Time-resolved Spectroscopy Research Articles

Quantum-enhanced time-domain spectroscopy.

The time-resolved detection of mid- to far-infrared electric fields absorbed and emitted by molecules is among the most sensitive spectroscopic approaches and has the potential to transform sensing in fields such as security screening, quality control, and medical diagnostics. However, the sensitivity of the standard detection approach, which relies on encoding the far-infrared electric field into amplitude modulation of a visible or near-infrared probe laser pulse, is limited by the shot noise of the latter. This constraint cannot be overcome without using a quantum resource. Here, we show that this constraint can be overcome using a two-mode squeezed state. Quantum-correlated ultrashort pulses, generated by parametric down-conversion, enhance the sensitivity of far-infrared detection beyond the classical limit, achieving a twofold reduction in measured noise. This advancement paves the way for further development of ultrafast quantum metrology, moving toward quantum-enhanced time-resolved electric field spectroscopy with sensitivities beyond the standard quantum limit.

Time-Resolved X-Ray Spectroscopy from the Atomic Orbital Ground State Up

Time-Resolved X-Ray Spectroscopy from the Atomic Orbital Ground State Up

Ultrafast Intersystem Crossing in Naturally Occurring Plant Pigments 5-Hydroxyflavones under Direct UV Excitation.

Flavonoids, a group of natural pigments, have attracted notable attention for their intrinsic fluorescent bioactive properties and potential therapeutic implications. Recent studies have suggested that the photoexcitation of specific flavonoids can also lead to the formation of triplet states, thereby potentially enhancing their applications in photoactivated antioxidant mechanisms. However, the crucial mechanism details about triplet state formation are still poorly understood. In this Letter, the ultrafast excited state relaxation mechanism for a series of 5-hydroxyflavone derivatives was studied by femtosecond time-resolved spectroscopy combined with quantum chemical calculations. Our results reveal the ultrafast ISC (kISC ≈ 1011 s-1) channel, which is sensitive to molecular structure and solvent environment, in 5-hydroxyflavones for the first time. Notably, the triplet excited state quantum yield of 4',7-dimethoxy-5-hydroxyflavone can reach up to 8% in acetonitrile solution. These results are essential for understanding the triplet state generation mechanism in 5-hydroxyflavone derivatives and could help the further development of 5-hydroxyflavone scaffold antioxidant agents.

Excited-State Proton Transfer Dynamics of Cyanonaphthol in Protic Ionic Liquids: Concerted Effects of Basicity of Anions and Alkyl Carbons in Cations.

Excited-state proton transfer (ESPT) reactions of 5-cyano-2-naphthol (5CN2) and 5,8-dicyano-2-naphthol (DCN2) were investigated in protic ionic liquids (PILs) composed of quaternary ammonium (NnnnH+) (n = 2, 4, or 8) and hexanoate (C5H11COO-) using time-resolved fluorescence spectroscopy. The effects of the number of alkyl carbons in the cation and the basicity of the anion on the reaction yield and dynamics were examined. In a series of [NnnnH][C5H11COO], fluorescence from the hydrogen-bonding complex (AHBX-*) of a proton-dissociated form (RO-*) with a solvent acid in the electronic excited state was observed between the fluorescence bands of an acidic form (ROH*) and an anionic form (RO-*) as in the case of [N222H][CF3COO] (Fujii et al., J. Phys. Chem B, 2017, 121, 6042). The yield, formation rate, and decay rate of AHBX-* were assessed by the steady-state fluorescence intensity ratio of AHBX-* to RO-* and by analysis of the time-resolved fluorescence spectra within several tens of nanoseconds. The stability of AHBX-* was significantly different from that in [N222H][CF3COO] and depended on the number of alkyl carbons in the cation. The formation of AHBX-* was quite fast compared to the case in [N222H][CF3COO] and almost close to the excitation pulse width. Excitation wavelength dependence of the fluorescence dynamics was observed within several hundred picoseconds for the series of [NnnnH][C5H11COO]. The origin was ascribed to the complex with solvent IL formed in the electronic ground state, as suggested by the density-functional theory (DFT) calculations.

Absolute line strength measurements of HO2 radical in the OO-stretching fundamental band between 1088 and 1124cm-1 using time-resolved dual-comb spectroscopy.

Absolute line strength measurements of hydroperoxyl (HO2) radical in the OO-stretching (ν3) fundamental band have been performed by means of mid-infrared time-resolved dual-comb spectroscopy. By employing two sets of dual-comb spectrometers, high-resolution time-resolved spectra of HO2 and HCl, formed in the photolysis reaction system of Cl2/CH3OH/O2, could be, respectively, measured near 1123 and 3059cm-1. With kinetic simulations, spectral analysis of both HO2 and HCl, as well as the accurate line strength of the HCl R(9) transition at 3059.316cm-1, an absolute line strength of the ν3 131,13 ← 121,12 F1,2 transitions in HO2 at 1122.983cm-1 was first determined to be 1.80 × 10-20cmmolecule-1 with a small uncertainty down to 4% under the conditions with low initial concentrations of Cl radical (1.63-1.81 × 1013moleculecm-3). Furthermore, broadband high-resolution spectra of the ν3 fundamental band of HO2 were recorded in the range of 1088-1124cm-1 with an average spectral resolution of 0.002cm-1. By contour fitting the measured broadband spectra with PGOPHER, the line strengths of hundreds of rovibrational transitions were obtained relative to the well-determined HO2 lines at 1122.983cm-1, and those values were observed to be higher than those tabulated in the HITRAN database by a factor of ∼2.8. Moreover, the absolute band strength of the ν3 fundamental band in HO2 was derived to be 22.3 km mol-1 with an uncertainty of 5%. This work providing precise and detailed spectral data would be crucial in revisiting the theoretical modeling of HO2 geometry and updating the database of the HO2 radical.

Insight into exciton polaritons of two-dimensional transition metal dichalcogenides with time-resolved spectroscopy

Insight into exciton polaritons of two-dimensional transition metal dichalcogenides with time-resolved spectroscopy

Selenium-Substitution Strategy for Enhanced Mobility, Tunable Bandgap, and Improved Electrochemical Energy Storage in Semiconducting Conjugated Coordination Polymers.

Conjugated coordination polymers (c-CPs), a novel class of organic-inorganic hybrid materials, are distinguished by their unique structural characteristics and exceptional charge transport properties. The electronic properties of these materials are critically determined by the constituting coordination atoms, with electron-rich selenol ligands emerging as promising candidates for constructing high-mobility semiconducting c-CPs. Currently,c-CPs incorporating selenium-substituted ligands remain scarce. In this study, we successfully synthesized a new tetraseleno-hydroxyquinone (TSHQ) ligand using a "4+2" design strategy and developed a semiconducting three-dimensional Ag-Se coordination polymer, Ag4TSHQ. Ag4TSHQ exhibits room-temperature electrical conductivity of up to 1.6 S/m and shares the same structural topology as Ag4TTHQ (TTHQ = tetrathiol-hydroxyquinone), enabling precise bandgap modulation from 0.6 eV to 1.5 eV via a mixed-ligand approach. Time-resolved terahertz spectroscopy reveals that the charge mobility of Ag4TSHQ in the dc limit is ~350 cm2/V⋅s, which is twice that of its sulfur counterpart, Ag4TTHQ. Furthermore, our evaluations of their electrochemical energy storage capabilities demonstrate that Ag4TSHQ effectively utilizes its redox potential, achieving a remarkable specific capacitance of up to 340 F/g-significantly outperforming Ag4TTHQ, which has a capacitance of 294 F/g. These findings underscore the potential of selenium-ligand-based c-CPs for optoelectronic applications and energy storage technologies.

Effect of growth dynamics on the structural, photophysical and pseudocapacitance properties of famatinite copper antimony sulphide colloidal nanostructures (including nanosheets).

Facile phase selective synthesis of copper antimony sulphide (CAS) nanostructures is important because of their tunable photoconductive and electrochemical properties. In this study, off-stoichiometric famatinite phase CAS (fCAS) quasi-spherical and quasi-hexagonal colloidal nanostructures (including nanosheets) of sizes, 2.4-18.0 nm were grown under variable conditions of temperature (60-200 °C), time and oleylamine capping ligand concentration using copper(II) acetylacetonate and antimony(III) diethyldithiocarbamate precursors. Data from powder X-ray diffraction, Raman spectroscopy and high-resolution scanning/transmission electron microscopy confirm the tetragonal structure of the famatinite phase. X-ray photoelectron spectroscopy, transmission electron microscopy and scanning electron microscopy-energy dispersive X-ray spectroscopy data suggest a correlation of particle size, morphology and composition of the off-stoichiometric fCAS nanostructures with growth temperature and time, and oleylamine concentration. The off-stoichiometric Cu3-aSb1+bS4±c (a, b, c - mole fractions) nanostructures being severely copper-deficient and antimony-rich, exhibit shallow-lying acceptor copper vacancy states, deep-lying donor states of antimony interstitials, sulphur vacancies and antimony-copper antisites and shallow-lying acceptor surface trapping states. These electronic states are likely implicated in tunable UV-visible absorption and bandgaps between 2.3 and 2.8 eV, and broad visible-NIR photoluminescence with fast recombination of radiative lifetimes between 0.2 and 6.2 ns, confirmed from absorption, steady-state and time-resolved photoluminescence spectroscopies. Additionally, cyclic voltammetry and electrochemical impedance spectroscopy confirm that electrodes of the fCAS nanostructures display slightly variable pseudocapacitance of charge-storage primarily via possible sodium ion intercalation with a high specific capacitance of ∼84 F g-1 obtained at a scan rate of 5 mV s-1. Overall, these results show the influence of composition, in particular point defects, phase quality and morphology on the optical and pseudocapacitance properties of fCAS nanostructures, suitable as solar absorbers or electrodes for energy storage devices.

Nanosecond Nanothermometry in an Electron Microscope.

Thermal transport in nanostructures plays a critical role in modern technologies. As devices shrink, techniques that can measure thermal properties at nanometer and nanosecond scales are increasingly needed to capture transient, out-of-equilibrium phenomena. We present a novel pump-probe photon-electron method within a scanning transmission electron microscope (STEM) to map temperature dynamics with unprecedented spatial and temporal resolutions. By combining focused laser-induced heating and synchronized time-resolved monochromated electron energy-loss spectroscopy (EELS), we track phonon, exciton, and plasmon signals in various materials, including silicon nitride, aluminum thin film, and transition metal dichalcogenides. Our results demonstrate the technique's ability to follow temperature changes at the nanometer and nanosecond scales. The experimental data closely matched theoretical heat diffusion models, confirming the method's validity. This approach opens new opportunities to investigate transient thermal phenomena in nanoscale materials, offering valuable insights for applications in thermoelectric devices and nanoelectronics.

Time-resolved spectroscopy uncovers deprotonation-induced reconstruction in oxygen-evolution NiFe-based (oxy)hydroxides

Transition-metal layered double hydroxides are widely utilized as electrocatalysts for the oxygen evolution reaction (OER), undergoing dynamic transformation into active oxyhydroxides during electrochemical operation. Nonetheless, our understanding of the non-equilibrium structural changes that occur during this process remains limited. In this study, utilizing in situ energy-dispersive X-ray absorption spectroscopy and machine learning analysis, we reveal the occurrence of deprotonation and elucidate the role of incorporated iron in facilitating the transition from nickel-iron layered double hydroxide (NiFe LDH) into its active oxyhydroxide. Our findings demonstrate that iron substitution promotes deprotonation process within NiFe LDH, resulting in the preferential removal of protons from the specific bridged hydroxyl group (Ni2+-OH-Fe3+) linked to edge-sharing [NiO6] and [FeO6] octahedron. This deprotonation behavior drives the formation of high-valence Ni3+δ species (0 <δ < 1), which subsequently serve as the active sites, thereby ensuring efficient oxygen evolution activity. This approach offers high-resolution insights of dynamic structural evolution, overcoming the limitations of extended acquisition times and advancing our understanding of OER mechanisms.

A Cost-Effective Computational Strategy for the Electronic Layout Characterization of a Second Generation Light-Driven Molecular Rotary Motor in Solution.

Light-driven molecular rotary motors are nanometric machines able to convert light into unidirectional motions. Several types of molecular motors have been developed to better respond to light stimuli, opening new avenues for developing smart materials ranging from nanomedicine to robotics. They have great importance in the scientific research across various disciplines, but a detailed comprehension of the underlying ultrafast photophysics immediately after photo-excitation, that is, Franck-Condon region characterization, is not fully achieved yet. For this aim, it is first required to rely on an accurate description at ab initio level of the system in this potential energy region before performing any further step, that is, dynamics. Thus, we present an extensive investigation aimed at accurately describing the electronic structure of low-lying electronic states (electronic layout) of a molecular rotor in the Franck-Condon region, belonging to the class of overcrowded alkenes: 9-(2-methyl-2,3-dihydro-1H-cyclopenta[a]naphthalen-1-ylidene)-9H-fluorene. This system was chosen since its photophysics is very interesting for a more general understanding of similar compounds used as molecular rotors, where low-lying electronic states can be found (whose energetic interplay is crucial in the dynamics) and where the presence of different substituents can tune the HOMO-LUMO gap. For this scope, we employed different theory levels within the time-dependent density functional theory framework, presenting also a careful comparison adopting very accurate post Hartree-Fock methods and characterizing also the different conformations involved in the photocycle. Effects on the electronic layout of different functionals, basis sets, environment descriptions, and the role of the dispersion correction were all analyzed in detail. In particular, a careful treatment of the solvent effects was here considered in depth, showing how the implicit solvent description can be accurate for excited states in the Franck-Condon region by testing both linear-response and state-specific formalisms. As main results, we chose two cost-effective (accurate but relatively cheap) theory levels for the ground and excited state descriptions, and we also verified how choosing these different levels of theory can influence the curvature of the potential via a frequency analysis of the normal modes of vibrations active in the Raman spectrum. This theoretical survey is a crucial step towards a feasible characterization of the early stage of excited states in solution during photoisomerization processes wherein multiple electronic states might be populated upon the light radiation, leading to a future molecular-level interpretation of time-resolved spectroscopies.

Molecular engineering charge transfer and triplet exciton formation in donor-acceptor cocrystals.

Organic donor-acceptor (D-A) cocrystals are gaining attention for their potential applications in optoelectronic devices. This study explores the dynamics of charge transfer (CT) and triplet exciton formation in various D-A cocrystals. By examining a series of D-A cocrystals composed of coronene (COR), peri-xanthenoxanthene (PXX), and perylene (PER) donors paired with N,N-bis(3'-pentyl)perylene-3,4:9,10-bis(dicarboximide) (PDI), naphthalene-1,4:5,8-tetracarboxy-dianhydride (NDA), or pyrene-4,5,9,10-tetraone (PTO) acceptors, using transient absorption microscopy and time-resolved electron paramagnetic resonance spectroscopy, we find that the strength of the CT interaction influences the nature and yield of triplet excitons produced by CT state recombination. In particular, in the PER-PDI, COR-PTO, and PER-PTO cocrystals, localized triplet excitons are lower in energy than the CT state. By contrast, no localized triplet excitons are available to the CT states of the PXX-NDA, PER-NDA, and PXX-PTO cocrystals, and as a result, the CT states rapidly decay to ground state with no triplet formation. Moreover, density functional theory calculations show that the transition between delocalized CT states to a triplet state localized to a single donor or acceptor unit provides the source of spin-orbit coupling necessary when the triplet states are energetically accessible. These findings provide insights into the design of molecular materials with tailored exciton properties for optoelectronic applications.

The Q-Band Energetics and Relaxation of Chlorophylls a and b as Revealed by Visible-to-Near Infrared Time-Resolved Absorption Spectroscopy.

Chlorophyll (Chl) is the most abundant light-harvesting pigment of oxygenic photosynthetic organisms; however, the Q-band energetics and relaxation dynamics remain unclear. In this work, we have applied femtosecond time-resolved (fs-TA) absorption spectroscopy in 430-1,700 nm to Chls a and b in diluted pyridine solutions under selective optical excitation within their Q-bands. The results revealed distinct near-infrared absorption features of the Bx,y ← Qy and Bx,y ← Qx transitions in 930-1,700 nm, which together with the steady-state absorption in 400-700 nm unveiled the Qx(0,0)-state energy that lies 1,000 ± 400 and 600 ± 400 cm-1 above the Qy(0,0)-state for Chls a and b, respectively. In addition, the Qx-to-Qy internal conversion time constants are estimated to be less than 80 fs for Chls a and b. These findings may shed light on understanding the roles of the Chls in the primary excitation energy transfer reactions of photosynthesis.

Data Analysis Methods in Time-Resolved Fluorescence Spectroscopy: A Tutorial Review.

Fluorescence spectroscopy and related techniques benefit from exceptional sensitivity and have become engrained in a variety of fields from biosciences to materials sciences. Measuring time-domain fluorescence decays is nowadays a routine task in many laboratories across these different fields. Perhaps surprisingly, a correct data analysis of these fluorescence decay curves presents a formidable challenge and requires extensive insight in the problems associated with fitting this type of data. As a result, the reported analysis of these decays is usually limited to a non-linear least squares fit of a sum of a few exponential terms to the data. This review aims to expose the intricate field of data analysis in time-resolved fluorescence spectroscopy to a broader audience, from researchers interested in understanding the photophysics of their system to readers and reviewers trying to understand the merits of specific methods. Challenges associated with this type of kinetic experimental data are outlined and the clever analysis strategies devised by researchers across different disciplines are introduced and discussed in detail. A section on freely available scripts and software facilitating the analysis is included towards the end. We encourage the reader to try their hand at the worked examples.

Terahertz Saturable Absorption across Charge Separation in Photoexcited Monolayer Graphene/MoS2 Heterostructure.

Unveiling the nonlinear interactions between terahertz (THz) electromagnetic waves and free carriers in two-dimensional materials is crucial for the development of high-field and high-frequency electronic devices. Herein, we investigate THz nonlinear transport dynamics in a monolayer graphene/MoS2 heterostructure using time-resolved THz spectroscopy with intense THz pulses as the probe. Following ultrafast photoexcitation, the interfacial charge transfer establishes a nonequilibrium carrier redistribution, leaving free holes in the graphene and trapping electrons in the MoS2. When probed with intense THz pulses exceeding 34 kV/cm in a peak electric field, significant THz saturable absorption is observed over a period of 20 ps. Furthermore, the photoinduced change in the transmitted THz waveform, linked to the THz-driven nonlinear current, manifests as a substantial self-phase modulation. These nonlinear responses can be attributed to the competition between rapid carrier heating and slow carrier cooling via electron-electron and electron-phonon scattering in the charge-transfer-induced hole system of the graphene layer. This work demonstrates an integration of advantages arising from robust nonlinear absorption in graphene and enhanced photocarrier harvesting in transition metal dichalcogenides by exploiting heterostructure construction.

An ultrafast algorithm for ultrafast time-resolved coherent Raman spectroscopy

Time-resolved coherent Raman spectroscopy (CRS) is a powerful non-linear optical technique for quantitative, in-situ analysis of chemically reacting flows, offering unparalleled accuracy and exceptional spatiotemporal resolution. Its application to large polyatomic molecules, crucial for understanding reaction dynamics, has thus far been limited by the complexity of their rotational-vibrational Raman spectra. Progress in developing comprehensive spectral codes for these molecules, a longstanding goal, has been hindered by prohibitively long computation times required for their spectral synthesis. Here, we present an algorithm that achieves a million-fold improvement in computation time compared to existing methods. The algorithm demonstrates remarkable accuracy, with an approximation error below 0.1% across all tested probe delays, at both room temperature (296 K) and elevated temperatures (1500 K). This result could greatly expand the application of time-resolved CRS, particularly in plasma research, as well as in broader atmospheric and astrophysical sciences.

Modulating hot carrier relaxation and trapping dynamics in lead halide perovskite nanoplatelets by surface passivation.

Two-dimensional (2D) lead halide perovskite (LHP) nanoplatelets (NPLs) have recently emerged as promising materials for solar cells and light-emitting devices. The reduction of LHP dimensions introduces an abundance of surface defects, which can strongly influence the photophysical properties of these materials. However, an insightful understanding of the effect of surface defects on hot carrier (HC) relaxation, one of the important properties of LHP NPLs, is still inadequate. Herein, the HC relaxation and trapping dynamics in pristine and surface passivated two-layer (2L) CsPbBr3 NPLs have been investigated by using time-resolved spectroscopy. The results reveal that surface defects can trap HCs directly before they relax to the band edge, which accounts for the absence of the hot-phonon bottleneck (HPB) effect in LHP NPLs. After healing surface defects with a passivation agent, the relaxation time of HCs is extended from ∼73 to ∼130 fs in 2L CsPbBr3 NPLs, indicating that the channel of HCs trapped by the surface defects can be effectively blocked. Accordingly, the HPB effect is activated in surface-passivated CsPbBr3 NPLs. The finding of surface defect-related HC relaxation dynamics is important for guiding the development of high-performance LHP NPL devices related to HCs through surface defect engineering.

Time-resolved fluorescence of ANS dye as a sensor of proteins LLPS.

The explosive growth in the number of works addressing the phase separation of intrinsically disordered proteins has driven both the development of new approaches and the optimization of existing methods for biomolecular condensate visualization. In this work, we studied the potential use of the fluorescent dye ANS as a sensor for liquid-liquid phase separation (LLPS), focusing on visualizing condensates formed by the stress-granules scaffold protein G3BP1. Using fluorescence lifetime imaging microscopy (FLIM), we demonstrated that ANS can accumulate in RNA-induced G3BP1 condensates in aqueous solutions, but not in G3BP1 condensates formed under macromolecular crowding conditions in highly concentrated PEG solutions. We showed that the experimentally determined limiting fluorescence anisotropy (r0'), which characterizes the amplitude of high-frequency intramolecular mobility of ANS in aqueous solutions containing RNA-induced G3BP1 condensates, is half the value observed for ANS in aqueous G3BP1 solutions. Our results demonstrate the feasibility of using time-resolved fluorescence spectroscopy and microscopy of ANS for detecting LLPS of intrinsically disordered proteins in aqueous solutions.

Analytical sensitivity factors from distributions of time of flight of photons for near-infrared spectroscopy studies in multilayered turbid media.

In the last years, time-resolved near-infrared spectroscopy (TD-NIRS) has gained increasing interest as a tool for studying tissue spectroscopy with commercial devices. Although it provides much more information than its continuous wave counterpart, accurate models interpreting the measured raw data in real time are still lacking. We introduce an analytical model that can be integrated and used in TD-NIRS data processing software and toolkits in real time. This is based on the so-called sensitivity factors (SFs) of the distributions of time of flight (DTOFs) of photons measured in optically turbid and semi-infinite multilayered media, such as the human head. We derived analytical expressions for the SFs that link changes in the absorption coefficient of each layer to changes in the statistical moments of DTOFs acquired in a reflectance configuration. This was later validated with results from Monte Carlo (MC) simulations, which stand as the gold standard in terms of photon migration in biological tissue. Next, we designed a couple of simulated experiments depicting how the analytical SFs can be used to retrieve absorption changes in the particular case of a five-layered medium. Comparison between theory and simulations in 2-, 5-, and 10-layered media showed very good agreement (in most cases with weighted mean absolute percentage errors below 10%). Moreover, our derivations could be run in a few milliseconds (except for the extreme case of the variance SF in the 10-layered medium), which means a speedup of up to 10,000× with respect to MC simulations, with a much better spatial resolution and without their typically associated stochastic noise. In summary, our method achieves performances similar to those given by MC simulations, but orders of magnitude faster, which makes it very suitable for its implementation in real-time applications.

Determination of the carrier temperature in weakly confined semiconductor nanocrystals using time-resolved optical spectroscopy.

Many applications of nanocrystals rely on their use in light detection and emission. In recent years, nanocrystals with more relaxed carrier confinement, including so-called 'bulk' and 2D implementations, have made their stake. In such systems, the charge carriers generated after (photo-)excitation are spread over a semi-continuous density of states, behaviour controlled by the carrier temperature T. Current established methods to measure this dynamically changing temperature include transient absorption and luminescence spectroscopy, yet they very often fail to agree on the exact temperature leading to contradicting reports. Here, we show through a combined side-by-side experimental and theoretical study on state-of-art II-VI and perovskite nanocrystals under weak confinement that only transient PL can yield unambiguously the correct T. In particular, temperatures extracted from TA are heavily affected by the effective masses of the electron and hole bands involved leading to overestimations. Our results pave a way to a more robust extraction of carrier temperature and will help to consolidate ensuing structure-property relations derived from it.