Pump power dependence in resonance femtosecond stimulated Raman spectroscopy
Pump power dependence in resonance femtosecond stimulated Raman spectroscopy
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DOI：
10.1002/jrs.4354
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发表时间：
2013-09-01
影响因子：
2.5
通讯作者：
McCamant, David W.
中科院分区：
文献类型：
--
作者：
Lee, Joohyun;Challa, J. Reddy;McCamant, David W.
飞秒受激拉曼光谱（FSRS）是一种能够获得荧光物种和瞬态光化学中间体的共振拉曼光谱的新技术。与相关的瞬态红外吸收技术不同，FSRS信号对振动探测事件中使用的激光功率非常敏感。特别地，FSRS光谱对皮秒拉曼泵浦脉冲的强度高度敏感。我们测量了FSRS信号的功率依赖性，使用的脉冲能量从近似10（-9）到近似10（-5）J，在拉曼泵浦波长400 nm处具有摩尔吸光系数范围的分子，包括-胡萝卜素（ε（400）= 58300 M（-1）cm（-1））、对硝基苯胺（17800 M（-1）cm（-1））、硝基萘（247 M（-1）cm（-1））和二茂铁（57 M（-1）cm（-1））。我们发现，对于强吸收的分子系统，如-胡萝卜素和对硝基苯胺，基态（GS）FSRS信号实际上随着泵浦功率的增加而减少，泵浦注量高于10（-2）Jcm（-2），由于GS人口的耗尽。然而，对于弱吸收的物质，如硝基萘和二茂铁，信号随着泵浦能量密度的增加而线性增加，直到接近0.5Jcm（-2），此时溶质的双光子吸收引起泵浦脉冲的非线性吸收和FSRS信号的衰减。利用量子物理动力学模型对实验数据进行了定量模拟，并对模拟结果进行了分析，为共振FSRS实验中拉曼泵浦功率的选择提供了简单的指导。可接受的拉曼泵浦功率与聚焦光束面积成正比，并与样品的摩尔吸光系数成反比。版权所有（c）2013约翰威利父子有限公司
Femtosecond stimulated Raman spectroscopy (FSRS) has emerged as a powerful new technique that is capable of obtaining resonance Raman spectra of fluorescent species and transient photochemical intermediates. Unlike related transient infrared absorption techniques, the FSRS signal is quite sensitive to the laser power utilized in the vibrational probing event. In particular, FSRS spectra are highly sensitive to the intensity of the picosecond Raman-pump pulse. We have measured the power dependence of the FSRS signal using pulse energies from similar to 10(-9) to similar to 10(-5)J and molecules with a range of molar absorptivities at the Raman-pump wavelength of 400nm, including -carotene (epsilon(400)=58300M(-1)cm(-1)), para-nitroaniline (17800M(-1)cm(-1)), nitronaphthalene (247M(-1)cm(-1)) and ferrocene (57M(-1)cm(-1)). We show that for strongly absorbing molecular systems, such as -carotene and para-nitroaniline, the ground-state (GS) FSRS signal actually decreases with increasing pump power at pump fluences above similar to 10(-2)Jcm(-2), due to depletion of the GS population. However, for weakly absorbing species like nitronaphthalene and ferrocene, the signal increases linearly with increasing pump fluence until similar to 0.5Jcm(-2), at which point two-photon absorption by the solute induces nonlinear absorption of the pump pulse and attenuation of the FSRS signal. The data are quantitatively simulated with a photophysical kinetic model, and the results are analyzed to provide simple guidelines for acceptable Raman-pump powers in resonance FSRS experiments. The acceptable Raman-pump power is proportional to the focused beam area and depends inversely on the sample's molar absorptivity. Copyright (c) 2013 John Wiley & Sons, Ltd.