Development and Application of Ultrafast Circular Dichroism Spectroscopy Techniques

Chirality hold broad applications in life sciences,quantum devices,and various other areas.Traditionally,molecular chirality can be characterized by using steady-state circular dichroism spectroscopy.However,the techniques that can characterize excited state chirality are progressively capturing the public interest as it can provide the dynamic information for chirality generation and transfer.In this review,we focus on the theoretical background and the developmental history of femtosecond time-resolved circular dichroism spectroscopy(TRCD)techniques around the world.Additionally,we provide examples to showcase the utility of these techniques in the analysis of the dynamical molecular chemical structures,the investigation of molecular chirality generation,and the detection of electron spin dynamics in semiconductor quantum dots.

Investigation of ultrafast dynamics of CdTe quantum dots by femtosecond fluorescence up-conversion spectroscopy

The ultrafast carrier relaxation processes in CdTe quantum dots are investigated by femtosecond fluorescence upconversion spectroscopy.Photo-excited hole relaxing to the edge of the forbidden gap takes a maximal time of ～ 1.6 ps with exciting at 400 nm,depending on the state of the photo-excited hole.The shallow trapped states and deep trap states in the forbidden gap are confirmed for CdTe quantum dots.In addition,Auger relaxation of trapped carriers is observed to occur with a time constant of ～ 5 ps.A schematic model of photodynamics is established based on the results of the spectroscopy studies.Our work demonstrates that femtosecond fluorescence up-conversion spectroscopy is a suitable and effective tool in studying the transportation and conversion dynamics of photon energy in a nanosystem.

Determination of Iron in Water Solution by Time-Resolved Femtosecond Laser-Induced Breakdown Spectroscopy

The influence of the energy of femtosecond laser pulses on the intensity of Fe I （371.99 nm） emission line and the continuous spectrum of the plasma generated on the surface of Fe^3＋ water solution by a Ti： sapphire laser radiation with pulse duration 〈45 fs and energies up to 7 mJ is determined. A calibration curve was obtained for Fe3＋ concentration range from 0.5 g/L to the limit of detection in water solution, and its saturation was detected for concentrations above 0.25 g/L, which is ascribed to self-absorption. The 3σ- limit of detection obtained for Fe in water solution is 2.6 mg/L in the case of 7 mJ laser pulse energy. It is found that an increase of laser pulse energy insignificantly affects on LOD in the time-resolved LIBS and leads to a slight improvement of the limit of detection.

Ultrafast Investigation of Excited-State Dynamics in Trans-4-methoxyazobenzene Studied by Femtosecond Transient Absorption Spectroscopy

The ultrafast photoisomerization and excited-state dynamics of trans-4-methoxyazobenzene (trans-4-MAB) in solutions were investigated by femtosecond transient absorption spectroscopy and quantum chemistry calculations. After being excited to the S2 state, the two-dimensional transient absorptions spectra show that cis-4-MAB is produced and witnessed by the permanent positive absorption in 400-480 nm. Three decay components are determined to be 0.11, 1.4 and 2.9 ps in ethanol, and 0.16, 1.5 and 7.5 ps in ethylene glycol, respectively. The fast component is assigned to the internal conversion from the S2 to S1 state. The other relaxation pathways are correlated with the decay of the S1 state via internal conversion and isomerization, and the vibrational cooling of the hot S0 state of the cis-isomer. Comparing of the dynamics in different solvents, it is demonstrated that the photoisomerization pathway undergoes the inversion mechanism rather than the rotation mechanism.

High-Resolution Rb Two-Photon Transition Spectroscopy by a Femtosecond Frequency Comb via Pulses Control

We experimentally observe the high resolution direct frequency comb spectroscopy using counter-propagating broadband femtosecond pulses on two-photon transitions in room-temperature ^87 Rb atoms. The Doppler broad- ened background is effectively eliminated with the pulse shaping method and the spectrum modulation technique. The combination of the pulse shaping method and the spectra modulation technique provides a potential approachto reduce background of at least 99%.

Detection Techniques of Femtosecond Lasers

The measurement techniques of femtosecond spectroscopy are effective method to investigate ultrafast dynamics, they are widely used in the fields of physics, chemistry and biology. In this paper, the principle, experiment setup and the approaches to deal with the experiment data were presented. Then different measurement techniques such as transient absorption spectroscopy, photon echoes, optical Kerr effect and degenerate four-wave mixing were explained with special examples. At last, the application prospect of measurement techniques of femtosecond spectroscopy was forecasted.

Probing Effect of Solvation on Photoexcited Quadrupolar Donor-Acceptor-Donor Molecule via Ultrafast Raman Spectroscopy

The symmetric and quadrupolar donor-acceptordonor(D-A-D)molecules usually exhibit excitedstate charge redistribution process from delocalized intramolecular charge transfer(ICT)state to localized ICT state.Direct observation of such charge redistribution process in real-time has been intensively studied via various ultrafast time-resolved spectroscopies.Femtosecond stimulated Raman spectroscopy(FSRS)is one of the powerful methods which can be used to determine the excited state dynamics by tracking vibrational mode evolution of the specific chemical bonds within molecules.Herein,a molecule,4,4′-(buta-1,3-diyne-1,4-diyl)bis(N,N-bis(4-methoxyphenyl)aniline),that consists of two central adjacent alkyne(-C≡C-)groups as electron-acceptors and two separated,symmetric N,N-bis(4-methoxyphenyl)aniline at both branches as electron-donors,is chosen to investigate the excited-state photophysical properties.It is shown that the solvation induced excited-state charge redistribution in polar solvents can be probed by using femtosecond stimulated Raman spectroscopy.The results provide a fundamental understanding of photoexcitation induced charge delocalization/localization properties of the symmetric quadrupolar molecules with adjacent vibrational markers located at central position.

Simple aspects of femtosecond stimulated Raman spectroscopy

Femtosecond stimulated Raman spectroscopy （FSRS）, using an overlapping pair of narrow band Raman pump and broadband probe pulses with heterodyne detection along the probe pulse direction, is a new nonlinear spectroscopic technique to record vibrational spectra of even highly fluorescent molecules and to study vibrational dynamics on excited electronic states of molecules, as in photoisomerization. FSRS is described by diagrammatic third-order perturbation theory with wave packet analysis. The phase matching condition gives rise to forty-eight terms for FSRS, but the resonant condition reduces it to just eight terms, which can be depicted by Feynman dual time-line diagrams, or closed time path loop diagrams, or the complementary four-wave mixing energy level diagrams. The eight terms fall into four sets-SRS（I）, SRS（Ⅱ）, IRS（I）, IRS（Ⅱ）-where SRS stands for stimulated Raman scattering and IRS stands for inverse Raman scattering. The SRS（I） set can also account for spontaneous Raman scattering, but the remaining SRS（Ⅱ）, IRS（I） and IRS（Ⅱ） terms are only present in stimulated scattering with the presence of a probe field. The SRS（I） set accounts for the Stokes Raman lines while the IRS（I） term accounts for the anti-Stokes lines, relative to the Raman pump frequency, in the FSRS spectrum. The remaining SRS（Ⅱ） and IRS（Ⅱ） terms give rise to broad baselines. Using a harmonic oscillator model, analytic results are obtained for the four-time correlation functions in the third-order polarizations. The issue of high time and high frequency resolution in time-resolved FSRS spectra is discussed. Calculations are made with the theory to compare with experimental results for： （a） resonance FSRS of fluorescent Rhodamine 6G and （b） 2D-FSRS from a coherent vibrational state that has been prepared by an impulsive, off-resonant pump pulse on CDCl3. The calculated results compared well with experimental results, and in the case of 2D-FSRS on CDCl3 there is a dominant cascade effect contributing to the FSRS spectra.

Ultrafast proton transfer dynamics of 2-(2'-hydroxyphenyl)benzoxazole dye in different solvents

The excited-state intramolecular proton transfer of 2-(2-hydroxyphenyl)benzoxazole dye in different solvents is investigated using ultrafast femtosecond transient absorption spectroscopy combined with quantum chemical calculations.Conformational conversion from the syn-enol configuration to the keto configuration is proposed as the mechanism of excited-state intramolecular proton transfer.The duration of excited-state intramolecular proton transfer is measured to range from 50 fs to 200 fs in different solvents.This time is strongly dependent on the calculated energy gap between the N-S;and T-S;structures in the S;state.Along the proton transfer reaction coordinate,the vibrational relaxation process on the S;state potential surface is observed.The duration of the vibrational relaxation process is determined to be from8.7 ps to 35 ps dependent on the excess vibrational energy.

Femtosecond time-resolved difference absorption spectroscopy of all-trans-β-Apo-8′-carotenal

The femtosecond time-resolved difference absorption spectra of all-trans-β-Apo-8′-carotenal have been recorded and analyzed by the singular-value decomposition (SVD) method followed by global fitting using a sequential model for the excited-state energy relaxation. With this model, we have obtained the excited-state absorption spectra and the lifetimes of the corresponding excited states both in nonpolar solvent n-hexane and polar solvent methanol. Three excited states, namely S3(170fs), S2(2.32ps) and S1(26ps) in n-hexane, and two excited states S2(190fs) and S1(9.4ps) in methanol have been observed. The excited-state absorption spectra of all-trans-β-Apo-8′-carotenal in methanol display a red shift and broadeness, while the lifetime of S1 state becomes shorter. It is proposed that these effects are related to the presence of a carbonyl functional group that leads to the solvent effect on the excited-state energy level. At the same time, it is shown that the SVD method is a useful tool in resolving the time-resolved absorption spectra.

Hydrogen Bonding Effects on the Photophysical Properties of 2,3-dihydro-3-keto- 1H-pyrido[3,2,1-kl] phenothiazine

Time-dependent density functional theory （TDDFT） and femtosecond transient absorption spectroscopy were used to investigate the photophysical properties of 2,3-dihydro-3-keto-lH- pyrido[3,2,1-kl]phenothiazine （PTZ4） and 3-keto-lH-pyrido[3,2,1-kl]phenothiazine （PTZ5）. The calculated results obtained from TDDFT suggest that the red-shifts of the absorption spectra of these two fluorophores in methanol are due to the formation of hydrogen-bonded complexes at the ground state. Four conformers of PTZ4 were obtained by TDDFT. The two fluorescence peaks of PTZ4 in tetrahydrofuran （THF） came from the ICT states of the four conformers. The fluorescence of PTZ4 in THF showed a dependence on the excitation wavelength because of butterfly bending. The excited state dynamics of PTZ4 in THF and methanol were obtained by transient absorption spectroscopy. The lifetime of the excited PTZ4 in methanol was 53.8 ps, and its relaxation from the LE state to the ICT state was completed within several picoseconds. The short lifetime of excited PTZ4 in methanol was due to the formation of out-of-plane model hydrogen bonds between PTZ4 and methanol at the excited state.

Ultrahigh spatiotemporal resolved spectroscopy

We review the technique and research of the ultrahigh spatiotemporal resolved spectroscopy and its applications in the field of the ultrafast dynamics of mesoscopic systems and nanomaterials. Combining femtosecond time-resolved spectroscopy and scanning near-field optical microscopy (SNOM),we can obtain the spectra with ultrahigh temporal and spatial resolutions simultaneously. Some problems in doing so are discussed. Then we show the important applications of the ultrahigh spatiotemporal resolved spectroscopy with a few typical examples.

Ultrafast dephasing of interband transitions in semiconductors

Two basic types of dephasing mechanisms, carrier-carrier and carrier-phonon scattering including hole-hole and hole-phonon scattering are proposed in the theory of ultrafast polarization dephasing of continuum transitions in bulk semiconductors. The contribution of optical phonon scattering to the dephasing rate is the average of the scattering rates for electrons and holes. A weighting factor that reflects the change in the momentum of the particle in a collision is introduced into the usual integral of the carrier-carrier scattering rate to describe the contribution of carrier-carrier scattering to the dephasing rate for the case of static screening interaction. The theoretical calculations are in quantitative agreement with the reported experimental results.

Ultrafast exciton relaxation dynamics of PbS and core/shell PbS/CdS quantum dots

Optical properties and ultrafast exciton relaxation dynamics in PbS and core/shell PbS/CdS quantum dots(QDs) have been studied using UV-vis absorption and fluorescence spectroscopy as well as femtosecond(fs) transient absorption spectroscopy.The electronic absorption spectrum of the PbS QDs features broad absorption in the entire near IR-vis-UV region with a monotonic increase in intensity towards shorter wavelength.Relative to PbS,the absorption of the core/shell PbS/CdS QDs shows a slight blue shift in the 600?800 nm region,due to the decrease of the PbS crystal size caused by the synthetic process of the core/shell structure,and increased absorption near 400 nm due to the CdS shell.The PL of the PbS/CdS QDs was ~2.6 times more intense than that of the PbS QDs,due to surface passivation of PbS by CdS,and blue-shifted,attributable to smaller PbS size and thereby stronger quantum confinement in the core/shell QDs.Fs transient absorption measurements of both systems showed a strong transient absorption feature from 600 to 750 nm following excitation at 750 nm.The transient absorption decays can be fit to a biexponential with time constants of 8 and 100 ps for PbS and 6 and 80 ps for PbS/CdS.The amplitude and lifetime of the fast component were excitation intensity dependent,with the amplitude increasing more than linearly with increasing excitation intensity and the lifetime decreasing with increasing intensity.The fast decay is attributed to exciton-exciton annihilation and it occurs more readily for the PbS/CdS than the PbS QDs,which is attributed to a lower density of trap states in the core/shell QDs,as supported by their stronger PL.

Rebinding kinetics of dissociated amino acid ligand and carbon monoxide to ferrous microperoxidase-11 in aqueous solution

The rebinding kinetics of an amino acid ligand to ferrous microperoxidase-11 (MP11) after photolysis of aggregated ferrous MP11 was measured in aqueous solution with femtosecond transient visible absorption spectroscopy. The kinetics of CO rebinding to ferrous MP11 after photolysis of MP11CO was also measured in aqueous solution with femtosecond transient visible absorption spectroscopy. From these measurements, we found that either Val-11 or Lys-13 rebinds to ferrous MP11 exponentially with an 8 picosecond time constant in aggregated ferrous MP11 solution and that CO rebinds to ferrous MP11 nonexponentially with subnanosecond time scale in MP11CO solution. The kinetics of both the amino acid and CO rebinding to ferrous MP11 in MP11 system mimics that in carbon monoxide oxidation activator protein (CooA) or carboxymethyl cytochrome c (CmCytC) system. We also measured the kinetics of CO rebinding to ferrous MP11 in aqueous solution at different MP11CO concentrations and found that MP11CO concentration has an obvious effect on the kinetics of CO rebinding to ferrous MP11, where both the germinate yield and rate of CO rebinding to ferrous MP11 increase with the increase of MP11CO concentration. These findings suggested that the picosecond amino acid ligand rebinding process could disturb the proximal heme-ligand structure that possibly leads to the subnanosecond CO rebinding kinetics in MP11CO, CooACO and CmCytCCO systems.