相二阴离子的光电子成像研究。比较了纳秒和飞秒激光脉冲共振激发 S1 激发态后的光电子能谱和角分布，并评估了两种情况下排斥库仑势垒（RCB）的影响。光电子角分布显示了由于允许的电子激发导致的分子取向的影响，并且可以基于使用局部静态近似计算的 RCB 表面进行定性理解。飞秒和纳秒激发之间的比较显示出光电子光谱的显著差异。虽然飞秒实验证实通过 RCB 的隧穿是绝热的，但纳秒实验显示出在 RCB 最大值附近出现的宽光电子特征。这可以根据通过时间分辨光电子光谱确定的内部转换寿命来解释，该寿命约为 120 ps：由于这比纳秒激光脉冲快，因此可以通过 S1←S0 跃迁吸收多个光子，从而导致大量的内部能量并使电子直接在 RCB 上方脱离。通过非常低动能发射的光电子的存在，也推断出从单阴离子的碎裂和脱离。我们的结果强调了在解释其中激发共振态的多阴离子光电子光谱时的困难。
