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Femtosecond Stimulated Raman Line Shapes: Dependence on Resonance Conditions of Pump and Probe Pulses
FangChong
Author NameAffiliationE-mail
FangChong Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon, 97331-4003, U.S.A. fangchong@gmail.com 
Abstract:
Resonance enhancement has been increasingly employed in the emergent femtosecond stimulated Raman spectroscopy (FSRS) to selectively monitor molecular structure and dynamics with improved spectral and temporal resolutions and signal-to-noise ratios. Such joint efforts by the technique- and application-oriented scientists and engineers have laid the foundation for exploiting the tunable FSRS methodology to investigate a great variety of photosensitive systems and elucidate the underlying functional mechanisms on molecular time scales. During spectral analysis, peak line shapes remain a major concern with an intricate dependence on resonance conditions. Here, we present a comprehensive study of line shapes by tuning the Raman pump wavelength from red to blue side of the ground-state absorption band of the fluorescent dye rhodamine 6G in solution. Distinct line shape patterns in Stokes and anti-Stokes FSRS as well as from the low to high-frequency modes highlight the competition between multiple third-order and higher-order nonlinear pathways, governed by different resonance conditions achieved by Raman pump and probe pulses. In particular, the resonance condition of probe wavelength is revealed to play an important role in generating circular line shape changes through oppositely phased dispersion via hot luminescence (HL) pathways. Meanwhile, on-resonance conditions of the Raman pump could promote excited-state vibrational modes which are broadened and red-shifted from the coincident ground-state vibrational modes, posing challenges for spectral analysis. Certain strategies in tuning the Raman pump and probe to characteristic regions across an electronic transition band are discussed to improve the FSRS usability and versatility as a powerful structural dynamics toolset to advance chemical, physical, materials, and biological sciences.
Key words:  Femtosecond stimulated Raman spectroscopy, Resonance enhancement, Raman pump and probe pulses, Wavelength tunability, Dispersive line shapes, Stokes and anti-Stokes FSRS
FundProject:
飞秒受激拉曼光谱线形:基于泵浦及探测脉冲光的共振条件研究
FangChong
摘要:
在新兴的飞秒受激拉曼(FSRS)光谱中,共振增强效应逐渐地被广泛应用于针对性研究功能分子的结构及动力学。在以技术开发及应用为导向的科研人员的共同努力下,兼备时间及光谱高分辨率的FSRS技术已经被成功地应用于诸多的光敏体系以探究其在分子时间尺度上的工作原理。实际操作中,光谱分析常受制于微分线形以及共振条件的选择。本文用室温下化学染料罗丹明6G(R6G)的甲醇溶液为模型,系统性地研究了FSRS光谱线形基于窄带皮秒泵浦及宽带飞秒探测光波长的变化,并首次将拉曼泵浦光的波长从R6G的电子基态吸收峰的红侧连续调谐至蓝侧(648至479 nm)。我们观测到了在斯托克斯及反斯托克斯的FSRS谱图中的特征线形组合,以及在同一谱图中从低频到高频振动峰的线形变化,主要源自受泵浦及探测光波长共振状况影响的多个三阶及五阶非线性过程在飞秒至皮秒时域上的竞争。尤其是在观测到的循环线形的产生机理中,拉曼探测光的波长经由热发光(hot luminescence, 简写为HL)通道发挥了重要作用。与此同时,拉曼泵浦光与R6G的电子基态吸收峰的精准共振可以有效促进激发态振动模式,和基态相比,它们有增加的峰宽和红移的频率,并在谱图中与共振增强的基态振动模式并存。基于对上述实验的分析,本文讨论了受激拉曼泵浦及探测脉冲光波长的选择调谐策略,通过预共振以获取高信噪比的以吸收线形为主的电子基态和激发态的振动光谱信息,从而进一步将波长可调的FSRS发展成为化学,物理,材料及生物领域的强大且普适的结构动力学研究技术。
关键词:  飞秒受激拉曼光谱,共振增强,拉曼泵浦及探测脉冲光,波长可调,微分线形,斯托克斯及反斯托克斯FSRS
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