Tracking Ultrafast Structural Dynamics by Time-Domain Raman Spectroscopy

Abstract

In traditional Raman spectroscopy, narrow-band light is irradiated on a sample, and its inelastic scattering, i.e., Raman scattering, is detected. The energy difference between the Raman scattering and the incident light corresponds to the vibrational energy of the molecule, providing the Raman spectrum that contains rich information about the molecular-level properties of the materials. On the other hand, by using ultrashort optical pulses, it is possible to induce Raman-active coherent nuclear motion of the molecule and to observe the molecular vibration in real time. Moreover, this time-domain Raman measurement can be combined with femtosecond photoexcitation, triggering chemical changes, which enables tracking ultrafast structural dynamics in a form of "time-resolved" time-domain Raman spectroscopy, also known as time-resolved impulsive stimulated Raman spectroscopy. With the advent of stable, ultrashort laser pulse sources, time-resolved impulsive stimulated Raman spectroscopy now realizes high sensitivity and a wide detection frequency window from THz to 3000 cm −1 , and has seen success in unveiling the molecular mechanisms underlying the efficient functions of complex molecular systems. In this Perspective, we overview the present status of time-domain Raman spectroscopy, particularly focusing on its application to the study of femtosecond structural dynamics. We first explain the principle and a brief history of time-domain Raman spectroscopy and then describe the apparatus and recent applications to the femtosecond dynamics of complex molecular systems, including proteins, molecular assemblies, and functional materials. We also discuss future directions for time-domain Raman spectroscopy, which has reached a status allowing a wide range of applications.

Insight into the Adsorption Structure of TIPS-Pentacene on Noble Metal Surfaces

“…The plasmonic mechanism solely enhances Raman scattering for molecules in close proximity with said plasmons, so a 100 nm-thick film would predominately yield Raman scattering from molecules unassociated with the plasmons of the gold substrate. Indeed, this is reflected in our spectra, which are nearly identical to those of previous Raman studies of bulk TIPS-Pc. , …”

Excited-State Proton Transfer Dynamics in LSSmOrange Studied by Time-Resolved Impulsive Stimulated Raman Spectroscopy

“…TR-ISRS is a femtosecond time-domain Raman technique, in which a photochemical reaction is initiated by a femtosecond actinic pump pulse, and Raman active vibrations of the transient species are measured in the time domain using ultrashort Raman pump and Raman probe pulses. − Fourier transform of the observed time-domain Raman signals provides frequency-domain femtosecond time-resolved Raman spectra that contain rich structural information on the transients. TR-ISRS allows us to monitor structural dynamics on a femtosecond time scale, as has been shown in the studies of a variety of complex molecular systems, − including wtGFP . TR-ISRS data of LSSmOrange obtained in this study revealed that two structurally distinct excited states participate in ESPT of LSSmOrange.…”

Polymer Photoelectrodes for Solar Fuel Production: Progress and Challenges

“…Expanded analyses of EIS responses and other modulated electrooptical techniques applied to organic photoelectrodes will greatly improve our understanding. ,,, Additionally, coupling two or more spectroscopic techniques to explore and monitor the excited state structural and carrier populations, alongside the different photochemical/photophysical processes within these polymer photoelectrodes will provide rich information that will improve our understanding of their inner workings. For instance, performing integrated time-resolved Raman and TAS measurements on polymer photoelectrodes will afford us information on the excited state structural dynamics and charge carrier dynamics, respectively, which are key considerations for the architecture of next-generation efficient photoelectrodes.…”