Femtosecond Stimulated Raman Spectroscopy Research Articles

Studying Stimulated Raman Activity in Surface-Enhanced Femtosecond Stimulated Raman Spectroscopy by Varying the Excitation Wavelength.

We present the first multiwavelength surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS) study, as well as the first observation of anti-Stokes vibrational features in SE-FSRS spectra. We compare stimulated Raman loss (SRL) and stimulated Raman gain (SRG) signals at three pump wavelengths chosen to sample different portions of nanoparticle aggregate localized surface plasmon resonances. The SE-FSRS signals exhibit similar signal magnitudes in the SRL or SRG regions of the spectra regardless of Raman pump or probe wavelength. The spectral lineshapes, however, differ dramatically with excitation wavelengths. The observed trends in spectral line shape show a strong dependence on the relative position of the excitation fields with respect to the plasmon resonance but do not match predictions from any existing SE-FSRS theory. These results suggest the need for further theoretical efforts with complementary experimental studies of individual aggregates to remove the effects of inherent ensemble averaging.

Quantitative Determination of the Differential Raman Scattering Cross Sections of Glucose by Femtosecond Stimulated Raman Scattering.

Femtosecond stimulated Raman spectroscopy (FSRS) is a vibrational spectroscopy technique that has been used in a wide variety of applications: from transient vibrational signature tracking to amplifying weak normal Raman scattering signals. Presented here is an application of FSRS to quantify the differential Raman scattering cross sections (DRSCs) of glucose. In using FSRS to determine the DRSCs of multiple glucose vibrational modes, we demonstrate the applicability of both stimulated Raman loss (SRL) spectroscopy and stimulated Raman gain (SRG) FSRS. Using the two analogous FSRS techniques, SRG and SRL, we determine that the DRSCs of glucose excited at 514.5 nm range from a low of 5.0 ± 1.1 × 10-30 to a high of 8.9 ± 0.9 × 10-30 cm2 molecule-1 sr-1. This work establishes both the compatibility of SRL for measuring DRSCs and values for the DRSC of multiple vibrational modes of glucose.

Ultrafast Dynamics in Light-Driven Molecular Rotary Motors Probed by Femtosecond Stimulated Raman Spectroscopy.

Photochemical isomerization in sterically crowded chiral alkenes is the driving force for molecular rotary motors in nanoscale machines. Here the excited-state dynamics and structural evolution of the prototypical light-driven rotary motor are followed on the ultrafast time scale by femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption (TA). TA reveals a sub-100-fs blue shift and decay of the Franck-Condon bright state arising from relaxation along the reactive potential energy surface. The decay is accompanied by coherently excited vibrational dynamics which survive the excited-state structural evolution. The ultrafast Franck-Condon bright state relaxes to a dark excited state, which FSRS reveals to have a rich spectrum compared to the electronic ground state, with the most intense Raman-active modes shifted to significantly lower wavenumber. This is discussed in terms of a reduced bond order of the central bridging bond and overall weakening of bonds in the dark state, which is supported by electronic structure calculations. The observed evolution in the FSRS spectrum is assigned to vibrational cooling accompanied by partitioning of the dark state between the product isomer and the original ground state. Formation of the product isomer is observed in real time by FSRS. It is formed vibrationally hot and cools over several picoseconds, completing the characterization of the light-driven half of the photocycle.

Ultraviolet Light Makes dGMP Floppy: Femtosecond Stimulated Raman Spectroscopy of 2'-Deoxyguanosine 5'-Monophosphate.

The ultrafast dynamics of 2'-deoxyguanosine 5'-monophosphate after excitation with ultraviolet light has been studied with femtosecond transient absorption (TA) and femtosecond stimulated Raman spectroscopy (FSRS). TA kinetics and transient anisotropy spectra reveal a rapid relaxation from the Franck-Condon region, producing an extremely red-shifted stimulated emission band at ∼440 nm that is formed after 200 fs and subsequent relaxation for 0.8-1.5 ps, consistent with prior studies. Viscosity dependence shows that the initial relaxation, before 0.5 ps, is the same in water or viscous glycerol/water mixtures, but after 0.5 ps the dynamics significantly slow down in a viscous solution. This indicates that large amplitude structural changes occur after 0.5 ps following photoexcitation. FSRS obtained with both 480 and 600 nm Raman pump pulses observe very broad Raman peaks at 509 and 1530 cm-1, as well as a narrower peak at 1179 cm-1. All of the Raman peaks decay with 0.7-1.3 ps time constants. The 1530 cm-1 peak also shows an increasing inhomogeneous linewidth over the first 0.3 ps. Our TA and FSRS data are consistent with a structurally inhomogeneous population in the S1 (La) state and, in particular, with previous theoretical models in which out-of-plane distortion at C2 and the amine move the molecule toward a conical intersection with the ground state. These FSRS data are the first to directly observe the structural inhomogeneity imparted upon the excited-state population by the broad, flat potential energy surface of the S1 (La) state.

Ultrafast Intramolecular Proton Transfer of Alizarin Investigated by Femtosecond Stimulated Raman Spectroscopy.

We report time-resolved stimulated Raman spectra of alizarin in DMSO solution with 403 nm excitation. Upon photoexcitation, the intramolecular proton transfer reaction of alizarin occurs in 70-80 fs, which is confirmed by both the population growth and the frequency and bandwidth changes of skeletal vibrational modes of alizarin. Interestingly, the vibrational frequencies of ν(C═C) and ν(C═O) show opposite shifts during the reaction, which may implicate changes in the resonance structure of anthraquinone and the attached carbonyl group. Vibrational relaxation in the potential surface of the proton transferred tautomer of alizarin and the population decay occurring with two distinct time scales were also observed in addition to the solvation dynamics of DMSO solvent molecules.

Illuminating Photochemistry of an Excitation Ratiometric Fluorescent Protein Calcium Biosensor.

Fluorescent protein (FP)-based biosensors have become an important and promising tool to track metal ion movement inside living systems. Their working principles after light irradiation, however, remain elusive. To facilitate the rational design of biosensors, we dissect the fluorescence modulation mechanism of a newly developed excitation ratiometric green FP-based Ca2+ biosensor, GEX-GECO1, using femtosecond stimulated Raman spectroscopy (FSRS) in the electronic excited state. Upon 400 nm photoexcitation, characteristic vibrational marker bands at ∼1180 and 1300 cm-1 show concomitant decay and rise dynamics, probing the progression of an ultrafast excited state proton transfer (ESPT) reaction. The Ca2+-bound biosensor exhibits two distinct populations that undergo ESPT with ∼6 and 80 ps time constants, in contrast to one dominant population with a 25 ps time constant in the Ca2+-free biosensor. This result is supported by key structural constraints from molecular dynamics simulations with and without Ca2+. The blueshift of the ∼1265 cm-1 C-O stretch mode unravels the vibrational cooling dynamics of the protonated chromophore regardless of Ca2+ binding events. This unique line of inquiry reveals the essential structural dynamics basis of fluorescence modulation inside an excitation ratiometric protein biosensor, correlating the uncovered chromophore structural heterogeneity with different H-bonding configurations and intrinsic proton transfer rate in the photoexcited state.

Femtosecond stimulated Raman study of the photoactive flavoprotein AppABLUF

Femtosecond stimulated Raman spectroscopy (FSRS) is applied to study the photocycle of a blue light using flavin (BLUF) domain photoreceptor, AppABLUF. It is shown that FSRS spectra are sensitive to the light adapted state of the protein and probe its excited state dynamics. The dominant contribution to the most sensitive excited state Raman active modes is from flavin ring modes. However, TD-DFT calculations for excited state structures indicate that reproduction and assignment of the experimentally observed spectral shift will require high level calculations on the flavin in its specific protein environment.

Resonant femtosecond stimulated Raman spectroscopy with an intense actinic pump pulse: Application to conical intersections.

We theoretically investigate the feasibility of characterizing conical intersections with time-resolved resonant femtosecond stimulated Raman spectroscopy (FSRS) using an intense actinic pump pulse. We perform nonperturbative numerical simulations of FSRS signals for a three-electronic-state two-vibrational-mode model, which is inspired by the S2(ππ*)-S1(nπ*) conical intersection in pyrazine. Our results show that moderately strong actinic pulses increase the intensity of vibrational fingerprint lines in FSRS transients. They facilitate the extraction of useful spectroscopic information by enhancing peaks revealing the coupling and tuning modes of the conical intersection.

Tracking Ultrafast Vibrational Cooling during Excited-State Proton Transfer Reaction with Anti-Stokes and Stokes Femtosecond Stimulated Raman Spectroscopy.

Energy dissipation following photoexcitation is foundational to photophysics and chemistry. Consequently, understanding such processes on molecular time scales holds paramount importance. Femtosecond stimulated Raman spectroscopy (FSRS) has been used to study the molecular structure-function relationships but usually on the Stokes side. Here, we perform both Stokes and anti-Stokes FSRS to track energy dissipation and excited-state proton transfer (ESPT) for the photoacid pyranine in aqueous solution. We reveal biphasic vibrational cooling on fs-ps time scales during ESPT. Characteristic low-frequency motions (<800 cm-1) exhibit initial energy dissipation (∼2 ps) that correlates with functional events of forming contact ion pairs via H-bonds between photoacid and water, which lengthens to ∼9 ps in methanol where ESPT is inhibited. The interplay between photoinduced dissipative and reactive channels is implied. Thermal cooling to bulk solvent occurs on the ∼50 ps time scale. These results demonstrate the combined Stokes and anti-Stokes FSRS as a powerful toolset to elucidate structural dynamics.

Solvent-Templated Folding of Perylene Bisimide Macrocycles into Coiled Double-String Ropes with Solvent-Sensitive Optical Signatures.

A series of semirigid perylene bisimide (PBI) macrocycles with varied ring size containing two to nine PBI chromophores were synthesized in a one-pot reaction and their photophysical properties characterized by fluorescence, steady-state, and transient absorption spectroscopy as well as femtosecond stimulated Raman spectroscopy. These macrocycles show solvent-dependent conformational equilibria and excited-state properties. In dichloromethane, the macrocycles prevail in wide-stretched conformations and upon photoexcitation exhibit symmetry-breaking charge separation followed by charge recombination to triplet states, which photosensitize singlet oxygen formation. In contrast, in aromatic solvents folding of the macrocycles with a distinct odd-even effect regarding the number of PBI chromophore units was observed in steady-state and time-resolved absorption and fluorescence spectroscopy as well as femtosecond stimulated Raman spectroscopy. These distinctive optical properties are attributable to the folding of the even-membered macrocycles into exciton-vibrational coupled dimer pairs in aromatic solvents. Studies in a variety of aromatic solvents indicate that these solvents embed between PBI dimer pairs and accordingly template the folding of even-membered PBI macrocycles into ropelike folded conformations that give rise to solvent-specific exciton-vibrational couplings in UV-vis absorption spectra. As a consequence of the embedding of solvent molecules in the coiled double-string rope architecture, highly solvent specific intensity ratios are observed for the two lowest-energy exciton-vibrational bands, enabling assignment of the respective solvent simply based on the absorption spectra measured for the tetramer macrocycle.

Tuning calcium biosensors with a single-site mutation: structural dynamics insights from femtosecond Raman spectroscopy.

Fluorescent protein biosensors are popular reporters for biological processes and life sciences, but their fundamental working mechanisms remain unclear. To characterize the functional fluorescence events on their native timescales, we implemented wavelength-tunable femtosecond stimulated Raman spectroscopy (FSRS) to shed light on a blue-green emission-ratiometric fluorescent protein based Ca2+ biosensor with a single Pro377Arg mutation. The transient Raman modes of the embedded chromophore from ca. 1000-1650 cm-1 exhibit characteristic intensity and frequency dynamics which infer the underlying atomic motions and photochemical reaction stages. Our experimental study reveals the hidden structural inhomogeneity of the protein local environment upon Ca2+ binding with the mutated arginine residue trapping multiple chromophore subpopulations, which manifest distinct time constants of ∼16 and 90 ps for excited state proton transfer (ESPT) following 400 nm photoexcitation. The altered ESPT reaction pathways and emission properties of the Ca2+ biosensor represent the foundational step of rationally designing advanced fluorescent protein biosensors to tune their functionalities by site-specifically altering the local environment (e.g., the active site) of the embedded chromophore.

Spiers Memorial Lecture.Surface-enhanced Raman spectroscopy: from single particle/molecule spectroscopy to ångstrom-scale spatial resolution and femtosecond time resolution.

Four decades on, surface-enhanced Raman spectroscopy (SERS) continues to be a vibrant field of research that is growing (approximately) exponentially in scope and applicability while pushing at the ultimate limits of sensitivity, spatial resolution, and time resolution. This introductory paper discusses some aspects related to all four of the themes for this Faraday Discussion. First, the wavelength-scanned SERS excitation spectroscopy (WS-SERES) of single nanosphere oligomers (viz., dimers, trimers, etc.), the distance dependence of SERS, the magnitude of the chemical enhancement mechanism, and the progress toward developing surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS) are discussed. Second, our efforts to develop a continuous, minimally invasive, in vivo glucose sensor based on SERS are highlighted. Third, some aspects of our recent work in single molecule SERS and the translation of that effort to ångstrom-scale spatial resolution in ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) and single molecule electrochemistry using electrochemical (EC)-TERS will be presented. Finally, we provide an overview of analytical SERS with our viewpoints on SERS substrates, approaches to address the analyte generality problem (i.e. target molecules that do not spontaneously adsorb and/or have Raman cross sections <10-29 cm2 sr-1), SERS for catalysis, and deep UV-SERS.

Balancing the Effects of Extinction and Enhancement for Optimal Signal in Surface-Enhanced Femtosecond Stimulated Raman Spectroscopy

The field of ultrafast surface-enhanced Raman spectroscopy (SERS) is rapidly expanding; however, few applications for these new techniques have been demonstrated. One obstacle for the widespread application of ultrafast SERS is the addition of highly enhancing and scattering plasmonic substrates to already complex nonlinear spectroscopies. The competition between extinction and enhancement in ultrafast SERS techniques complicates the optimization of a number of experimental parameters. Here we study the concentration and path length dependences of signal quality in surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS). We find that in contrast to previous studies of spontaneous SERS which use signal magnitudes to define optimal experimental parameters, signal-to-noise ratios (SNRs) are the best measure of ideal experimental parameters in SE-FSRS. We report ideal concentrations and path lengths to use in transmissive geometry SE-FSRS experiments with colloidal nanoparticle substrates. Our re...

Monitoring Photochemical Reaction Pathways of Tungsten Hexacarbonyl in Solution from Femtoseconds to Minutes

Metal-organic complexes are widely used across disciplines for energy and biological applications, however, their photophysical and photochemical reaction coordinates remain unclear in solution due to pertaining molecular motions on ultrafast time scales. In this study, we apply transient absorption and tunable femtosecond stimulated Raman spectroscopy (FSRS) to investigate the UV photolysis of tungsten hexacarbonyl and subsequent solvent binding events. On the macroscopic time scale with UV lamp irradiation, no equilibrated intermediate is observed from W(CO)6 to W(CO)5(solvent), corroborated by vibrational normal mode calculations. Upon 267 nm femtosecond laser irradiation, the excited-state absorption band within ∼400-500 nm exhibits distinct dynamics in methanol, tetrahydrofuran, and acetonitrile on molecular time scales. In methanol, solvation of the nascent pentacarbonyl-solvent complex occurs in ∼8 ps and in tetrahydrofuran, 13 ps which potentially involves the associative oxygen-donating ligand rearrangement reaction. In contrast, a stimulated emission feature above 480 nm emerges after ∼1 ps in acetonitrile with a nitrogen-donating ligand. These structural dynamics insights demonstrate the combined resolving power of ultrafast electronic and stimulated Raman spectroscopy to elucidate photochemistry of functional organometallic complexes in solution. The delineated reaction pathways in relation to ligand nucleophilicity and solvent reorientation time provide the rational design principles for solution precursors in nanowrite applications.

Surface-Enhanced Femtosecond Stimulated Raman Spectroscopy at 1 MHz Repetition Rates.

Surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS) is an ultrafast Raman technique that combines the sensitivity of surface-enhanced Raman scattering with the temporal resolution of femtosecond stimulated Raman spectroscopy (FSRS). Here, we present the first successful implementation of SE-FSRS using a 1 MHz amplified femtosecond laser system. We compare SE-FSRS and FSRS spectra measured at 1 MHz and 100 kHz using both equal pump average powers and equal pump energies to demonstrate that higher repetition rates allow spectra with higher signal-to-noise ratios to be obtained at lower pulse energies, a significant advance in the implementation of SE-FSRS. The ability to use lower pulse energies significantly mitigates sample damage that results from plasmonic enhancement of high-energy ultrafast pulses. As a result of the improvements to SE-FSRS developed in this Letter, we believe that SE-FSRS is now poised to become a powerful tool for studying the dynamics of plasmonic materials and adsorbates thereon.

Perpendicular State of an Electronically Excited Stilbene: Observation by Femtosecond-Stimulated Raman Spectroscopy.

In the photoisomerization path of stilbene, a perpendicular state P on the S1 potential energy surface is expected just before internal conversion through a conical intersection S1/S0. For decades the observation of P was thwarted by a short lifetime τP in combination with slow population flow over a barrier. But these limitations can be overcome by ethylenic substitution. Following optical excitation of trans-1,1'-dicyanostilbene, P is populated significantly (τP = 27 ps in n-hexane) and monitored by an exited-state absorption band at 370 nm. Here we report stimulated Raman lines of P. The strongest, at 1558 cm-1, is attributed to stretching vibrations of the phenyl rings. Transient electronic states, resonance conditions, and corresponding Raman signals are discussed.

Origin of Dispersive Line Shapes in Plasmonically Enhanced Femtosecond Stimulated Raman Spectra

A theoretical model is described to explain the observed dispersive-like vibrational line shapes reported in previous studies of plasmonically enhanced (PE) femtosecond stimulated Raman spectroscopy (FSRS). These Raman line shapes are rationalized in terms of interference or heterodyne terms between the PE stimulated Raman response from the molecules located in the plasmonically enhancing regions of Au nanoparticles and the nonlinear emission/scattering due to the resonant surface plasmon resonance (SPR) of these nanoparticles. The treatment of the FSRS signal response follows the standard perturbative polarization approach apart from the familiar g4 field enhancement factor found in spontaneous SERS description, where g represents the local electromagnetic field enhancement due to proximity with the plasmonic nanoparticles. The SPR contribution is modeled as a PE resonant two-level system. The heterodyne cross-term between these two complex third-order responses is large because both of the components ar...

Femtosecond stimulated Raman spectroscopy as a tool to detect molecular vibrations in ground and excited electronic states.

We give a detailed theoretical analysis of the simplest variant of femtosecond stimulated Raman spectroscopy, where a picosecond Raman pump pulse and a femtosecond Raman probe pulse are applied resonantly to a chromophore in thermal equilibrium in the ground electronic state. We demonstrate that this technique is capable of the detection of dephasing-free Raman-like lines revealing vibrational modes not only in the electronic ground state but also in the excited electronic state of the chromophore. The analytical results obtained with simplifying assumptions for the shape of the laser pulses are substantiated by numerical simulations with realistic laser pulses, employing the equation-of-motion phase-matching approach.

Femtosecond Stimulated Raman Spectroscopy

AbstractThe front cover artwork is provided by the group of Prof. Richard A. Mathies (University of California at Berkeley, USA). The image sketches the first step in vision: the 11‐cis to all‐trans isomerization of retinal in the visual pigment rhodopsin, as revealed by femtosecond stimulated Raman spectroscopy. Read the full text of the Review at 10.1002/cphc.201600104.

Femtosecond Stimulated Raman Exposes the Role of Vibrational Coherence in Condensed-Phase Photoreactivity.

Femtosecond spectroscopy has revealed coherent wave packet motion time and time again, but the question as to whether these coherences are necessary for reactivity or merely a consequence of the experiment has remained open. For diatomic systems in the gas phase, such as sodium iodide, the dimensionality of the system requires coordinated atomic motion along the reaction coordinate. Coherent dynamics are also readily observed in condensed-phase multidimensional systems such as chromophores in proteins and solvated charge transfer dimers. Is precisely choreographed nuclear motion (i.e., coherence) required for reactivity in these systems? Can this coherence reveal anything about the reaction coordinate? In this Account, we describe our efforts to tackle these questions using femtosecond stimulated Raman spectroscopy (FSRS). Results of four exemplary systems are summarized to illustrate the role coherence can play in condensed-phase reactivity, the exploitation of vibrational coherence to measure vibrational anharmonicities, and the development of two-dimensional FSRS (2D-FSRS). We begin with rhodopsin, the protein responsible for vertebrate vision. The rhodopsin photoreaction is preternaturally fast: ground-state photoproduct is formed in less than 200 fs. However, the reactively important hydrogen out-of-plane motions as well as various torsions and stretches remain vibrationally coherent long after the reaction is complete, indicating that vibrational coherence can and does survive reactive internal conversion. Both the ultrashort time scale of the reaction and the observed vibrational coherence indicate that the reaction in rhodopsin is a vibrationally coherent process. Next we examine the functional excited-state proton transfer (ESPT) reaction of green fluorescent protein. Oscillations in the phenoxy C-O and imidazolinone C═N stretches in the FSRS spectrum indicated strong anharmonic coupling to a low-frequency phenyl wagging mode that gates the ESPT reaction. In this case, the coherence revealed not only itself but also the mode coupling that is necessary for reactivity. Curious as to whether vibrational coherence is a common phenomenon, we examined two simpler photochemical systems. FSRS studies of the charge transfer dimer tetramethylbenzene:tetracyanoquinodimethane revealed many vibrational oscillations with high signal-to-noise ratio that allowed us to develop a 2D-FSRS technique to quantitatively measure anharmonic vibrational coupling for many modes within a reacting excited state. Armed with this technique, we turned our attention to a bond-breaking reaction, the cycloreversion of a cyclohexadiene derivative. By means of 2D-FSRS, the vibrational composition of the excited-state transition state and therefore the reaction coordinate was revealed. In aggregate, these FSRS measurements demonstrate that vibrational coherences persist for many picoseconds in condensed phases at room temperature and can survive reactive internal conversion. Moreover, these coherences can be leveraged to reveal quantitative anharmonic couplings between a molecule's normal modes in the excited state. These anharmonic couplings are the key to determining how normal modes combine to form a reaction coordinate. It is becoming clear that condensed-phase photochemical reactions that occur in 10 ps or less require coordinated, coherent nuclear motion for efficient reactive internal conversion.