Accessing Excited State Molecular Vibrations by Femtosecond Stimulated Raman Spectroscopy

Orchestrating Nuclear Dynamics in a Permanganate Doped Crystal with Chirped Pump-Probe Spectroscopy

Pump-probe spectroscopy is a powerful tool to investigate light-induced dynamical processes in molecules and solids. Targeting vibrational excitations occurring on the time scales of nuclear motions is challenging, as pulse durations shorter than a vibrational period are needed to initiate the dynamics, and complex experimental schemes are required to isolate weak signatures arising from wavepacket motion in different electronic states. Here, we demonstrate how introducing a temporal delay between the spectral components of femtosecond beams, namely a chirp resulting in the increase of their duration, can counterintuitively boost the desired signals by 2 orders of magnitude. Measuring the time-domain vibrational response of permanganate ions embedded in a KClO4 matrix, we identify an intricate dependence of the vibrational response on pulse chirps and probed wavelength that can be exploited to unveil weak signatures of the doping ions─otherwise dominated by the nonresonant matrix─or to obtain vibrational excitations pertaining only to the excited state, suppressing ground-state contributions.

Two-Dimensional Electronic Spectroscopy Resolves Relative Excited-State Displacements

Knowledge of relative displacements between potential energy surfaces (PES) is critical in spectroscopy and photochemistry. Information on displacements is encoded in vibrational coherences. Here we apply ultrafast two-dimensional electronic spectroscopy in a pump-probe half-broadband (HB2DES) geometry to probe the ground- and excited-state potential landscapes of cresyl violet. 2D coherence maps reveal that while the coherence amplitude of the dominant 585 cm-1 Raman-active mode is mainly localized in the ground-state bleach and stimulated emission regions, a 338 cm-1 mode is enhanced in excited-state absorption. Modeling these data with a three-level displaced harmonic oscillator model using the hierarchical equation of motion-phase matching approach (HEOM-PMA) shows that the S1 ← S0 PES displacement is greater along the 585 cm-1 coordinate than the 338 cm-1 coordinate, while Sn ← S1 displacements are similar along both coordinates. HB2DES is thus a powerful tool for exploiting nuclear wavepackets to extract quantitative multidimensional, vibrational coordinate information across multiple PESs.

Mastering Femtosecond Stimulated Raman Spectroscopy: A Practical Guide

Femtosecond stimulated Raman spectroscopy (FSRS) is a powerful nonlinear spectroscopic technique that probes changes in molecular and material structure with high temporal and spectral resolution. With proper spectral interpretation, this is equivalent to mapping out reactive pathways on highly anharmonic excited-state potential energy surfaces with femtosecond to picosecond time resolution. FSRS has been used to examine structural dynamics in a wide range of samples, including photoactive proteins, photovoltaic materials, plasmonic nanostructures, polymers, and a range of others, with experiments performed in multiple groups around the world. As the FSRS technique grows in popularity and is increasingly implemented in user facilities, there is a need for a widespread understanding of the methodology and best practices. In this review, we present a practical guide to FSRS, including discussions of instrumentation, as well as data acquisition and analysis. First, we describe common methods of generating the three pulses required for FSRS: the probe, Raman pump, and actinic pump, including a discussion of the parameters to consider when selecting a beam generation method. We then outline approaches for effective and efficient FSRS data acquisition. We discuss common data analysis techniques for FSRS, as well as more advanced analyses aimed at extracting small signals on a large background. We conclude with a discussion of some of the new directions for FSRS research, including spectromicroscopy. Overall, this review provides researchers with a practical handbook for FSRS as a technique with the aim of encouraging many scientists and engineers to use it in their research.

Photogrammetry of Ultrafast Excited-State Intramolecular Proton Transfer Pathways in the Fungal Pigment Draconin Red

Proton transfer processes of organic molecules are key to charge transport and photoprotection in biological systems. Among them, excited-state intramolecular proton transfer (ESIPT) reactions are characterized by quick and efficient charge transfer within a molecule, resulting in ultrafast proton motions. The ESIPT-facilitated interconversion between two tautomers (PS and PA) comprising the tree fungal pigment Draconin Red in solution was investigated using a combination of targeted femtosecond transient absorption (fs-TA) and excited-state femtosecond stimulated Raman spectroscopy (ES-FSRS) measurements. Transient intensity (population and polarizability) and frequency (structural and cooling) dynamics of -COH rocking and -C=C, -C=O stretching modes following directed stimulation of each tautomer elucidate the excitation-dependent relaxation pathways, particularly the bidirectional ESIPT progression out of the Franck-Condon region to the lower-lying excited state, of the intrinsically heterogeneous chromophore in dichloromethane solvent. A characteristic overall excited-state PS-to-PA transition on the picosecond timescale leads to a unique "W"-shaped excited-state Raman intensity pattern due to dynamic resonance enhancement with the Raman pump-probe pulse pair. The ability to utilize quantum mechanics calculations in conjunction with steady-state electronic absorption and emission spectra to induce disparate excited-state populations in an inhomogeneous mixture of similar tautomers has broad implications for the modeling of potential energy surfaces and delineation of reaction mechanisms in naturally occurring chromophores. Such fundamental insights afforded by in-depth analysis of ultrafast spectroscopic datasets are also beneficial for future development of sustainable materials and optoelectronics.

Ultrafast Spectroscopies of Nitrophenols and Nitrophenolates in Solution: From Electronic Dynamics and Vibrational Structures to Photochemical and Environmental Implications

Nitrophenols are a group of small organic molecules with significant environmental implications from the atmosphere to waterways. In this work, we investigate a series of nitrophenols and nitrophenolates, with the contrasting ortho-, meta-, and para-substituted nitro group to the phenolic hydroxy or phenolate oxygen site (2/3/4NP or NP^-), implementing a suite of steady-state and time-resolved spectroscopic techniques that include UV/Visible spectroscopy, femtosecond transient absorption (fs-TA) spectroscopy with probe-dependent and global analysis, and femtosecond stimulated Raman spectroscopy (FSRS), aided by quantum calculations. The excitation-dependent (400 and 267 nm) electronic dynamics in water and methanol, for six protonated or deprotonated nitrophenol molecules (three regioisomers in each set), enable a systematic investigation of the excited-state dynamics of these functional "nanomachines" that can undergo nitro-group twisting (as a rotor), excited-state intramolecular or intermolecular proton transfer (donor-acceptor, ESIPT, or ESPT), solvation, and cooling (chromophore) events on molecular timescales. In particular, the meta-substituted compound 3NP or 3NP^- exhibits the strongest charge-transfer character with FSRS signatures (e.g., C-N peak frequency), and thus, does not favor nitroaromatic twist in the excited state, while the ortho-substituted compound 2NP can undergo ESIPT in water and likely generate nitrous acid (HONO) after 267 nm excitation. The delineated mechanistic insights into the nitro-substituent-location-, protonation-, solvent-, and excitation-wavelength-dependent effects on nitrophenols, in conjunction with the ultraviolet-light-induced degradation of 2NP in water, substantiates an appealing discovery loop to characterize and engineer functional molecules for environmental applications.

Excited-state resonance Raman spectroscopy probes the sequential two-photon excitation mechanism of a photochromic molecular switch

Some diarylethene molecular switches have a low quantum yield for cycloreversion when excited by a single photon, but react more efficiently following sequential two-photon excitation. The increase in reaction efficiency depends on both the relative time delay and the wavelength of the second photon. This paper examines the wavelength-dependent mechanism for sequential excitation using excited-state resonance Raman spectroscopy to probe the ultrafast (sub-30 fs) dynamics on the upper electronic state following secondary excitation. The approach uses femtosecond stimulated Raman scattering (FSRS) to measure the time-gated, excited-state resonance Raman spectrum in resonance with two different excited-state absorption bands. The relative intensities of the Raman bands reveal the initial dynamics in the higher-lying states, S_n, by providing information on the relative gradients of the potential energy surfaces that are accessed via secondary excitation. The excited-state resonance Raman spectra reveal specific modes that become enhanced depending on the Raman excitation wavelength, 750 or 400 nm. Many of the modes that become enhanced in the 750 nm FSRS spectrum are assigned as vibrational motions localized on the central cyclohexadiene ring. Many of the modes that become enhanced in the 400 nm FSRS spectrum are assigned as motions along the conjugated backbone and peripheral phenyl rings. These observations are consistent with earlier measurements that showed higher efficiency following secondary excitation into the lower excited-state absorption band and illustrate a powerful new way to probe the ultrafast dynamics of higher-lying excited states immediately following sequential two-photon excitation.

Absolute excited state molecular geometries revealed by resonance Raman signals

Ultrafast reactions activated by light absorption are governed by multidimensional excited-state (ES) potential energy surfaces (PESs), which describe how the molecular potential varies with the nuclear coordinates. ES PESs ad-hoc displaced with respect to the ground state can drive subtle structural rearrangements, accompanying molecular biological activity and regulating physical/chemical properties. Such displacements are encoded in the Franck-Condon overlap integrals, which in turn determine the resonant Raman response. Conventional spectroscopic approaches only access their absolute value, and hence cannot determine the sense of ES displacements. Here, we introduce a two-color broadband impulsive Raman experimental scheme, to directly measure complex Raman excitation profiles along desired normal modes. The key to achieve this task is in the signal linear dependence on the Frank-Condon overlaps, brought about by non-degenerate resonant probe and off-resonant pump pulses, which ultimately enables time-domain sensitivity to the phase of the stimulated vibrational coherences. Our results provide the tool to determine the magnitude and the sensed direction of ES displacements, unambiguously relating them to the ground state eigenvectors reference frame.

Noises investigations and image denoising in femtosecond stimulated Raman scattering microscopy

In the literature of SRS microscopy, the hardware characterization usually remains separate from the image processing. In this article, we consider both these aspects and statistical properties analysis of image noise, which plays the vital role of joining links between them. Firstly, we perform hardware characterization by systematic measurements of noise sources, demonstrating that our in-house built microscope is shot noise limited. Secondly, we analyze the statistical properties of the overall image noise, and we prove that the noise distribution can be dependent on image direction, whose origin is the use of a lock-in time constant longer than pixel dwell time. Finally, we compare the performances of two widespread general algorithms, that is, singular value decomposition and discrete wavelet transform, with a method, that is, singular spectrum analysis (SSA), which has been adapted for stimulated Raman scattering images. In order to validate our algorithms, in our investigations lipids droplets have been used and we demonstrate that the adapted SSA method provides an improvement in image denoising.

Femtosecond photodecarbonylation of photo-ODIBO studied by stimulated Raman spectroscopy and density functional theory

Photo-oxa-dibenzocyclooctyne (Photo-ODIBO) undergoes photodecarbonylation under UV excitation to its bright S₂ state, forming a highly reactive cyclooctyne, ODIBO. Following 321 nm excitation with sub-50 fs actinic pulses, the excited state evolution and cyclopropenone bond cleavage with CO release were characterized using femtosecond stimulated Raman spectroscopy and time-dependent density functional theory Raman calculations. Analysis of the photo-ODIBO S₂ CO Raman band revealed multi-exponential intensity, peak splitting and frequency-shift dynamics. This suggests a stepwise cleavage of the two C-C bonds in the cyclopropenone structure that is completed within <300 fs after excitation. Evidence of intramolecular vibrational relaxation on the S₂ state, concurrent with photodecarbonylation, with dynamics matching previous electronic transient absorption spectroscopy, was also observed. This confirms an excited state, as opposed to ground state, photodecarbonylation mechanism resulting in a vibronically excited photoproduct, ODIBO.

Excited-State Energy Surfaces in Molecules Revealed by Impulsive Stimulated Raman Excitation Profiles

Photophysical and photochemical processes are ruled by the interplay between transient vibrational and electronic degrees of freedom, which are ultimately determined by the multidimensional potential energy surfaces (PESs). Differences between ground and excited PESs are encoded in the relative intensities of resonant Raman bands, but they are experimentally challenging to access, requiring measurements at multiple wavelengths under identical conditions. Herein, we perform a two-color impulsive vibrational scattering experiment to launch nuclear wavepacket motions by an impulsive pump and record their coupling with a targeted excited-state potential by resonant Raman processes with a delayed probe, generating in a single measurement background-free vibrational spectra across the entire sample absorption. Building on the interference between the multiple pathways resonant with the excited-state manifold that generate the Raman signal, we show how to experimentally tune their relative phase by varying the probe chirp, decoding nuclear displacements along different normal modes and revealing the multidimensional PESs. Our results are validated against time-dependent density functional theory.

Tracking Ultrafast Structural Dynamics in a Dual-Emission Anti-Kasha-Active Fluorophore Using Femtosecond Stimulated Raman Spectroscopy

The anti-Kasha process provides the possibility of using high-energy excited states to develop novel applications. Our previous research (Nature communications, 2020, 11, 793) has demonstrated a dual-emission anti-Kasha-active fluorophore for bioimaging application, which exhibits near-infrared emissions from the S₁ state and visible anti-Kasha emissions from the S₂ state. Here, we applied tunable blue-side femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption spectroscopy, assisted by quantum calculations, to reveal the anti-Kasha dual emission mechanism, in which the emergence of two fluorescing states is due to the retardation of internal conversion from the S₂ state to the S₁ state. It has been demonstrated that the facts of anti-Kasha high-energy emission are commonly attributed to a large energy gap between the two excited states, leading to a decrease in the internal conversion rate due to a poor Franck-Condon factor. In this study, analysis of the calculation and FSRS experimental results provide us further insight into the dual-emission anti-Kasha mechanism, where the observation of hydrogen out-of-plane Raman modes from FSRS suggested that, in addition to the energy-gap law, the initial photoinduced molecular conformational change plays a key role in influencing the rate of internal conversion.