Picosecond Time Region Research Articles

Peptide-bridged bis-porphyrin compounds: A photophysical and molecular dynamics study

Covalently linked peptide-porphyrin compounds are most suitable systems for fundamental studies aiming to the comprehension of the mechanisms driving photoinduced energy/electron transfer processes. Mimicking photosynthetic units, the porphyrin groups act as antenna moieties while the peptide chain is the active medium through which energy and/or electron funneling occur. In this contribution we studied the transfer of excitation between two identical tetraphenylporphyrin groups connected by short peptide chains of different length formed by non-coded conformationally constrained α-amino acids, i.e., Cα-methylvaline. The photophysical events following porphyrin photoexcitation were characterized from the microsecond to the picosecond time region by time-resolved spectroscopy techniques. Ultrafast transient absorption measurements revealed the presence of a transient species that we assign to a self-trapped exciton migrating through the peptide chain. The exciton species propagates the electronic coupling between the two porphyrin groups giving rise to a characteristic bisignate band measured by circular dichroism experiments. Molecular dynamics simulations strongly suggest that the long lifetime (hundreds of picoseconds) of the exciton species is determined by the rigidity of the Cα-methylvaline residues, that inhibited energy relaxation pathways coupled to torsional motions of the peptide chain.

Excitonic effects in the linear and nonlinear optical properties of ZnO@WS2 heterojunction films under photoexcitation

The excitation effects of linear and nonlinear optical properties for the ZnO@WS2 heterojunction (ZWH) films under photoexcitation were studied. We found that intrinsic absorption and radiation of ZWH films from bandgap and defects of ZnO and WS2. "M" and "W" type of Z-scan curve represent mechanism of TPA/3PA induced the ground state/excited state SA or RSA. The transient absorption spectra corresponding to free carriers, trapped carriers, and state filling were observed in the picosecond time region for samples. Our results revealed the carrier relaxation and recombination processes in WS2 and ZWH films, identifying the critical role of stoichiometry in photo-induced phenomena.

Coherent Vibration and Femtosecond Dynamics of the Platinum Complex Oligomers upon Intermolecular Bond Formation in the Excited State

AbstractFemtosecond time‐resolved absorption and picosecond time‐resolved emission measurements were carried out for highly concentrated aqueous solutions of K2[Pt(CN)4] to investigate excited‐state dynamics of the [Pt(CN)42−] oligomers formed with metallophilic interactions. Time‐resolved absorption spectra exhibit complicated dynamics that are represented with five time constants. Among them, the 90‐ps and 400‐ps dynamics were assigned to the S1 → T1 intersystem crossing of the trimer and tetramer coexisting in the solution by comparison with the fluorescence decays. Clear oscillations of transient absorption were observed in the first few picoseconds, and the frequency‐detected‐wavelength 2D analysis revealed that the 135‐cm−1 and 65‐cm−1 oscillations arise from the Pt–Pt stretch motions of the S1 trimer and S1 tetramer, respectively. The obtained time‐resolved spectroscopic data provide a clear view of the excited‐state dynamics of the [Pt(CN)42−] oligomers in the femto‐/picosecond time region.

Coherent Vibration and Femtosecond Dynamics of the Platinum Complex Oligomers upon Intermolecular Bond Formation in the Excited State.

Femtosecond time-resolved absorption and picosecond time-resolved emission measurements were carried out for highly concentrated aqueous solutions of K2 [Pt(CN)4 ] to investigate excited-state dynamics of the [Pt(CN)4 2- ] oligomers formed with metallophilic interactions. Time-resolved absorption spectra exhibit complicated dynamics that are represented with five time constants. Among them, the 90-ps and 400-ps dynamics were assigned to the S1 → T1 intersystem crossing of the trimer and tetramer coexisting in the solution by comparison with the fluorescence decays. Clear oscillations of transient absorption were observed in the first few picoseconds, and the frequency-detected-wavelength 2D analysis revealed that the 135-cm-1 and 65-cm-1 oscillations arise from the Pt-Pt stretch motions of the S1 trimer and S1 tetramer, respectively. The obtained time-resolved spectroscopic data provide a clear view of the excited-state dynamics of the [Pt(CN)4 2- ] oligomers in the femto-/picosecond time region.

D-π-A type conjugated pyrene derivatives: Third-order nonlinear absorption and excited-state dynamics

Two novel conjugated pyrene derivatives (C1 and C2) with D-π-A structures were synthesized to realize broadband reverse saturable absorption (RSA) under femtosecond laser excitation (ranging from 515 to 700 nm) resulting from two-photon absorption (TPA) and TPA-induced excited-state absorption. Furthermore, these two derivatives showed RSA on the picosecond and nanosecond time regions (532 nm). Quantum chemical calculations indicated that two mechanisms for the HOMO-LUMO excitation of the two molecules exist simultaneously: intramolecular charge transfer (which is considered to trigger the nonlinear optical response) and π-π* transition. It was found that the π-π* transition localized in the pyrene group can contribute to the increase in the nonlinear absorption of pyrene derivatives. Femtosecond time-resolved transient absorption spectrum provided information on the dynamics of the locally excited singlet state (1LE) and the charge-transfer state (1CT) for both compounds. The results show that these pyrene derivatives are promising candidates for future optoelectronic and RSA applications.

Picosecond rotation of a ring-shaped optical lattice by using a chirped vortex-pulse pair.

A novel method of ultrafast rotation of a ring-shaped optical lattice in the picosecond time region was proposed and demonstrated. Our ring-lattice generator was assembled by a pair of linearly chirped pulses with a time delay, a high-order birefringent retarder, and an axially symmetric polarization element. Using a mode-locked Ti:sapphire laser oscillator as a light source, stable two-, four-, and six-petaled ring-lattice rotations were demonstrated with the rotation periods of 1.6, 3.2, and 4.8ps, respectively. Our method has the potential to open up a new technique to resonantly excite propagating quasi-particles together with their coherent enhancement.

Ultrafast carrier dynamics in SnOx thin films

We studied the carrier dynamics in a series of SnOx thin films using femtosecond transient absorption (TA) spectroscopy. The observed carrier relaxation was found to be strongly dependent on thin film stoichiometry. The TA spectra corresponding to free carriers, trapped carriers, and state filling were observed in the picosecond time region for SnO2, SnOx, and SnO film, respectively. The TA decay kinetics of all films were best fit with a tri-exponential decay model with fast (1 ps), medium (∼10 ps), and slow (ns) components. Our results revealed the carrier relaxation and recombination processes in SnOx thin films, identifying the critical role of stoichiometry in photo-induced phenomena.

Slow intraband relaxation and localization of photogenerated carriers inCuIn1−xGaxSe2thin films: Evidence for the existence of long-lived high-energy carriers

The dynamics of free carriers in polycrystalline ${\mathrm{CuIn}}_{1\ensuremath{-}x}$${\mathrm{Ga}}_{x}$${\mathrm{Se}}_{2}$ (CIGS) thin films were studied using picosecond time-resolved photoluminescence (PL) and femtosecond transient-absorption (TA) measurements. The PL spectrum and the TA decay component due to the band-to-band recombination of free carriers were observed in the picosecond time region. From excitation-photon-energy-dependent TA measurements, we identified a slow intraband relaxation of free carriers in the CIGS thin films. Collectively, the combination of PL and TA experiments reveal a global feature of energy relaxation and recombination processes of free carriers in the femtosecond to nanosecond time regions.

Spectrally tailored supercontinuum generation from single-mode-fiber amplifiers

Spectral filtering of an all-normal-dispersion Yb-doped fiber laser was demonstrated effective for broadband supercontinuum generation in the picosecond time region. The picosecond pump pulses were tailored in spectrum with 1 nm band-pass filter installed between two single-mode fiber amplifiers. By tuning the spectral filter around 1028 nm, four-wave mixing was initiated in a photonic crystal fiber spliced with single-mode fiber, as manifested by the simultaneous generation of Stokes wave at 1076 nm and anti-Stokes wave at 984 nm. Four-wave mixing took place in cascade with the influence of stimulated Raman scattering and eventually extended the output spectrum more than 900 nm of 10 dB bandwidth. This technique allows smooth octave supercontinuum generation by using simple single-mode fiber amplifiers rather than complicated multistage large-mode-area fiber amplifiers.

The Early Steps in the Photocycle of a Photosensor Protein Sensory Rhodopsin I from Salinibacter ruber

Light absorption by the photoreceptor microbial rhodopsin triggers trans-cis isomerization of the retinal chromophore surrounded by seven transmembrane α-helices. Sensory rhodopsin I (SRI) is a dual functional photosensory rhodopsin both for positive and negative phototaxis in microbes. By making use of the highly stable SRI protein from Salinibacter ruber (SrSRI), the early steps in the photocycle were studied by time-resolved spectroscopic techniques. All of the temporal behaviors of the Sn←S1 absorption, ground-state bleaching, K intermediate absorption, and stimulated emission were observed in the femto- to picosecond time region by absorption spectroscopy. The primary process exhibited four dynamics similar to other microbial rhodopsins. The first dynamics (τ1 ∼ 54 fs) corresponds to the population branching process from the Franck-Condon region to the reactive (S1(r)) and nonreactive (S1(nr)) S1 states. The second dynamics (τ2 = 0.64 ps) is the isomerization process of the S1(r) state to generate the ground-state 13-cis form, and the third dynamics (τ3 = 1.8 ps) corresponds to the internal conversion of the S1(nr) state. The fourth component (τ3' = 2.5 ps) is assignable to the J-decay (K-formation). This reaction scheme was further supported by the results of fluorescence spectroscopy. To investigate the protein response(s), the spectral changes of the tryptophan bands were monitored by ultraviolet resonance Raman spectroscopy. The intensity change following the K formation in the chromophore structure (τ ∼ 17 ps) was significantly small in SrSRI as compared with other microbial rhodopsins. We also analyzed the effect(s) of Cl(-) binding on the ultrafast dynamics of SrSRI. Compared with a chloride pump Halorhodopsin, Cl(-) binding to SrSRI was less effective for the excited-state dynamics, whereas the binding altered the structural changes of tryptophan following the K-formation, which was the characteristic feature for SrSRI. On the basis of these results, a primary photoreaction scheme of SrSRI together with the role of chloride binding is proposed.

Dynamic Structure of a Molecular Liquid S0.5Cl0.5: Ab initio Molecular-Dynamics Simulations

The static and dynamic structures of a molecular liquid S0.5Cl0.5 consisting of Cl–S–S–Cl (S2Cl2) type molecules are studied by means of ab initio molecular dynamics simulations. Both the calculated static and dynamic structure factors are in good agreement with experimental results. The dynamic structures are discussed based on van-Hove distinct correlation functions, molecular translational mean-square displacements (TMSD) and rotational mean-square displacements (RMSD). In the TMSD and RMSD, there are ballistic and diffusive regimes in the sub-picosecond and picosecond time regions, respectively. These time scales are consistent with the decay time observed experimentally. The interaction between molecules in the liquid is also discussed in comparison with that in another liquid chalcogen–halogen system Se0.5Cl0.5.

Real-Time Observation of Tight Au–Au Bond Formation and Relevant Coherent Motion upon Photoexcitation of [Au(CN)2–] Oligomers

Structural dynamics involving tight Au-Au bond formation of excited-state oligomers [Au(CN)(2)(-)](n) was studied using picosecond/femtosecond time-resolved emission and absorption spectroscopy. With selective excitation of the trimer ([Au(CN)(2)(-)](3)) in aqueous solutions, transient absorption due to the excited-state trimer was observed around 600 nm. This transient exhibited a significant intensity increase (τ = 2.1 ps) with a blue shift in the early picosecond time region. Density functional theory (DFT) and time-dependent DFT calculations revealed that the observed spectral changes can be ascribed to a structural change from a bent to a linear staggered structure in the triplet excited-state trimer. The transient absorption also exhibited a clear modulation of the peak position, reflecting coherent nuclear wave packet motion induced by photoexcitation. The frequencies of the coherent motions are 66 and 87 cm(-1), in very good accord with the frequencies of two Au-Au stretch vibrations in the excited state of the trimer calculated by DFT. Time-resolved emission spectra in the subnanosecond time region showed that association of the excited-state trimer with the ground-state monomer proceeds with τ = 2.0 ns, yielding the excited-state tetramer.

Ultrashort-bunch Electron RF Gun

Two ultrashort electron-bunch photocathode RF guns in picosecond (ps) and femtosecond (fs) time region are reviewed. The ps gun was developed and used successfully to generate an ultralow-emittance and high-charge electron beam with normalized emittance of 1.2 mm-mrad at bunch charge of 1 nC. The low-emittance electron beam has been used successfully to produce a 100-fs high-charge electron single bunch with a booster linear accelerator and a magnetic bunch compressor. An ultrafast pulse radiolysis with time resolution of 240 fs was constructed using the fs electron beam for the study of ultrafast reactions in radiation chemistry and biology. The fs gun, developed under the KEK/Osaka University collaboration with a new structure cavity and many improvements, was succeeded in producing directly a near-relativistic 100-fs electron beam with energy of 1-3 MeV. Femtosecond time-resolved electron diffraction has been constructed using the fs RF gun for the study of photon-induced phase transition in materials. Some experimental results of pulse radiolysis and MeV electron diffraction in fs time region were reported and discussed.

Coherent dynamics and ultrafast excited state relaxation of blue copper protein; plastocyanin

Ultrafast transient absorption measurements in the femtosecond to picosecond time region were carried out for a blue copper protein, plastocyanin (Pc). To compare the dynamical profiles after photoexcitation upon the ligand-to-metal-charge-transfer (LMCT) band and the d-d transition band, the pump wavelength was set at wavelengths of 597 and 895 nm, respectively. The results were nearly identical, indicating that the transition from the LMCT to the lower ligand field (LF) states takes place in an ultrafast time regime of less than 40 fs. Subsequently, relaxation in the LF state occurs with a time constant of 90 fs and the system returns to the ground state with that of 250 fs. The longest time constant of 1.8 ps was attributed to the vibrational cooling in the ground state. Several wavepacket motions were observed, including Franck-Condon type motion at approximately 510 nm and a Herzberg-Teller type motion at 660-720 nm. Critically damped low-frequency oscillation of approximately 30 cm(-1) was also observed with both excitation wavelengths with the strongest amplitude around 600 nm. This oscillation could be due to the motion of the protein that is ballistically stimulated by ultrafast relaxation.

Pulse radiolysis study of ion-species effects on the solvated electron in alkylammonium ionic liquids

The spectra and kinetic behavior of solvated electrons (e sol −) in alkyl ammonium ionic liquids (ILs), i.e. N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEMMA-TFSI), N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEMMA-BF 4), N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI), N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13-TFSI), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P13-TFSI), and N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P14-TFSI) were investigated by the pulse radiolysis method. The e sol − in each of the ammonium ILs has an absorption peak at 1100 nm, with molar absorption coefficients of 1.5–2.3×10 4 dm 3 mol −1 cm −1. The e sol − decayed by first order with a rate constant of 1.4–6.4×10 6 s −1. The reaction rate constant of the solvated electron with pyrene (Py) was 1.5–3.5×10 8 dm 3 mol −1 s −1 in the various ILs. These values were about one order of magnitude higher than the diffusion-controlled limits calculated from measured viscosities. The radiolytic yields ( G-value) of the e sol − were 0.8–1.7×10 −7 mol J −1. The formation rate constant of e sol − in DEMMA-TFSI was 3.9×10 10 s −1. The dry electron (e dry −) in DEMMA-TFSI reacts with Py with a rate constant of 7.9×10 11 dm 3 mol −1 s −1, three orders of magnitude higher than that of the e sol − reactions. The G-value of the e sol − in the picosecond time region is 1.2×10 −7 mol J −1. The capture of e dry − by scavengers was found to be very fast in ILs.

Photoisomerization dynamics study on cis-azobenzene derivative using ultraviolet-to-visible tunable femtosecond pulses

We performed the transient absorption measurement and the first rate equation (RE) analysis for cis isomer of 4-carboxy-2′,6′-dimethylazobenzene to clarify the quantitative difference between the photoisomerization process and the thermal relaxation process from the S 1 C excited state. The RE analysis enabled us to determine the cis-to- trans photoisomerization rate per each pump pulse to be 3% under the condition of the 430 nm, 150 fs pump pulse with energy of 200 nJ. Moreover, the signal due to the yielded trans molecules appearing in the transient absorption was assigned from the following observed result: the transient absorbance change at the 380 nm probe mostly decreased within 300 fs after the 430 nm pulse pumping and then slowly decreased to zero, while the absorbance change at the 350 nm probe had a positive constant component in the over one picosecond time region. The RE analysis showed that this constant component is due to the yielded trans molecules, and its positive value is due to the fact that the absorption cross-section of the S 0 T -to- S 2 T transition in their trans molecules is larger than that of the S 0 C -to- S 2 C transition in the original cis molecules.

Dynamics of the charge transferred states relevant to magnetic phase transition in rubidium manganese hexacyanoferrate

Photoinduced charge transfer dynamics in the photomagnetic material RbMn[Fe(CN) 6], which exhibits a magnetic phase transition with a large hysteresis loop (230–300 K), has been investigated by observing the CN − stretching modes, which are sensitive to the valences of the adjacent transition metal ions. Mid-infrared transient absorption measurements were performed between 2013 and 2179 cm −1 to observe the transient and persistent products. The sample in the high-temperature phase was excited by 400 nm laser pulses at the ligand to metal charge transfer band near the high-temperature end of the hysteresis loop. Bleach of the Fe 3+–CN −–Mn 2+ band representing a decrease of the high-temperature phase and increases of the Fe 2+–CN −–Mn 3+ and Fe 2+–CN −–Mn 2+ bands were observed in picosecond time region, indicating a transient production of charge transferred states.

Hydrogen-Bonding Dynamics in Aqueous Solutions of Amides and Acids: Monomer, Dimer, Trimer, and Polymer

Hydrogen-bonding dynamics in aqueous solutions of series of amides and acids have been investigated by means of femtosecond Raman-induced Kerr effect spectroscopy and ab initio quantum chemistry calculation. The amides and acids studied here are acetamide, 1,3-propanedicarboxamide, 1,3,5-pentanetricaroxamide, polyacrylamide with Mw=1500, acetic acid, 1,3-propanedicarboxylic acid, 1,3,5-pentanetricarboxylic acid, and poly(acrylic acid) with Mw=2000. The femtosecond damped transient feature for aqueous amide solutions, which arises from the intermolecular hydrogen bonds of amide and water, becomes clearer with the larger molecular weight of amide. A characteristic vibrational band at about 100 cm(-1) is assigned as the hydrogen-bonding vibrational mode and the ab initio quantum chemistry calculation result indicates that at least two waters, which make up the hydrogen-bonding network with amide, are necessary for this mode. The hydrogen-bonding vibrational mode at about 100 cm(-1) in aqueous amide solutions shifts to the higher frequency with the larger molecular weight amide in consequence of the stronger intermolecular interaction between amide and water. The evidence likely comes from the stronger hydrophobic interaction for polymer than oligomers and monomer. In the picosecond time region, an extra slow relaxation process with a time constant of about 60 ps has been found in the aqueous polymer solutions. The relaxation is assigned as a local motion of the constitutional repeat unit of polymers from comparison with monomer and oligomers.

Subpicosecond pulse radiolysis in liquid methyl-substituted benzene derivatives

The early processes of radiation chemistry in the picosecond time region in methyl-substituted benzene derivatives have been investigated using subpicosecond pulse radiolysis. In o-xylene, a fairly slow geminate ion recombination was observed within 50 ps after the electron beam irradiation; this is due to the smaller electron mobility. The kinetic traces were analyzed using the Smoluchowski equation with exponential and modified-Gaussian (YGP) functions as the distribution of thermalized electrons. Only exponential functions well reproduced the experimental data within 50 ps after the electron pulse.

Photoinduced Charge Separation and Charge Recombination in the [60]Fullerene−Diphenylbenzothiadiazole−Triphenylamine Triad: Role of Diphenylbenzothiadiazole as Bridge

Photoinduced electron-transfer processes of the newly synthesized [60]fullerene-diphenylbenzothiadiazole-triphenylamine (C60-PBTDP-TPA) triad in polar and nonpolar solvents have been studied by using time-resolved transient absorption and fluorescence measurements from picosecond to microsecond regions. By fluorescence lifetime measurements in picosecond time regions, excitation of the charge-transfer transition of the PBTDP-TPA moiety in C60-PBTDP-TPA induces energy transfer to the C60 moiety generating 1C60*-PBTDP-TPA, competitively with charge separation generating C60*--PBTDP-TPA*+. From 1C60*-PBTDP-TPA, which is generated directly and indirectly, charge separation occurs generating C60*--PBTDP-TPA*+ in polar solvents. The C60*--PBTDP-TPA*+ formed via the singlet excited states decayed within a few nanoseconds as revealed by the picosecond transient absorption spectra. In the nanosecond time region, C60*--PBTDP-TPA*+ is produced slowly, probably via 3C60*-PBTDP-TPA. Lifetimes of such slowly generated C60*--PBTDP-TPA*+ were longer than 1 micros, which are the longest values among the C60-bridge-TPA triad systems reported hitherto at room temperature. Roles of the PBTDP-TPA moiety with twisted intermolecular charge-transfer character playing as energy donor and electron donor in addition to the bridge have been disclosed.