Oscar Breland G, Chen Cheng, Liu Weimin, Zhu Liangdong, Fang Chong
Department of Chemistry, Oregon State University , 153 Gilbert Hall, Corvallis, Oregon 97331, United States.
Department of Physics, Oregon State University , 301 Weniger Hall, Corvallis, Oregon 97331, United States.
J Phys Chem A. 2017 Jul 27;121(29):5428-5441. doi: 10.1021/acs.jpca.7b04404. Epub 2017 Jul 17.
Tracking molecular motions in real time remains a formidable challenge in science and engineering fields because the experimental methodology requires simultaneously high spatial and temporal resolutions. Building on early successes and future potential of femtosecond stimulated Raman spectroscopy (FSRS) as a structural dynamics technique, we present a comprehensive study of stimulated Raman line shapes of a photosensitive molecule in solution with tunable Raman pump and probe pulses. Following femtosecond 400 nm electronic excitation, the model photoacid pyranine exhibits dynamic and mode-dependent Raman line shapes when the Raman pump is tuned from the red side toward and across the excited-state absorption (ESA) band (e.g., from S) with varying resonance conditions. On the anti-Stokes FSRS side, low-frequency modes below ∼1000 cm exhibit a line shape change from gain to dispersive to loss, whereas the dispersive intermediate is much less notable for high-frequency modes. The characteristic mode frequency blue shift involving vibrationally hot states in S with time constants of ∼9.6 and 58.6 ps reveals the sensitivity of anti-Stokes FSRS to vibrational cooling and solvation. This work lays the foundation for expanding tunable FSRS technology on both the Stokes and anti-Stokes sides to investigate a variety of photoinduced processes in solution with sufficient resolution to expose functional motions and increased sensitivity to monitor vibrational cooling.
在科学和工程领域，实时追踪分子运动仍然是一项艰巨的挑战，因为实验方法需要同时具备高空间分辨率和高时间分辨率。基于飞秒受激拉曼光谱（FSRS）作为一种结构动力学技术的早期成功和未来潜力，我们对溶液中一种光敏分子的受激拉曼线形进行了全面研究，采用了可调谐的拉曼泵浦和探测脉冲。在400nm飞秒电子激发之后，当拉曼泵浦从红侧向激发态吸收（ESA）带调谐并穿过该带（例如从S）时，模型光酸吡喃在不同共振条件下呈现出动态的、与模式相关的拉曼线形。在反斯托克斯FSRS一侧，低于~1000cm的低频模式呈现出线形从增益到色散再到损耗的变化，而对于高频模式，色散中间态则不太明显。涉及S中振动热态的特征模式频率蓝移，其时间常数约为9.6和58.6ps，揭示了反斯托克斯FSRS对振动冷却和溶剂化的敏感性。这项工作为在斯托克斯和反斯托克斯两侧扩展可调谐FSRS技术奠定了基础，以便以足够的分辨率研究溶液中各种光诱导过程，从而揭示功能运动，并提高监测振动冷却的灵敏度。
