Femtosecond Pump-probe Spectroscopy Research Articles

Ultrafast charge transfer in atomically thin MoS₂/WS₂ heterostructures.

Van der Waals heterostructures have recently emerged as a new class of materials, where quantum coupling between stacked atomically thin two-dimensional layers, including graphene, hexagonal-boron nitride and transition-metal dichalcogenides (MX2), give rise to fascinating new phenomena. MX2 heterostructures are particularly exciting for novel optoelectronic and photovoltaic applications, because two-dimensional MX2 monolayers can have an optical bandgap in the near-infrared to visible spectral range and exhibit extremely strong light-matter interactions. Theory predicts that many stacked MX2 heterostructures form type II semiconductor heterojunctions that facilitate efficient electron-hole separation for light detection and harvesting. Here, we report the first experimental observation of ultrafast charge transfer in photoexcited MoS2/WS2 heterostructures using both photoluminescence mapping and femtosecond pump-probe spectroscopy. We show that hole transfer from the MoS2 layer to the WS2 layer takes place within 50 fs after optical excitation, a remarkable rate for van der Waals coupled two-dimensional layers. Such ultrafast charge transfer in van der Waals heterostructures can enable novel two-dimensional devices for optoelectronics and light harvesting.

Tautomerism in porphycenes: analysis of rate-affecting factors.

Double hydrogen transfer occurring in both ground and the lowest electronically excited singlet states was studied for a series of 19 differently substituted porphycenes. The rates of tautomerization have been determined using femtosecond pump-probe spectroscopy with polarized light. The values vary by over 3 orders of magnitude, suggesting the importance of tunneling. Good correlation exists between the values of the rates and the parameters characterizing the strength of two intramolecular hydrogen bonds: proton NMR shift, distance between the hydrogen-bonded nitrogen atoms, and the NH stretching frequency. While hydrogen-bond strength is the main factor determining the rate of double hydrogen transfer, other factors, such as static and dynamic symmetry breaking and the population of low-frequency vibrations also have to be taken into account.

Comprehensive study of ultrafast excited-state proton transfer in water and D2O providing the missing RO(-)···H(+) ion-pair fingerprint.

Steady-state and time-resolved optical techniques were employed to study the photoprotolytic mechanism of a general photoacid. Previously, a general scheme was suggested that includes an intermediate product that, up until now, had not been clearly observed experimentally. For our study, we used quinone cyanine 7 (QCy7) and QCy9, the strongest photoacids synthesized so far, to look for the missing intermediate product of an excited-state proton transfer to the solvent. Low-temperature steady-state emission spectra of both QCy7 and QCy9 clearly show an emission band at T < 165 K in H2O ice that could be assigned to ion-pair RO(-)*···H3O(+), the missing intermediate. Room-temperature femtosecond pump-probe spectroscopy transient spectra at short times (t < 4 ps) also shows the existence of transient absorption and emission bands that we assigned to the RO(-)*···H3O(+) ion pair. The intermediate dissociates on a time scale of 1 ps and about 1.5 ps in H2O and D2O samples, respectively.

ChemInform Abstract: The Role of πσ* States in the Photochemistry of Heteroaromatic Biomolecules and their Subunits: Insights from Gas‐Phase Femtosecond Spectroscopy

AbstractReview: 225 refs.

Study of photophysical properties of a series of polymethine dyes by femtosecond laser photolysis

The excited-state dynamics of 12 symmetric polymethine dyes in acetone was studied with femtosecond pump–probe spectroscopy. Four groups of compounds with different terminal heterocycles were studied: indoles, indoles with cyclic group attached to the nitrogen atom, benzimidazoles and benzo[cd] indoles. Within each group, the length of polymethine chain between the terminal groups was varied. The influence of electron-donating ability of the terminal groups and the length of the polymethine chain on the excited-state dynamics of the compounds was evaluated. The length of the polymethine chain was found to have an obvious effect both on the distance between the S1 ← S0 and Sn ← S1 transition bands and the lifetime of the excited state. The latter parameter is largely affected by the nature of terminal groups, most importantly, their electron-donating ability. In addition to experimental results, quantum chemical calculations were performed to provide with complementary insights on the excited states of investigated compounds.

Dynamics of coherent acoustic phonons in thin films ofCoSb3and partially filledYbxCo4Sb12skutterudites

Skutterudites are considered as interesting material for thermoelectric applications. Filling foreign atoms into the cagelike structure of a ${\mathrm{CoSb}}_{3}$ skutterudite is beneficial to its thermoelectric properties by increasing phonon scattering while maintaining the electrical conductivity. In this paper we demonstrate the generation and detection of coherent acoustic phonons in thin films of ${\mathrm{CoSb}}_{3}$ and partially filled ${\mathrm{Yb}}_{x}{\mathrm{Co}}_{4}{\mathrm{Sb}}_{12}$ skutterudites using femtosecond pump-probe spectroscopy. By using a pulse-echo method, the longitudinal sound velocity of amorphous and polycrystalline ${\mathrm{CoSb}}_{3}$ thin films is obtained. For partially filled ${\mathrm{Yb}}_{x}{\mathrm{Co}}_{4}{\mathrm{Sb}}_{12}$ thin films, an obvious decrease of the longitudinal sound velocity is observed at high filling fraction. Concomitantly, the high frequency acoustic phonon modes are strongly damped as the $\mathrm{Yb}$ filling fraction increases, which gives direct evidence for acoustic phonon scattering processes. It is shown that the reduction of lattice thermal conductivity after Yb filling is mainly achieved by the strong scattering of acoustic phonons.

Roles of allene-group in an intramolecular charge transfer character of a short fucoxanthin homolog as revealed by femtosecond pump-probe spectroscopy

Ultrafast excited state dynamics of the short fucoxanthin homolog with four conjugated double bonds and its allene-modified analog in various solvents have been studied by femtosecond pump-probe spectroscopy. The results clarified that the short fucoxanthin homolog exhibits a strong intramolecular charge transfer character in an excited manifold even in a nonpolar solvent. Then, the data show that a modification of the allene-group induced the lifetime shortening of the lowest singlet state of the short fucoxanthin homolog. This implies that the allene-group enhances the ICT character of the short fucoxanthin homolog.

Characterization of the intramolecular transfer state of marine carotenoid fucoxanthin by femtosecond pump–probe spectroscopy

Fucoxanthin, containing a carbonyl group in conjugation with its polyene backbone, is a naturally occurring pigment in marine organisms and is essential to the photosynthetic light-harvesting function in brown alga and diatom. Fucoxanthin exhibits optical characteristics attributed to an intramolecular charge transfer (ICT) state that arises in polar environments due to the presence of the carbonyl group. In this study, we report the spectroscopic properties of fucoxanthin in methanol (polar and protic solvent) observed by femtosecond pump-probe measurements in the near-infrared region, where transient absorption associated with the optically allowed S2 (1(1)B u (+) ) state and stimulated emission from the strongly coupled S1/ICT state were observed following one-photon excitation to the S2 state. The results showed that the amplitude of the stimulated emission of the S1/ICT state increased with decreasing excitation energy, demonstrating that the fucoxanthin form associated with the lower energy of the steady-state absorption exhibits stronger ICT character.

Disentangling electrons and lattice nonlinear optical response in metal-dielectric Bragg filters

We investigate by broadband femtosecond pump-probe spectroscopy the transient optical response of a metal-dielectric Bragg filter (MDBF) made of thin layers of gold and fused silica. A quantitative comparison of the experimental results with a semiclassical model with no fitting parameters allows us to disentangle all the different contributions to the transient optical response of the MDBF, which include the ultrafast dynamics of thermalized and nonthermalized electrons as well as of lattice phonons excited in the metal layers. Furthermore, we provide a quantitative comparison of the optical response of the MDBF with that of a single gold layer under comparable excitation conditions. The results clearly show the advantages and disadvantages of the multilayer configuration in terms of all-optical-modulation capability.

Femtosecond pump–probe spectroscopy of graphene oxide in water

Transient absorption properties of aqueous graphene oxide (GO) have been studied by use of femtosecond pump–probe spectroscopy. Excited state absorption and photobleaching are observed in the wide spectral range. The observed fast three lifetime components are attributed to the absorption of upper excited states and localized states, which is confirmed by both laser induced absorption and transmission kinetics. The longest time component is assigned to the lowest excited state of GO, which mainly originates from the sp2 domains. With the increase of the excitation power, two-quantum absorption occurs, which results in an additional rise-time component of the observed transients.

Formation and decay of P680 (PD1–PD2)+PheoD1− radical ion pair in photosystem II core complexes

Under physiological conditions (278K) femtosecond pump-probe laser spectroscopy with 20-fs time resolution was applied to study primary charge separation in spinach photosystem II (PSII) core complexes excited at 710nm. It was shown that initial formation of anion radical band of pheophytin molecule (Pheo−) at 460nm is observed with rise time of ~11ps. The kinetics of the observed rise was ascribed to charge separation between Chl (chlorophyll a) dimer, primary electron donor in PSII (P680*) and Pheo located in D1 protein subunit (PheoD1) absorbing at 420nm, 545nm and 680nm with formation of the ion-radical pair P680+PheoDI−. The subsequent electron transfer from PheoD1− to primary plastoquinone electron acceptor (QA) was accompanied by relaxation of the 460-nm band and occurred within ~250ps in good agreement with previous measurements in Photosystem II-enriched particles and bacterial reaction centers. The subtraction of the P680+ spectrum measured at 455ps delay from the spectra at 23ps or 44ps delay reveals the spectrum of PheoDI−, which is very similar to that measured earlier by accumulation method. The spectrum of PheoDI− formation includes a bleaching (or red shift) of the 670nm band indicating that Chl-670 is close to PheoD1. According to previous measurements in the femtosecond–picosecond time range this Chl-670 was ascribed to ChlD1 [Shelaev, Gostev, Vishnev, Shkuropatov, Ptushenko, Mamedov, Sarkisov, Nadtochenko, Semenov and Shuvalov, J. Photochemistry and Photobiology, B: Biology 104 (2011) 45–50]. Stimulated emission at 685nm was found to have two decaying components with time constants of ~1ps and ~14ps. These components appear to reflect formation of P680+ChlD1− and P680+PheoD1−, respectively, as found earlier. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.

Dynamics of photodissociation of XeF2 in organic solvents

Transient electronic absorption measurements with 1 ps time resolution follow XeF2 photoproducts in acetonitrile and chlorinated solvents. Ultraviolet light near 266 nm promptly breaks one Xe-F bond, and probe light covering 320-700 nm monitors the products. Some of the cleaved F atoms remain in close proximity to an XeF fragment and perturb the electronic states of XeF. The time evolution of a perturbed spectral feature is used to monitor the FXe-F complex population, which decays in less than 5 ps. Decay can occur through geminate recombination, diffusive separation or reaction of the complex with the solvent.

The role of πσ* states in the photochemistry of heteroaromatic biomolecules and their subunits: insights from gas-phase femtosecond spectroscopy

In an effort to illuminate why nature has chosen a particular set of bio-molecular ‘building-blocks’ for life, recent years have seen an up-surge of gas-phase spectroscopy experiments aimed at understanding why the DNA/RNA nucleobases, aromatic amino acids and their corresponding chromophore subunits, exhibit a resistance to photochemical damage (photostability) following the absorption of ultraviolet (UV) radiation. The research considered in this Perspective article specifically focuses on the role of electronically excited dissociative 1πσ* states (formed through the photo-induced promotion of an electron in a σ* ← π molecular orbital transition), which have been implicated to contribute to this photostable behaviour. In particular, we review the application of gas-phase femtosecond pump-probe spectroscopies to gain insights into 1πσ* state driven relaxation dynamics in a number of heteroaromatic biomolecules and their UV chromophore subunits. This Perspective article also discusses how the information obtained from these studies can be used as a ‘stepping-stone’ for extending this research to larger, more complex biomolecules and ultimately, more realistic systems in solution.

Anomalous temperature dependence of the vibrational lifetime of the OD stretch vibration in ice and liquid water

The temperature dependence of the vibrational T1 lifetime of the OD stretch vibration of HDO in H2O ice was measured with femtosecond mid-IR pump-probe spectroscopy. We found an increase of T1 from 480 ± 40 fs at 25 K to 860 ± 60 fs at 265 K. These lifetimes are remarkably shorter than the vibrational lifetime of the OD stretch vibration of HDO in H2O in the liquid phase, which has a value of 1.7 ± 0.1 ps at 274 K and increases to 2.24 ± 0.09 at 343 K. The observed temperature dependence of T1 can be well explained from a relaxation mechanism in which the OD vibration relaxes via energy transfer to the bend-libration combination tones of H2O and HDO.

Femtosecond spectroscopy of acoustic frequency combs in the 100-GHz frequency range in Al/Si membranes

Acousticfrequencycombsareopticallyexcitedanddetectedinsiliconmembranescoveredwiththinaluminum layers by femtosecond pump-probe spectroscopy. The various frequency combs consist of 11 up to 45 modes ranging in frequency from 10 up to 500 GHz. Evaluating the different modes of the combs allows us to quantify the dynamic properties of this two-layer system with great precision. Deviations of the frequencies of higher modes from a linear relation can be quantitatively understood. The time domain traces show clearly defined pulses which are detected in regular time intervals after each roundtrip in the acoustic cavity formed by the membrane and the metal film. By analyzing the individual reflected pulses and their evolution in time, damping times for the whole frequency range are determined. We analytically derive a deviation of the individual comb modes from integer values of the fundamental frequency which is corroborated by the experiments.

Excitation of rotons in parahydrogen crystals: The laser-induced-molecular-alignment mechanism

Solid parahydrogen (p-H2) is known to support long-lived coherences, of the order of 100 ps, which enables high-resolution spectroscopy in the time domain. Rotational Raman-type excitations to sublevels of J = 2 are delocalized due to electric-quadrupole–quadrupole coupling in p-H2 crystals, and the resulting states can be characterized as rotons. Wave packets of rotons exhibit molecular alignment with respect to laboratory coordinates. Here the concept of field-free molecular alignment, induced by strong ultrashort laser pulses, is extended into a molecular solid case. We derive a solid-state analog for the gas-phase alignment measure and illustrate the time-dependent alignment degree in p-H2 crystals by numerical simulations. To underscore the Raman gain effect of the solid, general properties of the field-free alignment are revisited by comparing gaseous p-H2 with N2. The interplay between the polarization direction of the excitation pulses and the axis directionality of the crystal is shown to affect the alignment dynamics via the spatial (M = 0,±1,±2) composition of the roton wave packets. We simulate experimental traces by incorporating the induced alignment degree in the calculation of heterodyne-detected realization of femtosecond pump-probe optical Kerr effect spectroscopy. With the help of dispersed, two-dimensional resolved images of the calculated signal we reproduce the experiment as a whole. To that end, the effects of probe chirp, shape, and power must be explored in detail. We find good agreement with previous experiments and unravel the ambiguity of tracing back the wave-packet composition from the signal; in particular, we find that the effect of quantum phase factors of all the components should be taken into account when explaining the signal properties.

Reading Charge Transport from the Spin Dynamics on the Surface of a Topological Insulator

Resolving the conductance of the topological surface states (TSSs) from the bulk contribution has been a great challenge for studying the transport properties of topological insulators. By developing a nonperturbative diffusion equation that describes fully the spin-charge dynamics in the strong spin-orbit coupling regime, we present a proposal to read the charge transport information of TSSs from its spin dynamics which can be isolated from the bulk contribution by the time-resolved second harmonic generation pump-probe measurement. We demonstrate the qualitatively different Dyaknov-Perel spin relaxation behavior between the TSSs and the two-dimensional spin-orbit coupling electron gas. The decay time of both in-plane and out-of-plane spin polarization is naturally proved to be identical to the charge transport time. The out-of-plane spin dynamics is shown to be in the experimentally reachable regime of the femtosecond pump-probe spectroscopy and thereby we suggest experiments to detect the charge transport properties of the TSSs from their unique spin dynamics.

Transient Absorption Kinetics Associated with Higher Exciton States in Semiconducting Single-Walled Carbon Nanotubes: Relaxation of Excitons and Phonons

We investigate the transient absorption kinetics due to excitons associated with the first and higher transition states in semiconducting single-walled carbon nanotubes at high excitation densities by means of femtosecond pump–probe spectroscopy. The time evolutions of absorption changes associated with the first, second, and third excitonic transition bands exhibit similar decay behavior; the decay is much longer than the exciton population decay observed by time-resolved luminescence measurements. From spectral shape analysis of the transient absorption spectra, it is found that the absorption change in the short time regime (<∼2 ps) is mainly due to the bimolecular Auger recombination process, reflecting the exciton population decay. The long-lasting decay behavior over ∼1 ns is attributed to the thermalization process of phonons generated during the inter- and intraband relaxation of excitons promoted to higher states by the Auger recombination process and the subsequent thermal diffusion to the surrounding surfactant and solvent.

Nonlinear absorption of light pulses under conditions of two-photon resonance in bulk crystals and nanostructures in the femtosecond pump-probe spectroscopy mode

A theory of nonlinear absorption of femtosecond light pulses by bulk crystals and nanostructures of different dimensions in the pump-probe spectroscopy mode has been developed. Expressions for the energy absorbed from the probe pulse under conditions of two-photon resonances are derived for transitions between the discrete or band electronic states and between the size-quantization subband states in quantum wells and quantum wires. The dependences of the energy absorbed from the probe pulse on the two-photon resonance detuning and on the time delay between the pulses are analyzed. The dependence of the power absorbed from the probe pulse on the time is also obtained.

Photophysics of diiodine-substituted fluorinated boron–dipyrromethene: A time resolved study

In this letter nanosecond laser flash photolysis and femtosecond pump-probe spectroscopy are applied to clarify the nature, spectral and kinetic properties of transient species formed upon the excitation of 4,4-difluoro-2,6-diiodo-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene in ethanol solution. Picosecond dynamics observed after the excitation of this compound can be adequately interpreted as the following sequence of processes: ultrafast (0.6ps) internal S2−S1 conversion, vibrational relaxation of the hot S1 state (7ps) and intersystem crossing (115ps) to the long-lived T1 state (φТ=0.56). The latter species decays in a microsecond timescale via oxygen quenching and T–T annihilation leading to the recovery of the ground state.