Time-resolved Raman Spectroscopy Research Articles

Ultrafast Spectroscopy at the Central Laser Facility

In this article, we will examine ultrafast spectroscopy techniques and applications, covering time-resolved infrared (TR-IR) spectroscopy, time resolved visible (TA) spectroscopy, two-dimensional infrared (2D-IR) spectroscopy, Kerr-gated Raman spectroscopy, time-resolved Raman and surface sum-frequency generation (SSFG) spectroscopy. In addition to introducing each technique, we will cover some basics, such as what kinds of lasers are used and discuss how these techniques are applied to study a diversity of chemical problems such as photocatalysis, photochemistry, electrocatalysis, battery electrode characterisation, zeolite characterisation and protein structural dynamics.

Ultrafast Polarization-Resolved Phonon Dynamics in Monolayer Semiconductors.

Monolayer transition metal dichalcogenide semiconductors exhibit unique valleytronic properties interacting strongly with chiral phonons that break time-reversal symmetry. Here, we observed the ultrafast dynamics of linearly and circularly polarized E'(Γ) phonons at the Brillouin zone center in single-crystalline monolayer WS2, excited by intense, resonant, and polarization-tunable terahertz pulses and probed by time-resolved anti-Stokes Raman spectroscopy. We separated the coherent phonons producing directional sum-frequency generation from the incoherent phonon population emitting scattered photons. The longer incoherent population lifetime than what was expected from coherence lifetime indicates that inhomogeneous broadening and momentum scattering play important roles in phonon decoherence at room temperature. Meanwhile, the faster depolarization rate in circular bases than in linear bases suggests that the eigenstates are linearly polarized due to lattice anisotropy. Our results provide crucial information for improving the lifetime of chiral phonons in two-dimensional materials and potentially facilitate dynamic control of spin-orbital polarizations in quantum materials.

Operando Raman spectroscopy uncovers hydroxide and CO species enhance ethanol selectivity during pulsed CO2 electroreduction

Pulsed CO2 electroreduction (CO2RR) has recently emerged as a facile way to in situ tune the product selectivity, in particular toward ethanol, without re-designing the catalytic system. However, in-depth mechanistic understanding requires comprehensive operando time-resolved studies to identify the kinetics and dynamics of the electrocatalytic interface. Here, we track the adsorbates and the catalyst state of pre-reduced Cu2O nanocubes ( ~ 30 nm) during pulsed CO2RR using sub-second time-resolved operando Raman spectroscopy. By screening a variety of product-steering pulse length conditions, we unravel the critical role of co-adsorbed OH and CO on the Cu surface next to the oxidative formation of Cu-Oad or CuOx/(OH)y species, impacting the kinetics of CO adsorption and boosting the ethanol selectivity. However, a too low OHad coverage following the formation of bulk-like Cu2O induces a significant increase in the C1 selectivity, while a too high OHad coverage poisons the surface for C-C coupling. Thus, we unveil the importance of co-adsorbed OH on the alcohol formation under CO2RR conditions and thereby, pave the way for improved catalyst design and operating conditions.

Accelerated molecular vibrational decay and suppressed electronic nonlinearities in plasmonic cavities through coherent Raman scattering

Molecular vibrations and their dynamics are of outstanding importance for electronic and thermal transport in nanoscale devices as well as for molecular catalysis. The vibrational dynamics of <100 molecules are studied through three-color time-resolved coherent anti-Stokes Raman spectroscopy using plasmonic nanoantennas. This isolates molecular signals from four-wave mixing (FWM) while using exceptionally low nanowatt powers to avoid molecular damage via single-photon lock-in detection. FWM is found to be strongly suppressed in nanometer-wide plasmonic gaps compared to plasmonic nanoparticles. Simultaneous time-resolved incoherent anti-Stokes Raman spectroscopy allows us to separate the contributions of vibrational population decay (T1) and dephasing (T2). With increasing illumination intensity, the ultrafast vibrational dephasing rates of biphenyl-4-thiol molecules are accelerated at least tenfold, while phonon population decay rates remain constant. The extreme plasmonic field enhancement within nanogaps opens up prospects for measuring single-molecule vibrationally coupled dynamics and diverse molecular optomechanics phenomena. Published by the American Physical Society 2024

Magnesite formation from nesquehonite slurry at 90 °C using some soluble Mg salts: Eitelite as an atypical transient mineral phase

The production of engineered magnesite (MgCO3) under mild conditions remains an important fundamental challenge in order to store permanently the industrial captured CO2. Because, the magnesite is the more-known-stable carbonate at the Earth surface conditions and it is ready-to-use as profitable construction materials. In the present study, time-resolved Raman spectroscopy experiments have been conducted to characterize magnesite formation from nesquehonite slurry at 90 °C in an ionic carbonate alkaline medium (CO32–/HCO3– close to 1) using four different soluble Mg salts (acetate, sulphate, nitrate and chloride of magnesium). The investigated experimental conditions have revealed that only nesquehonite (MgCO3·3H2O) via un amorphous phase transformation is produced at room temperature. However, magnesite was systematically formed from nesquehonite slurry when temperature increase to 90 °C (heat-ageing step). Here, hydromagnesite (Mg5(CO3)4(OH)2·4H2O) and eitelite (Na2Mg(CO3)2) with long lifetimes were the main characterized transient phases prior to magnesite formation depending of the counterions. Moreover, eitelite rare mineral has not been reported as transient phase during magnesite formation. In summary, the imposed carbonate concentration, carbonate speciation, counterions, competitive divalent cations and pH have played a critical role on the magnesite formation at 90 °C. The discovered experimental mild conditions are promising; however, 2–3 days were required in single Mg system, except when dissolved calcium is added (Ca/Mg≈0.15) in the chloride counterion system; for such case only one hour is required to produce Ca-magnesite and protodolomite at 90 °C, i.e., the production of anhydrous Mg-Ca carbonates phases in very reduced time. In this latter case, the monohydrocalcite (CaCO3·H2O) and the nesquehonite precursors produced at room temperature were rapidly transformed via concurrent dissolution into Ca-magnesite (CaxMg1-xCO3) and protodolomite (Ca0.5Mg0.5CO3). In conclusion, these novel insights are relevant for fundamental (direct monitoring of multi-nucleation events in complex ionic and/or slurries systems) and applied (permanent storage of CO2) research on the formation of anhydrous Mg carbonates (e.g., magnesite and dolomite) debated in the last two centuries.

Real-time temperature monitoring technology for dynamic combustion processes using dual-probe femtosecond CARS

We propose a fast responsive and non-invasive optical temperature measurement method which can be used to monitor the temperature of air-fed flames in real time. This method is based on femtosecond time-resolved coherent anti-Stokes Raman spectroscopy (fs-CARS). Two probe pulses with different wavelengths are employed in the CARS system to generate two CARS signals, and the intensity ratio of the two CARS signals has a definite numerical relationship with temperature and therefore can be used to rapidly measure the flame temperature. The temperature of methane/air swirl flames was monitored by the dual-probe fs-CARS thermometry with a sampling rate of 100 Hz. For the steady swirl flame, the measured average temperature in the inner recirculation zone is 1551 K with a standard deviation of 3.7%. While, at a higher methane/air equivalent ratio of 0.74, the flame cannot remain steady, and the temperature frequently jumps from around 1700 K to 2000 K, causing the vortex to expand and break. A significant advantage of the dual-probe fs-CARS thermometry is that the temperature of the flame can be obtained immediately during the measurement without any post-processing. Therefore, this method is a potential candidate for accurately monitoring the real-time temperature of turbulent combustion.

Nonequilibrium DMFT approach to time-resolved Raman spectroscopy

Nonequilibrium DMFT approach to time-resolved Raman spectroscopy

Spectroscopic Techniques to Unravel Mechanistic Details in Light‐Induced Transformations and Photoredox Catalysis

AbstractThe rapid development of photo(redox) catalysis within the last decades is remarkable to the extent that the utilization of light‐driven processes in organic chemistry has become a credible alternative to current thermal processes. Such advances offer tremendous opportunities of collaborations between scientific realms that can have a drastic impact on the development of the field. In this concept article, a special emphasis is placed on spectroscopic techniques that are used, or could be used, for light‐induced transformations and photoredox catalysis applications. These include spectroelectrochemistry, UV‐VIS, IR and X‐Ray transient absorption spectroscopy, laser pulsed radiolysis (PR), photo‐induced chemically induced dynamic nuclear polarization (Photo‐CIDNP), photoacoustic spectroscopy, time‐resolved Raman spectroscopy (TRRS), time‐resolved Electron Paramagnetic Resonance (TREPR) and time‐resolved dielectric loss spectroscopy (TRDL). The theoretical background behind each technique is briefly introduced followed by selected relevant examples from the literature.

Time-resolved Raman spectroscopy for monitoring the structural evolution of materials during rapid compression.

Rapid compression experiments performed using a dynamic diamond anvil cell (dDAC) offer the opportunity to study compression rate-dependent phenomena, which provide critical knowledge of the phase transition kinetics of materials. However, direct probing of the structure evolution of materials is scarce and so far limited to the synchrotron based x-ray diffraction technique. Here, we present a time-resolved Raman spectroscopy technique to monitor the structural evolutions in a subsecond time resolution. Instead of applying a shutter-based synchronization scheme in previous work, we directly coupled and synchronized the spectrometers with the dDAC, providing sequential Raman data over a broad pressure range. The capability and versatility of this technique are verified by in situ observation of the phase transition processes of three rapid compressed samples. Not only the phase transition pressures but also the transition pathways are reproduced with good accuracy. This approach has the potential to serve as an important complement to x-ray diffraction applied to study the kinetics of phase transitions occurring on time scales of seconds and above.

Carboxyl-Alkyl Functionalized Conjugated Polyelectrolytes for High Performance Organic Electrochemical Transistors.

Contemporary design principles for organic mixed ionic electronic conductors (OMIECs) are mostly based on the ethylene glycol moiety, which may not be representative of the OMIEC class as a whole. Furthermore, glycolated polymers can be difficult to synthesize and process effectively. As an emerging alternative, we present a series of polythiophenes functionalized with a hybrid carboxyl-alkyl side chain. By variation of the alkyl spacer length, a comprehensive evaluation of both the impact of carboxylic acid functionalization and alkyl spacer length was conducted. COOH-functionalization endows the polymer with preferential intrinsic low-swelling behavior and water processability to yield solvent-resistant conjugated polyelectrolytes while retaining substantial electroactivity in aqueous environments. Advanced in situ techniques, including time-resolved spectroelectrochemistry and Raman spectroscopy, are used to interrogate the materials' microstructure, ionic-electronic coupling, and operational stability in devices. To compare these materials' performance to state-of-the-art technology for the design of OMIECs, we benchmarked the materials and demonstrated significant application potential in both planar and interdigitated organic electrochemical transistors (OECTs). The polythiophene bearing carboxyl-butyl side chains exhibits greater electrochemical performance and faster doping kinetics within the polymer series, with a record-high OECT performance among conjugated polyelectrolytes ([μC*]pOECT = 107 ± 4 F cm-1 V-1 s-1). The results provide an enhanced understanding of structure-property relationships for conjugated polyelectrolytes operating in aqueous media and expand the materials options for future OMIEC development. Further, this work demonstrates the potential for conjugated polymers bearing alkyl-COOH side chains as a path toward robust OMIEC designs that may facilitate further facile (bio)chemical functionalization for a range of (bio)sensing applications.

G phonon linewidth and phonon-phonon interaction in p-type doped CVD graphene crystals

We employed conventional and time-resolved Raman spectroscopy to study the effects of electron-phonon and phonon-phonon interactions on the linewidth of the G phonon in both unintentionally and intentionally doped polycrystalline chemical vapor deposited (CVD) graphene samples. We directly measured the G phonon lifetime, observing a ≈14% decrease with doping up to EF = 270 meV. The anticipated reduction of G phonon linewidth due to a decrement in electron-phonon contribution deviates from first principles calculations. CVD samples exhibit a ≈30% decrease in the electron-phonon coupling constant, λΓ(EF = 0), compared to exfoliated graphene samples. Additionally, phonon-defect scattering makes a significant contribution to the G band linewidth in CVD graphene samples, owing to their lower crystal quality compared to exfoliated graphene.

What is cooking in your kitchen: seeing "invisible" with time-resolved coherent anti-Stokes Raman spectroscopy.

Cooking oil is a critical component of human food and its main component, lipid, is influential to health, but assessing its authenticity and quality can be challenging due to its complex chemical composition. In this study, we introduce a novel application of time-resolved coherent anti-Stokes Raman scattering (T-CARS) spectroscopy for detecting adulteration and understanding the mechanisms of lipid oxidation in various cooking oils. Our research surpasses the limitations of conventional spontaneous Raman spectroscopy, demonstrating that intra-molecular interactions from unsaturated bonds in triglycerides significantly influence vibrational dephasing time. We observed that these dephasing times, although diverse initially, converge to a similar value after heating cycles. Notably, a longer vibrational dephasing of the CH2 symmetric stretching mode was found to correlate with a higher lipid oxidation rate. These findings underscore the potential of T-CARS in identifying and characterizing subtle molecular interactions, offering a transformative approach to understanding molecular dynamics. This research paves the way for broader applications of T-CARS across fields such as chemistry, biomedicine, and material science, marking a significant advancement in the development of innovative analytical techniques.

In situ time-resolved Raman spectroscopy of nitromethane under static and dynamic compression

Energetic materials are extensively used as propellants in rockets demanding the understanding of their chemical and thermal stability for safe storage and transportation as well as ease of decomposition. Nitromethane (NM) is one such material with significant performance advantage over other mono propellants. In this manuscript, we report the detailed molecular-level behavior of NM under static and dynamic compression. Dynamic compression experiments were performed up to ∼6.4 GPa using a 2 J/8 ns Nd: YAG laser coupled with time-resolved Raman spectroscopy (TRRS) setup. Static compression experiments were performed up to ∼20 GPa using a diamond anvil cell. During laser-driven shock compression, NM undergoes three phase transitions at 1.1, 2.5, and 3.4 GPa. However, in the case of static compression, the corresponding phase transitions were observed at 0.3, 1.3–1.8, and 2.5 GPa. TRRS was also performed at 300 mJ (1.47 GW/cm2), 500 mJ (2.45 GW/cm2), and 800 mJ (3.9 GW/cm2) and intensity ratios of shocked and un-shocked Raman peaks were utilized to experimentally calculate the shock velocities, which were determined to be 2.66 ± 0.09, 3.58 ± 0.40, and 3.83 ± 0.60 km/s, respectively. These experimental results were corroborated with the one-dimensional (1D) radiation hydrodynamics simulations, performed to obtain shock pressure. The shock velocities at these laser intensities were calculated to be 2.98, 3.69, and 3.92 km/s, respectively, which are in reasonably close agreement with our observed results.

Revealing Kinetics of Paracetamol Crystallization Using Time-Resolved Raman Spectroscopy, Orthogonal Time-Lapse Photography, and Non-Negative Matrix Factorization (OSANO)

Revealing Kinetics of Paracetamol Crystallization Using Time-Resolved Raman Spectroscopy, Orthogonal Time-Lapse Photography, and Non-Negative Matrix Factorization (OSANO)

Energy Transfer Characteristics of Lipid Bilayer Membranes of Liposomes Examined with Picosecond Time-Resolved Raman Spectroscopy.

A number of biochemical reactions proceed inside biomembranes. Since the rate of a chemical reaction is influenced by chemical properties of the surrounding environment, it is important to examine the chemical environment inside the biomembranes. Although the energy transfer characteristics are a basic and important property of a reaction medium, experimental investigation of the thermal conducting capabilities of the biomembranes is a challenging task. We have examined the energy transfer characteristics of lipid bilayer membranes of liposomes, a good model system for the biomembrane, with picosecond time-resolved Raman spectroscopy. The cooling kinetics of the first excited singlet (S1) state of trans-stilbene solubilized within the lipid bilayer membranes is observed as a peak shift of the 1570 cm-1 Raman band of S1 trans-stilbene. The cooling rate constant of S1 trans-stilbene is obtained in six lipid bilayer membranes formed by phospholipids with different hydrocarbon chains, DSPC, DPPC, DMPC, DLPC, DOPC, and egg-PC. We estimate the thermal diffusivity of the lipid bilayer membranes with a known correlation between the cooling rate constant and the thermal diffusivity of the solvent. The thermal diffusivity estimated for the liquid-crystal-phase lipid bilayer membranes is 8.9 × 10-8 to 9.4 × 10-8 m2 s-1, while that for the gel-phase lipid bilayer membranes is 8.4 × 10-8 to 8.5 × 10-8 m2 s-1. The difference in thermal diffusivity between the two phases is explained by a one-dimensional diffusion equation of heat.

Selective Analysis of Redox Processes at the Electrode Interface with Time-Resolved Raman Spectroscopy.

Electrochemistry and electrochemical reactions are increasingly important in the transition to a sustainable chemical industry. The electron transfer that drives such reactions takes place within nanometers of the electrode surface, and follow-up chemical reactions take place within the diffusion layer. Hence, understanding electrochemical reactions requires time-, potential-, and spatially resolved analysis. The confocal nature of Raman spectroscopy provides high spatial resolution, in addition to detailed information on molecular structure. The intrinsic weakness of nonresonant Raman scattering, however, is not sensitive enough for relatively minor changes to the solution resulting from reactions at the electrode interface. Indeed, the limit of detection is typically well above the concentrations used in electrochemical studies. Here, we show that surface-enhanced Raman scattering (SERS) and resonance Raman (rR) spectroscopy allow for spatially and time-resolved analysis of solution composition at (<1-2 nm) and near (within 5 μm) the electrode surface, respectively, in a selective manner for species present at low (<1 mM) concentrations. We show changes in concentration of species at the electrode surface, without the need for labels, specific adsorption, or resonance enhancement, using a SERS-active gold electrode prepared readily by electrochemical surface roughening. A combination of smooth and roughened gold electrodes is used to distinguish between surface and resonance enhancement using the well-known redox couples ferrocene and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). We discuss the impact of specific adsorption on the spectral analysis with the ruthenium(II) polypyridyl complex, [Ru(bpy)3]2+. The dual function of the electrode (surface enhancement and electron transfer) in the analysis of solution processes is demonstrated with the reversible oxidation of TMA (4,N,N-trimethylaniline), where transient soluble species are identified in real time, with rapid spectral acquisition, making use of localized enhancement. We anticipate that this approach will find use in elucidating electro(catalytic) reactions at electrode interfaces.

Immobilization of Phosphamide on the TiO2 Surface for Heterogeneous Phase Catalytic Appel Reaction

Global consumption of triphenylphosphine (Ph3P) for phosphorus-mediated organic synthesis and production of the dead-end triphenylphosphine oxide (Ph3PO) waste is exceptionally high. Recycling Ph3PO and/or use of it as a reaction mediator gained significant attention. On the other end, phosphamides, traditionally used as a flame redundant, are stable analogues to Ph3PO. Herein, via a low temperature condensation reaction of methyl 4-(aminomethyl)benzoate (AMB) and diphenyl phosphinic chloride (DPPC), methyl 4-((N,N-diphenylphosphinamido)methyl)benzoate (1) has been synthesized and hydrolysis of the ester functional group of 1 leads to a phosphamide with a carboxylate terminal, 4-((N,N-diphenylphosphinamido)methyl)benzoic acid (2). The presence of phosphamide functionality (NH─P═O) in 2 can be confirmed by its characteristic Raman vibration at 999 cm-1 with expected P-N and P═O bonds distances from the single-crystal X-ray structure. In-situ hydrolysis of [Ti(OiPr)4] in the presence of 2 followed by hydrothermal heating results in immobilization of 2 on a ca. 5 nm TiO2 surface (2@TiO2). The covalent attachment of 2 via coordination through the carboxylate terminal to the TiO2 nanocrystal's surface has been established via multiple spectroscopic and microscopic studies. 2@TiO2 is further used as the heterogeneous mediator for the catalytic Appel reaction, halogenation of alcohol (typically mediated by phosphine), with a fair catalytic conversion and a recorded TON up to 31. The major advantage of the heterogeneous approach studied herein is the recovery of used 2@TiO2 from the reaction mixture via centrifugation only leaving the organic product in the supernatant, which is limiting in Ph3P-mediated homogeneous catalysis. Time-resolved Raman spectroscopy confirms amino phosphine as the active species formed in-situ during the catalytic Appel reaction. Post-catalytic characterization of the material recovered after catalysis from the reaction mixture confirms the chemical integrity and that can further be utilized for another two catalytic runs. The developed reaction scheme showcases the use of a phosphamide as a reactive analogue to Ph3PO for an organic reaction in a heterogeneous approach, and the same strategy can be explored further as a general scheme for other phosphorus-mediated reactions.

Local Built-In Field at the Sub-nanometric Heterointerface Mediates Cascade Electrochemical Conversion of Lithium-sulfur Batteries.

Heterogeneous catalytic mediators have been proposed to play a vital role in enhancing the multiorder reaction and nucleation kinetics in multielectron sulfur electrochemistry. However, the predictive design of heterogeneous catalysts is still challenging, owing to the lack ofin-depth understanding of interfacial electronic states and electron transfer on cascade reaction in Li-S batteries. Here, a heterogeneous catalytic mediator based on monodispersed titanium carbide sub-nanoclusters embedded in titanium dioxide nanobelts is reported. The tunable catalytic and anchoring effects of the resulting catalyst are achieved by the redistribution of localized electrons caused by the abundant built-in fields in heterointerfaces. Subsequently, the resulting sulfur cathodes deliver an areal capacity of 5.6 mAh cm-2 and excellent stability at 1 C under sulfur loading of 8.0mg cm-2 . The catalytic mechanism especially on enhancing the multiorder reaction kinetic of polysulfides is further demonstrated via operando time-resolved Raman spectroscopy during the reduction process in conjunction with theoretical analysis.

CMOS SPAD Line Sensor With Fine-Tunable Parallel Connected Time-to-Digital Converters for Raman Spectroscopy

A 256-channel single-photon avalanche diode (SPAD) line sensor was designed for time-resolved Raman spectroscopy in 110-nm CMOS technology. The line sensor consists of an 8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> 256 SPAD array and 256 parallel connected time-to-digital converters (TDCs). The adjustable temporal resolution and dynamic range of TDCs are 25.6–65 ps and 3.2–8.2 ns, respectively. The median timing skew along 256 channels is 43.7 ps, and TDC bin boundaries can be fine-tuned at the ps-level to enable precise timing skew compensation. The sensor is capable of real-time dark count measurement (two dark measurements for each excitation pulse) that gives accurate data for dark count compensation without any increment in measurement time. The maximum excitation pulse rate with real-time dark count measurement is 680 kHz. Raman spectra of six different samples were measured to prove the performance of the sensor in time-resolved Raman spectroscopy.

Surface hydroxide promotes CO2 electrolysis to ethylene in acidic conditions

Performing CO2 reduction in acidic conditions enables high single-pass CO2 conversion efficiency. However, a faster kinetics of the hydrogen evolution reaction compared to CO2 reduction limits the selectivity toward multicarbon products. Prior studies have shown that adsorbed hydroxide on the Cu surface promotes CO2 reduction in neutral and alkaline conditions. We posited that limited adsorbed hydroxide species in acidic CO2 reduction could contribute to a low selectivity to multicarbon products. Here we report an electrodeposited Cu catalyst that suppresses hydrogen formation and promotes selective CO2 reduction in acidic conditions. Using in situ time-resolved Raman spectroscopy, we show that a high concentration of CO and OH on the catalyst surface promotes C-C coupling, a finding that we correlate with evidence of increased CO residence time. The optimized electrodeposited Cu catalyst achieves a 60% faradaic efficiency for ethylene and 90% for multicarbon products. When deployed in a slim flow cell, the catalyst attains a 20% energy efficiency to ethylene, and 30% to multicarbon products.