Institute of Chemistry and The Farkas Center for Light-Induced Processes, The Hebrew University, Jerusalem 91904, Israel.
Phys Chem Chem Phys. 2010 Mar 7;12(9):2149-63. doi: 10.1039/b920356g. Epub 2010 Jan 15.
The well-documented propensity of negatively-chirped pulses to enhance resonant impulsive Raman scattering has been rationalized in terms of a one pulse pump-dump sequence which "follows" the evolution of the excited molecules and dumps them back at highly displaced configurations. The aim of this study was to extend the understanding of this effect to molecules with many displaced vibrational modes in the presence of condensed surroundings. In particular, to define an optimally chirped pulse, to investigate what exactly it "follows" and to discover how this depends on the molecule under study. To this end, linear chirp effects on vibrational coherences in poly-atomics are investigated experimentally and theoretically. Chirped pump-impulsive probe experiments are reported for Sulforhodamine-B ("Kiton Red"), Betaine-30 and Oxazine-1 in ethanol solutions with <10 fs resolution. Numerical simulations, including numerous displaced modes and electronic dephasing, are conducted to reproduce experimental results. Through semi-quantitative reproduction of experimental results in all three systems we show that the effect of group velocity dispersion (GVD) on the buildup of ground state wave-packets depends on the pulse spectrum, on the displacements of vibrational modes upon excitation, on the detuning of the excitation pulses from resonance, and on electronic dephasing rates. Akin to scenarios described for frequency-domain resonance Raman, within the small-displacement regime each mode responds to excitation chirp independently and the optimal GVD is mode-specific. Highly-displaced modes entangle the dynamics of excitation in different modes, requiring a multi-dimensional description of the response. Rapid photochemistry and ultrafast electronic dephasing narrow the window of opportunity for coherent manipulations, leading to a reduced and similar optimal chirp for different modes. Finally, non-intuitive coherent aspects of chirp "following" are predicted in the small-displacement and slow-dephasing regime, which remain to be observed in experiment.
负啁啾脉冲增强共振脉冲拉曼散射的良好记录的倾向已经根据一个单脉冲泵-泄序列得到了合理化，该序列“跟随”激发分子的演化，并将它们回放到高度位移的构型中。本研究的目的是将这种效应的理解扩展到在凝聚环境存在下具有许多位移振动模式的分子中。特别是，定义一个最佳啁啾脉冲，研究它到底“跟随”什么，并发现它如何依赖于所研究的分子。为此，在线性啁啾效应对多原子振动相干性的影响进行了实验和理论研究。报道了用于 Sulforhodamine-B（“Kiton Red”）、Betaine-30 和 Oxazine-1 的啁啾泵-脉冲探测实验，分辨率小于 10 fs。进行了包括许多位移模式和电子退相的数值模拟，以重现实验结果。通过在所有三个系统中对半定量地重现实验结果，我们表明群速度色散（GVD）对基态波包的建立的影响取决于脉冲谱、激发时振动模式的位移、激发脉冲与共振的失谐以及电子退相速率。类似于在频域共振拉曼中描述的情况，在小位移范围内，每个模式独立响应激发啁啾，最佳 GVD 是模式特异性的。高度位移模式使不同模式中的激发动力学纠缠在一起，需要对响应进行多维描述。快速光化学和超快电子退相缩小了相干操纵的机会窗口，导致不同模式的最佳啁啾减少且相似。最后，在小位移和慢退相的情况下，预测了啁啾“跟随”的非直观相干方面，这些方面仍有待在实验中观察到。
