Ultrafast Pump-probe Experiments Research Articles

Revisiting the benzene excimer using [2,2] paracyclophane model system: Experiment and theory.

We report high-level calculations of the excited states of [2,2]-paracyclophane (PCP), which was recently investigated experimentally by ultrafast pump-probe experiments on oriented single crystals [Haggag et al., ChemPhotoChem 6 e202200181 (2022)]. PCP, in which the orientation of the two benzene rings and their range of motion are constrained, serves as a model for studying benzene excimer formation. The character of the excimer state and the state responsible for the brightest transition are similar to those of the benzene dimer. The constrained structure of PCP allows one to focus on the most important degree of freedom, the inter-ring distance. The calculations explain the main features of the transient absorption spectral evolution. This brightest transition of the excimer is polarized along the inter-fragment axis. The absorption of the light polarized in the plane of the rings reveals the presence of other absorbing states of Rydberg character, with much weaker intensities. We also report new transient absorption data obtained by a broadband 8fs pump, which time-resolve strong modulations of the excimer absorption. The combination of theory and experiment provides a detailed picture of the evolution of the electronic structure of the PCP excimer in the course of a single molecular vibration.

Influence of pump laser fluence on ultrafast myoglobin structural dynamics.

High-intensity femtosecond pulses from an X-ray free-electron laser enable pump-probe experiments for the investigation of electronic and nuclear changes during light-induced reactions. On timescales ranging from femtoseconds to milliseconds and for a variety of biological systems, time-resolved serial femtosecond crystallography (TR-SFX) has provided detailed structural data for light-induced isomerization, breakage or formation of chemical bonds and electron transfer1,2. However, all ultrafast TR-SFX studies to date have employed such high pump laser energies that nominally several photons were absorbed per chromophore3-17. As multiphoton absorption may force the protein response into non-physiological pathways, it is of great concern18,19 whether this experimental approach20 allows valid conclusions to be drawn vis-à-vis biologically relevant single-photon-induced reactions18,19. Here we describe ultrafast pump-probe SFX experiments on the photodissociation of carboxymyoglobin, showing that different pump laser fluences yield markedly different results. In particular, the dynamics of structural changes and observed indicators of the mechanistically important coherent oscillations of the Fe-CO bond distance (predicted by recent quantum wavepacket dynamics21) are seen to depend strongly on pump laser energy, in line with quantum chemical analysis. Our results confirm both the feasibility and necessity of performing ultrafast TR-SFX pump-probe experiments in the linear photoexcitation regime. We consider this to be a starting point for reassessing both the design and theinterpretation of ultrafast TR-SFX pump-probe experiments20 such that mechanistically relevant insight emerges.

Sub-50 fs temporal resolution in an FEL-optical laser pump-probe experiment at FLASH2.

High temporal resolution is essential for ultra-fast pump-probe experiments. Arrival time jitter and drift measurements, as well as their control, become critical especially when combining XUV or X-ray free-electron lasers (FELs) with optical lasers due to the large scale of such facilities and their distinct pulse generation processes. This paper presents the application of a laser pulse arrival time monitor that actively corrects the arrival time of an optical laser relative to the FEL's main optical clock. Combined with post-analysis single pulse jitter correction this new approach improves the temporal resolution for pump-probe experiments significantly. Benchmark measurements on photo-ionization of xenon atoms performed at FLASH beamline FL26, demonstrate a sub-50 fs FWHM overall temporal resolution.

Weyl semimetal assisted nonreciprocal modification of Flat-top optical pulses via defective photonic band-gap structures

We study the non-reciprocal modification of flat-top optical pulses via a one-dimensional photonic band-gap structure with Weyl semimetal-based defect layers due to their wide range of applications, such as high-speed communication, nonlinear optical switching, and ultrafast pump–probe experiments. We apply the transfer matrix method to obtain the transmission spectrum of the structure. Also, the Fourier transform technique is used to investigate the effect of the propagation direction of the incoming pulse on the time profile of the outgoing pulse. Then we examine the effect of the carrier frequency and duration of the incoming pulse on the length, energy, and magnetic field distribution of the outgoing pulse. It is shown that the time profile of an incoming flat-top pulse may modify to a nearly flat-top, single-peak, or oscillatory multi-peak time profile depending on the carrier frequency, length, and propagation direction of the incoming pulse.

A New Insight into the Singlet Oxygen Mechanism for Photodynamic Therapy.

Modern photodynamic therapy has been built on the mechanism of the interaction between the photosensitizer (porphyrin derivatives) and oxygen to produce singlet oxygen, which relies on energy transfer from the triplet excited state (T1) of porphyrin to the excited state of oxygen. In this process, the energy transfer from the singlet excited state (S1) of porphyrin to oxygen is believed to be not pronounced as the rapid decay of S1 and the large energy mismatch. Here, we have evidenced the existence of an energy transfer between S1 and oxygen, which can contribute to the production of singlet oxygen. For hematoporphyrin monomethyl ether (HMME), the Stern-Volmer constant of S1 (KSV') is 0.023 kPa-1, according to the oxygen concentration-dependent steady fluorescence intensities. In addition, fluorescence dynamic curves of S1 under various oxygen concentrations have also been measured through ultrafast pump probe experiments to further verify our results.

Ion/Water Network Structural Dynamics in Highly Concentrated Lithium Chloride and Lithium Bromide Solutions Probed with Ultrafast Infrared Spectroscopy

The structural dynamics of highly concentrated LiCl and LiBr aqueous solutions were observed from 1-4 to 1-16 water molecules per ion pair using ultrafast polarization-selective pump-probe (PSPP) experiments on the OD stretch of dilute HOD. At these high salt concentrations, an extended ion/water network exists with complex structural dynamics. Population decays from PSPP experiments highlight two distinct water components. From the frequency-dependent amplitudes of the decays, the spectra of hydroxyls bound to halides and to water oxygens are obtained, which are not observable in the FT-IR spectra. PSPP experiments also measure frequency-dependent water orientational relaxation. At short times, wobbling dynamics within a restricted angular cone occurs. At high concentrations, the cone angles are dependent on frequency (hydrogen bond strength), but at higher water concentrations (>10 waters per ion pair), there is no frequency dependence. The average cone angle increases as the ion concentration decreases. The slow time constant for complete HOD orientational relaxation is independent of concentration but slower in LiCl than in LiBr. Comparison to structural MD simulations of LiCl from the literature indicates that the loss of the cone angle wavelength dependence and the increase in the cone angles as the concentration decreases occur as the prevalence of large ion/water clusters gives way to contact ion pairs.

Theory of optical generation and detection of propagating magnons in an antiferromagnet

We report a theory of optical generation and detection of the propagating spin waves in antiferromagnetic materials relevant for ultrafast pump-probe experiments. We derive and solve the equations of motion for antiferromagnetic spins in response to the light-induced effective magnetic field in the linear regime. Different forms of the excitation and the properties of the generated spin waves are analyzed. We theoretically show the selective detection of the spin waves by the magneto-optical Kerr effect. The developed formalism is readily applicable to inform future experiments on antiferromagnetic optomagnonics.

Coherent Phonons in Antimony: An Undergraduate Physical Chemistry Solid-State Ultrafast Laser Spectroscopy Experiment

Ultrafast laser pump-probe spectroscopy is an important and growing field of physical chemistry that allows the measurement of chemical dynamics on their natural timescales, but undergraduate laboratory courses lack examples of such spectroscopy and the interpretation of the dynamics that occur. Here we develop and implement an ultrafast pump-probe spectroscopy experiment for the undergraduate physical chemistry laboratory course at the University of California Berkeley. The goal of the experiment is to expose students to concepts in solid-state chemistry and ultrafast spectroscopy via classic coherent phonon dynamics principles developed by researchers over multiple decades. The experiment utilizes a modern high-repetition rate 800 nm femtosecond Ti:Sapphire laser, split pulses with a variable time delay, and sensitive detection of transient reflectivity signals. The experiment involves minimal intervention from students and is therefore easy and safe to implement in the laboratory. Students first perform an intensity autocorrelation measurement on the femtosecond laser pulses to obtain their temporal duration. Then, students measure the pump-probe reflectivity of a single-crystal antimony sample to determine the period of coherent phonon oscillations initiated by an ultrafast pulse excitation, which is analyzed by fitting to a sine wave. Due to the disruption of in-person instruction caused by the COVID-19 pandemic, during those semesters students were provided the data they would have obtained during the experiment to analyze at home. Evaluation of student written reports reveals that the learning goals were met, and that students gained an appreciation for the field of ultrafast laser-induced chemistry.

Valley selectivity of soft x-ray excitations of core electrons in two-dimensional transition metal dichalcogenides

Optical properties of semiconducting monolayer transition metal dichalcogenides have received a lot of attention in recent years, following the discovery of the valley selective optical population of either ${\text{K}}_{+}$ or ${\text{K}}_{\ensuremath{-}}$ valleys at the direct band gap, depending on the polarization of the incoming light. We use group theoretical selection rules, as well as ab initio DFT calculations, to investigate whether this valley selectivity effect is also present in x-ray optical transitions from the flat core level of the transition metal atom to the valence and conduction band K valleys. Valley selectivity is predicted for $s, {p}_{1/2}$, and ${p}_{3/2}$ edges in transitions to and from the valence band edges with circularly polarized radiation. Possible novel applications to the diagnostics of valleytronic properties and intervalley dynamics are investigated and the feasibility of ultrafast pump-probe and Kerr rotations experiments with suitable soft-x-ray free-electron laser sources is discussed.

Tetrahydroxyphthalocyanine as a potential nonlinear optical material

We have synthesized a new zinc(II)-phthalocyanine (ZnPc) incorporating hydroxyl groups at the peripheral positions. This new zinc(II)-tetra-hydroxyphthalocyanine (1) has been characterized by high-resolution MALDI TOF MS, UV-Vis, FT-IR (ATR), and 1H- and 13C-NMR techniques, and further by the electrochemical measurements. DFT and TD-DFT computations were performed to model plausible structure and also analyze the electronic structure and optical properties of 1. The effects of the polarity and hydrogen bonding ability of the hydroxyl substituents on the peripheral positions of 1 on the optical and electrochemical properties are reported in this study. The redox behavior of 1 in dimethyl sulfoxide (DMSO) was determined by voltammetry and colorimetry supported in situ spectroelectrochemistry. The nonlinear absorption and optical limiting (OL) properties of 1 were measured in tetrahydrofuran (THF) by utilizing the open aperture Z-scan technique with nanosecond pulses at 532 nm. The sample demonstrated good reverse saturable absorption and OL behaviors. The ultrafast pump-probe experiments revealed the intersystem crossing (ISC) mechanism (triplet-triplet transition) and the nonlinear absorption mechanism of the compound. The results indicate that this new phthalocyanine (1) can be considered a potential candidate for low-power OL applications.

Time-Resolved Imaging of Stochastic Cascade Reactions over a Submillisecond to Second Time Range at the Angstrom Level

Many chemical reactions, such as multistep catalytic cycles, are cascade reactions in which a series of transient intermediates appear and disappear stochastically over an extended period. The mechanisms of such reactions are challenging to study, even in ultrafast pump-probe experiments. The dimerization of a van der Waals dimer of [60]fullerene producing a short carbon nanotube is a typical cascade reaction and is probably the most frequently studied in carbon materials chemistry. As many as 23 intermediates were predicted by theory, but only the first stable one has been verified experimentally. With the aid of fast electron microscopy, we obtained cinematographic recordings of individual molecules at a maximum frame rate of 1600 frames per second. Using Chambolle total variation algorithm processing and automated cross-correlation image matching analysis, we report on the identification of several metastable intermediates by their shape and size. Although the reaction events occurred stochastically, varying the lifetime of each intermediate accordingly, the average lifetime for each intermediate structure could be obtained from statistical analysis of many cinematographic images for the cascade reaction. Among the shortest-living intermediates, we detected one that lasted less than 3 ms in three independent cascade reactions. We anticipate that the rapid technological development of microscopy and image processing will soon initiate an era of cinematographic studies of chemical reactions and cinematic chemistry.

Light-Tunable Charge Density Wave Orders in MoTe2 and WTe2 Single Layers

By using constrained density functional theory modeling, we demonstrate that ultrafast optical pumping unveils hidden charge orders in group VI monolayer transition metal ditellurides. We show that irradiation of the insulating 2H phases stabilizes multiple transient charge density wave orders with light-tunable distortion, periodicity, electronic structure and bandgap. Moreover, optical pumping of the semimetallic 1T' phases generates a transient charge ordered metallic phase composed of 2D diamond clusters. For each transient phase we identify the critical fluence at which it is observed and the specific optical and Raman fingerprints to directly compare with future ultrafast pump-probe experiments. Our work demonstrates that it is possible to stabilize charge density waves even in insulating 2D transition metal dichalcogenides by ultrafast irradiation.

Experiment and theory elucidate the pathways for H3+formation in the ultrafast double ionization in methanol

Experiment and theory elucidate the pathways for H₃⁺formation in the ultrafast double ionization in methanol

Toward witnessing molecular exciton entanglement from spectroscopy

Entanglement is a defining feature of quantum mechanics that can be a resource in engineered and natural systems, but measuring entanglement in experiment remains elusive especially for large chemical systems. Most practical approaches require determining and measuring a suitable entanglement witness which provides some level of information about the entanglement structure of the probed state. A fundamental quantity of quantum metrology is the quantum Fisher information (QFI) which is a rigorous witness of multipartite entanglement that can be evaluated from linear response functions for certain states. In this work, we explore measuring the QFI of molecular exciton states of the first-excitation subspace from spectroscopy. In particular, we utilize the fact that the linear response of a pure state subject to a weak electric field over all possible driving frequencies encodes the variance of the collective dipole moment in the probed state, which is a valid measure for QFI. The systems that are investigated include the molecular dimer, $N$-site linear aggregate with nearest-neighbor coupling, and $N$-site circular aggregate, all modeled as a collection of interacting qubits. Our theoretical analysis shows that the variance of the collective dipole moment in the brightest dipole-allowed eigenstate is the maximum QFI. The optical response of a thermally equilibrated state in the first-excitation subspace is also a valid QFI. Theoretical predictions of the measured QFI for realistic linear dye aggregates as a function of temperature and energetic disorder due to static variations of the host matrix show that 2- to 3-partite entanglement is realizable. This work lays some groundwork and inspires measurement of multipartite entanglement of molecular excitons with ultrafast pump-probe experiments.

Advanced calculations of X-ray spectroscopies with FEFF10 and Corvus.

The real-space Green's function code FEFF has been extensively developed and used for calculations of X-ray and related spectra, including X-ray absorption (XAS), X-ray emission (XES), inelastic X-ray scattering, and electron energy-loss spectra. The code is particularly useful for the analysis and interpretation ofthe XAS fine-structure (EXAFS) and the near-edge structure (XANES) inmaterials throughout the periodic table. Nevertheless, many applications, suchas non-equilibrium systems, and the analysis of ultra-fast pump-probe experiments, require extensions of the code including finite-temperature and auxiliary calculations of structure and vibrational properties. To enable these extensions, we have developed in tandem a new version FEFF10 and new FEFF-based workflows for the Corvus workflow manager, which allow users to easily augment the capabilities of FEFF10 via auxiliary codes. This coupling facilitates simplified input and automated calculations of spectra based on advanced theoretical techniques. The approach is illustrated with examples of high-temperature behavior, vibrational properties, many-body excitations in XAS, super-heavy materials, and fits of calculated spectra to experiment.

Broadband ultrashort light generation from a narrowband seed

Light beams with broad spectral bandwidth are indispensable for the generation of ultrashort light pulses. Here, the generation of broadband ultrashort light pulses from narrowband seeds in transparent media is demonstrated. Broadband light pulses with a spectral width of hundreds of nanometers can be generated seeding with narrowband light pulses. The full spectral bandwidths of the narrowband seeds are several nanometers. Broadband ultrashort pulses from about 500 nm to 1020 nm are obtained seeding with a light beam at around 900 nm with 9 nm full spectral bandwidth. The temporal and spatial parameters of the generated broadband pulses are measured and analyzed. A new method for the generation of wavelength tunable broadband ultrashort lights is presented. The broadband light beam generation phenomenon can be explained by ping-pong four wave mixing (PPFWM) process, which helps us to understand all the optical parametric processes visually. The generated broadband pulses have potential applications in ultrafast spectroscopy and ultrafast pump–probe experiments.

Temperature dependent electron–phonon coupling of Au resolved via lattice dynamics measured with sub-picosecond infrared pulses

The detailed understanding of energy transfer between hot electrons and lattice vibrations at non-cryogenic temperatures relies primarily upon the interpretation of ultrafast pump–probe experiments, where thermo-optical models provide insight into the relationship between optical response and temperature of the respective sub-systems; in one of the more studied materials, gold, the Drude model provides this relationship. In this work, we investigate the role of intra- and interband contributions applied to transient optical responses in ultrafast pump–probe experiments using both experiments and first-principle calculations, with probe wavelengths spanning from UV wavelengths into the infrared. We find that during conditions of electron–phonon equilibrium, the Drude model is not applicable to visible wavelengths due to interband transitions. Instead, at probe wavelengths far from these interband transitions (e.g., infrared wavelengths), the optical response is linearly proportional to the temperature of the phonon sub-system and is no longer obfuscated by Fermi-smearing, thus greatly simplifying the extraction of the electron–phonon coupling factor. Our intraband-probe measurements on the electron–phonon coupling factor of Au are in excellent agreement with analytical models and ab initio calculations; we observe a constant electron–phonon coupling factor up to electron temperatures of at least ∼2000 K.

Terahertz Polaron Oscillations of Electrons Solvated in Liquid Water.

The terahertz (THz) response of solvated electrons in liquid water is studied in nonlinear ultrafast pump-probe experiments. Free electrons with concentrations from c_{e}=4 to 140×10^{-6} moles/liter are generated by high-field THz or near-infrared multiphoton excitation. The time-resolved change of the dielectric function as mapped by broadband THz pulses exhibits pronounced oscillations persisting up to 30ps. Their frequency increases with electron concentration from 0.2 to 1.5THz. The oscillatory response is assigned to impulsively excited coherent polarons involving coupled electron and water shell motions with a frequency set by the local electric field.

Large-scale, power-efficient Au/VO2 active metasurfaces for ultrafast optical modulation

Abstract Active metasurfaces, in which the optical property of a metasurface device can be controlled by external stimuli, have attracted great research interest recently. For optical switching and modulation applications, high-performance active metasurfaces need to show high transparency, high power efficiency, as well as ultrafast switching and large-scale fabrication capability. This paper reports Au/VO2-based active metasurfaces meeting the requirements above. Centimeter-scale Au/VO2 metasurfaces are fabricated by polystyrene sphere colloidal crystal self-assembly. The devices show optical modulation on-off ratio up to 12.7 dB and insertion loss down to 3.3 dB at 2200 nm wavelength in the static heating experiment, and ΔT/T of 10% in ultrafast pump-probe experiments. In particular, by judiciously aligning the surface plasmon resonance wavelength to the pump wavelength of the femtosecond laser, the enhanced electric field at 800 nm is capable to switch off the extraordinary optical transmission effect at 2200 nm in 100 fs time scale. Compared to VO2 thin-film samples, the devices also show 50% power reduction for all-optical modulation. Our work provides a practical way to fabricate large-scale and power-efficient active metasurfaces for ultrafast optical modulation.

Accuracy of XAS theory for unraveling structural changes of adsorbates: CO on Ni(100)

Studying surface reactions using ultrafast optical pump and x-ray probe experiments relies on accurate calculations of x-ray spectra of adsorbates for the correct identification of the spectral signatures and their dynamical evolution. We show that experimental x-ray absorption can be well reproduced for different binding sites in a static prototype system CO/Ni(100) at a standard density functional theory generalized-gradient-approximation level of theory using a plane-wave basis and pseudopotentials. This validates its utility in analyzing ultrafast x-ray probe experiments. The accuracy of computed relative core level binding energies is about 0.2 eV, representing a lower limit for which spectral features can be resolved with this method. We also show that the commonly used Z + 1 approximation gives very good core binding energy shifts overall. However, we find a discrepancy for CO adsorbed in the hollow site, which we assign to the significantly stronger hybridization in hollow bonding than in on-top.