超快二维电子光谱(特邀)创刊六十周年特邀
1 引言
近几十年来,物理化学动力学领域的研究兴趣,逐渐从宏观世界微秒以上的动态过程,转向了微观世界飞秒到纳秒的超快过程。超快瞬态吸收光谱作为研究微观世界超快动态过程的重要手段之一,已被广泛应用于物理、化学、生物与材料等学科中,为揭示低维材料、光合作用、光催化与光伏材料等的光-物质相互作用与激发态动力学机理提供了重要的实验证据[1]。超快瞬态光谱利用一束超短激光脉冲将样品激发到激发态,然后再利用另一束具有给定时间延迟T的超快激光脉冲探测样品该时刻的光谱。通过探测不同时刻T的超快瞬态吸收光谱,即可反演出样品的激发态动力学。然而,随着应用的推广与研究的深入,解析复杂系统的动力学机理成为超快光谱领域亟待解决的重要问题。复杂系统往往具有不同组分光谱重叠以及不同动力学过程速率相近的特点。如何区分混叠的组分以及动力学过程成为了超快光谱领域的瓶颈之一[2-4]。
针对该问题,科拉拉多博尔德分校的Jonas教授团队于1998年提出了超快二维(2D)电子光谱技术[5]。该技术的原理与超快瞬态吸收光谱类似,但不同的是,其比超快瞬态吸收光谱多一个激发波长的维度,从而可将复杂体系的动力学信息,记录在一张如
图 1. 二维电子光谱示意图[6]。(a)二维电子光谱脉冲序列;(b)二维电子光谱图及其探测信息
Fig. 1. Illustration of 2DES[6]. (a) Pulse sequence in the 2DES; (b) two-dimensional electronic spectra and its detection information
近十五年来,超快二维电子光谱领域得到了飞速的发展。2007年,Fleming教授等[8]通过二维电子光谱在氯硫细菌的Fenna-Matthews-Olson捕光复合物中观察到了量子相干信号,并提出该信号来源于电子态之间的量子相干。由于该问题的重要性,许多理论与实验工作者开展了相关方面的研究,对该领域的发展起了极大的促进作用。相关理论与实验成果可参考相关文献与书籍[9-13]。至今为止:在仪器方面,二维光谱的探测范围已从近红外拓展到了紫外、THz波段;其探测手段从传统的透射光谱探测拓展到了荧光探测以及光电流探测;其维度也从时间与波长维度,拓展到了空间维度[6,14](
不少综述与书籍已详细介绍了二维光谱的基本原理及其在交叉学科领域中的应用[3,9,19-20],本文作者在近期也发表了相关方面的中文综述[6,21]。因此,本文将结合课题组的研究方向,从技术角度对二维电子光谱的进展进行梳理与总结,并结合国内外现状,讨论该领域在未来发展中的挑战与机遇,以期为感兴趣的读者提供一些参考信息,从而推动国内二维电子光谱领域的发展。
2 二维电子光谱进展
二维电子光谱是一种三阶非线性光谱技术。如
式中:R(τa,τb,τc)为样品的响应函数;E1、E2为泵浦激光脉冲电场;E3为探测脉冲电场。该信号通过与本机振荡激光脉冲混合,以外差探测的方式被光谱仪与阵列探测器所探测。通过扫描两束泵浦脉冲之间的时间延迟,并探测不同相干时间下的光谱,然后将光谱信号对相干延迟时间做傅里叶变换,即可得到记录T时刻激发态动力学信息的二维电子光谱图。通过探测不同延迟时间T的二维电子光谱图,得到一幅记录了完整激发态动力学过程的三维图谱。
二维电子实验技术的难点在于:1)需要对相干时间进行高于光学周期精度的扫描(如获取600 nm附近的二维电子光谱,其光学周期为2 fs,时间扫描精度要求高于1 fs);2)在数小时的探测过程中,激光脉冲需要保持相对的相位稳定性。因此,在发展初期,二维电子光谱领域主要集中于发展新型的相干时间扫描技术来提升装置的相位稳定性,从而实现高信噪比的光谱数据探测。如
与此同时,随着可调谐激光光源生成与脉冲压缩技术的发展,二维电子光谱也开展了探测波段拓展方面的研究(
图 3. 二维光谱的探测光谱窗口(虚线框内为相对成熟的技术,虚线框外为尚在发展的技术)[14]
Fig. 3. Spectral windows of 2DES (mature techniques are shown inside the dashed box while the developing ones remain outside) (reprinted from Ref. [14], with the permission of AIP Publishing)
然而,由于复杂体系不同组分间往往存在电子态跃迁光谱重叠的问题,因此,探测单独的电子态跃迁光谱往往无法有效解耦不同路径的激发态动力学过程。针对该问题,2015年科研人员先后发明了二维电子-振动光谱[27]与二维振动-电子光谱[28][
针对不同复杂系统的动力学研究,新型多维光谱技术在近年来被不断发明[
图 4. 新型二维光谱技术。(a)荧光探测二维电子光谱[24];(b)二维电子Stark光谱[47];(c)光电流探测二维电子光谱[49]
Fig. 4. New probing dimensions of 2D spectroscopy. (a) Spatially-resolved fluorescence-detected 2DES[24]; (b) 2D Stark spectroscopy (reprinted with the permission from Ref. [47], copyright © 2017 American Chemical Society); (c) photocurrent 2DES (reprinted with the permission from Ref. [49], copyright © 2021 American Chemical Society)
3 二维电子光谱展望
二维电子光谱作为一种重要的激发态动力学探测工具,已被广泛应用于物理、化学、材料与生物等学科领域中,在研究光合膜蛋白、光伏材料、低维材料与极化基元等体系的能级结构与激发态动力学机理中发挥了重要作用。然而,随着应用领域的拓展与前沿基础研究的深入,二维电子光谱也遇到了新的机遇与挑战。
3.1 技术挑战
首先,尽管二维光谱领域的课题组很多,但是由于其技术门槛的限制,大部分课题组尚只有单一波段的二维光谱装置[14]。构建全光谱段的二维光谱系统,对于研究复杂体系的光生物、光化学与光物理过程具有重要意义。如何降低二维光谱的技术门槛,使二维光谱仪器低成本化,光谱仪器设计的模块化与智能化,是推动二维光谱技术进一步发展以及拓展其应用领域的关键。
其次,二维光谱的数据分析方法仍存在着一定的局限。如在常用的全局-目标分析中,尚无法兼容Stokes位移等过程,同时也缺少对特征光谱峰位置与线宽的限制[53-54]。此外,时间分辨荧光光谱与稳态吸收光谱等其他实验数据也可为动力学过程分析提供重要证据。理论上来讲,基于超快二维光谱提出的动力学模型,需符合所有的实验数据。然而,现在尚缺乏一种可同时拟合二维光谱数据与其他实验数据的有效手段。
最后,红外探测器的限购极大地限制了国内二维电子-振动光谱与二维红外光谱领域的进一步发展。尽管上转换技术可规避红外探测器缺失的局限[55],然而其最高理论检出限将受限于非线性过程的效率。如何实现红外探测器的自主研发,以及如何在近期尽可能地提升上转换探测的信噪比与灵敏度,是解决该技术问题的关键。
3.2 前沿基础研究
相干动力学调控是光生物与光化学领域的一个重要议题[56-57]。二维电子光谱作为一种探测激发态相干动力学的重要手段,在阐明量子相干信号来源及相干调控机理方面发挥了重要作用。一方面,许多课题组利用二维电子光谱分别在光合作用、有机光伏材料与金属配合物等体系中[56,58]发现了激发态的量子相干过程。另一方面,偏振二维电子光谱等实验方法[59]与双边-费曼图[60]等数据分析方法被提出,用于区分电子态、振动态与电子-振动态之间的相干动力学过程,从而为揭示相干动力学调控机理提供线索。然而,如何从实验上直接观测到量子相干对某一激发态过程的影响仍是一个挑战。研究表明,电子态相干与电子-振动态相干被认为是调控激发态动力学的有效手段[56]。然而,这两类相干信号在大部分体系中的存在时间较短(百飞秒量级),无法有效调控皮秒时间尺度以上的传能与电荷分离等动力学过程。因此,领域内仍在寻找合适的体系与方法来延长量子相干的持续时间,继而实现激发态动力学过程的有效调控[61-62]。该领域的突破一方面依赖于合成化学与材料化学,另一方面则要求进一步发展二维光谱技术来实现对激发态量子相干动力学的更深入解析。
二维电子光谱的另一个重要新兴研究领域是激子极化基元。极化基元是一种光与物质强相互作用下生成的新粒子,它具有一半光子与一半激发态的性质。最简单的极化基元制备方法可通过将一个分子放置于一个共振频率与分子激发态频率相同的腔体内实现。极化基元可通过光-物质相互作用调控其能级结构,并具有远程能量传输的能力。因此,其被认为在光化学与光电器件中有着重要的潜在应用前景[63]。近年来,二维红外光谱在揭示分子振动极化基元的能级结构与远程传能机理中发挥了重要作用[64-65]。现存挑战包括如何表征与解析极化基元中的暗态分子及其动力学过程[66-69]。偏振二维光谱或可为表征暗态激发态动力学提供重要的实验依据。进一步发展二维光谱的数据分析方法,即在拟合中引入已知的暗态分子光谱与动力学信息,抑或可为深入揭示暗态动力学提供重要线索。相对来说,激子极化基元动力学机理的研究还相对较少。激子极化基元可通过调控光相互作用改变其激发态动力学过程,因此被认为在光化学与光催化领域具有重要的潜在应用前景[63,70]。此外,将激子极化基元与光合膜蛋白等光生物系统结合,可通过调控其能级结构改变激发态动力学,从而为揭示光合膜蛋白中的激发态动力学机理以及设计新型人工光合作用系统提供新信息[71]。二维电子光谱作为表征极化基元动力学的重要工具,将在该方向的研究中发挥重要作用。
4 结束语
总而言之,超快二维电子光谱在近二十年来得到了飞速的发展,其探测光谱窗口从近红外拓展到了紫外到THz,其时间窗口从飞秒到纳秒拓展到了飞秒到毫秒。此外,荧光探测二维电子光谱、空间分辨二维电子光谱、二维电子Stark光谱与光电流探测二维电光谱等技术被不断发明,这极大地增强了二维光谱的探测能力。新技术的发展一方面,将为光物理与光化学的研究提供重要的探测手段,为探索曾经因实验条件限制而无法研究的科学问题提供可能。另一方面,如何降低二维电子光谱的技术门槛,以及发展更好的二维光谱数据分析方法,让其成为像核磁共振一样的共性工具,将成为拓展该技术进一步应用的另一个重要挑战。
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Article Outline
肖常涛, 宋寅, 赵维谦. 超快二维电子光谱(特邀)[J]. 激光与光电子学进展, 2024, 61(1): 0130002. Changtao Xiao, Yin Song, Weiqian Zhao. Ultrafast Two-Dimensional Electronic Spectroscopy (Invited)[J]. Laser & Optoelectronics Progress, 2024, 61(1): 0130002.