Transient Bleaching Research Articles

Unusually Long Lifetime of Excited Charge-Transfer State of All-Inorganic Binuclear TiOMnII Unit Anchored on Silica Nanopore Surface

The lifetime and back electron transfer kinetics of an all-inorganic, oxo-bridged heterobinuclear TiIVOMnII group covalently anchored on a silica nanopore surface was investigated by transient optical absorption spectroscopy. Mesoporous silica particles of type SBA-15 loaded with TiOMn sites (1 wt %) were suspended in an index matching liquid for performing spectroscopy in transmission mode. Upon excitation of the TiIVOMnII → TiIIIOMnIII metal-to-metal charge-transfer transition (MMCT) by a visible laser pulse of 8 ns duration, a transient bleach was observed whose intensity versus pump wavelength dependence agreed with the MMCT absorption profile. The decay kinetics is well described by a superposition of first-order rates with a mean time constant of 1.8 ± 0.3 μs (room temperature). The dispersion of γ = 2 ± 0.2 (Albery model) is attributed to variations in the local silica coordination environment reflecting the disordered, amorphous nature of the silica nanopore surface. The result constitutes the first observation of the electron transfer kinetics of an all-inorganic heterobinuclear group. It is proposed that the microsecond lifetime, unusually long for such as small charge transfer chromophore, originates from strong polarization of the local and remote silica environment upon light-triggered electron transfer from Mn to Ti. This results in a substantial reorganization barrier for back electron transfer. The long lifetime makes oxo-bridged heterobinuclear units anchored on silica surfaces efficient visible light photocatalysts and suitable as charge-transfer chromophores for driving multielectron catalysts in artificial photosynthetic systems.

Exciton Dynamics in CdS−Ag2S Nanorods with Tunable Composition Probed by Ultrafast Transient Absorption Spectroscopy

Electron relaxation dynamics in CdS−Ag2S nanorods have been measured as a function of the relative fraction of the two semiconductors, which can be tuned via cation exchange between Cd2+ and Ag+. The transient bleach of the first excitonic state of the CdS nanorods is characterized by a biexponential decay corresponding to fast relaxation of the excited electrons into trap states. This signal completely disappears when the nanorods are converted to Ag2S but is fully recovered after a second exchange to convert them back to CdS, demonstrating annealing of the nonradiative trap centers probed and the robustness of the cation exchange reaction. Partial cation exchange produces heterostructures with embedded regions of Ag2S within the CdS nanorods. Transient bleaching of the CdS first excitonic state shows that increasing the fraction of Ag2S produces a greater contribution from the fast component of the biexponential bleach recovery, indicating that new midgap relaxation pathways are created by the Ag2S material. Transient absorption with a mid-infrared probe further confirms the presence of states that preferentially trap electrons on a time scale of 1 ps, 2 orders of magnitude faster than that of the parent CdS nanorods. These results suggest that the Ag2S regions within the heterostucture provide an efficient relaxation pathway for excited electrons in the CdS conduction band.

A spectroscopic and photochemical study of Ag +-, Cu 2+-, Hg 2+-, and Bi 3+-doped Cd xZn 1−xS nanoparticles

A combination of stationary and time-resolved absorption and photoluminescence spectroscopy with flash and steady-state photolysis of Ag +, Cu 2+, Hg 2+, or Bi 3+-doped Cd x Zn 1− x S nanoparticles was used to assess the nature of the doping influence upon the optical properties of Cd x Zn 1− x S nanoparticles. The relationships between the type and the concentration of a dopant and the dynamics of the photoinduced processes in the doped nanoparticles are derived and discussed. A correlation is found between the magnitude of doping-induced changes in the intensity and decay dynamics of the deep trap photoluminescence and an enhancement of the transient bleaching recovery and acceleration of the photocorrosive degradation of the doped Cd x Zn 1− x S NPs compared to the undoped ones. The impact of the dopant upon the intensity of the luminescence and microsecond transient bleaching bands was found to grow substantially from Ag + to Cu 2+, Hg 2+ and Bi 3+. The same trend was found to hold for the acceleration of the steady-state photochemical corrosion of doped Cd x Zn 1− x S nanoparticles. The differences among the effect of the dopant ions studied were interpreted in terms of the depth and charge of surface states created by Cd 2+ (Zn 2+) substitution by a dopant.

Simultaneous Transient Absorption and Transient Electrical Measurements on Operating Dye-Sensitized Solar Cells: Elucidating the Intermediates in Iodide Oxidation

Simultaneously acquired transient absorption, photocurrent, and photovoltage signals are used to characterize operational dye-sensitized solar cells (DSSCs) under 1 sun illumination. At open circuit, the biphasic decay of both photovoltage and 980 nm absorption have identical decay profiles. The data show that, in devices incorporating N719 [(Bu4N)2[Ru(dcbpyH)2(NCS)2] (dcbpy = 4,4′-dicarboxy-2,2′-bipyridyl) as the sensitizer, the 950−1020 nm absorption can be assigned to electrons alone. This differs from some previous assignments. At short circuit, the comparison of the integrated photocurrent transient, electron absorption, and the absorption of the oxidized dye (830 nm) can be used to show relative loss of photocurrent to oxidized dyes. In addition, we show that, under operational conditions, there is no evidence for long-lived di-iodine radicals (I2•−). Further, we show that the decay of the transient bleach at 560 nm, previously assigned to the bleach of the ground-state dye, contains additional long-lived bleach components that correlate with the electron transient. This calls into question the use of the bleach transients for kinetic studies of dye regeneration. The use of simultaneous transient absorption and electrical signals significantly increases the information available from transient absorption studies, at little extra cost in time or equipment.

Direct and Inverse Auger Processes in InAs Nanocrystals: Can the Decay Signature of a Trion Be Mistaken for Carrier Multiplication?

A complete and detailed theoretical investigation of the main processes involved in the controversial detection and quantification of carrier multiplication (CM) is presented, providing a coherent and comprehensive picture of excited state relaxation in InAs nanocrystals (NCs). The observed rise and decay times of the 1S transient bleach are reproduced, in the framework of the Auger model, using an atomistic semiempirical pseudopotential method, achieving excellent agreement with experiment. The CM time constants for small core-only and core/shell nanocrystals are obtained as a function of the excitation energy, assuming an impact-ionization-like process. The resulting lifetimes at energies close to the observed CM onset are consistent with the upper limits deduced experimentally from PbSe and CdSe samples. Most interestingly, as the Auger recombination lifetimes calculated for charged excitons are found to be of a similar order of magnitude to those computed for biexcitons, both species are expected to exhibit the fast decay component in NC population dynamics so far attributed exclusively to the presence of biexcitons and therefore identified as the signature of CM occurrence in high-energy low-pump-fluence spectroscopic studies. However, the ratio between trions and biexcitons time constants is found to be larger than the typical experimental accuracy. It is therefore concluded that, in InAs NCs, it should be experimentally possible to discriminate between the two species and that the origin of the observed discrepancies in CM yields is unlikely to lay in the presence of charged excitons.

Terahertz pulse induced intervalley scattering in photoexcited GaAs

Nonlinear transient absorption bleaching of intense few-cycle terahertz (THz) pulses is observed in photoexcited GaAs using opticalpump--THz-probe techniques. A simple model of the electron transport dynamics shows that the observed nonlinear response is due to THz-electric- field-induced intervalley scattering over sub-picosecond time scales as well as an increase in the intravalley scattering rate attributed to carrier heating. Furthermore, the nonlinear nature of the THz pulse transmission at high peak fields leads to a measured terahertz conductivity in the photoexcited GaAs that deviates significantly from the Drude behavior observed at low THz fields, emphasizing the need to explore nonlinear THz pulse interactions with materials in the time domain.

Excitation relaxation in copper selenide nanowires

AbstractUltrafast nonequilibrium charge carrier relaxation in highly ordered Cu2–x Se nanowires of 8 nm, 13 nm and 25 nm diameter was investigated by means of femtosecond pump–probe absorption spectroscopy. Transient absorption bleaching was observed in the region of the near infrared absorption band, whereas an induced absorption dominated at higher energies. The transient absorption kinetics is almost independent of the excitation and probe wavelength and shows a biexpoenetial charge carrier recombination with the excitation intensity dependent decay rates. The initial ultrafast relaxation, which gets slower at higher excitation intensities, is followed by the slower decay component emerging at high intensities. These relaxation peculiarities are discussed in terms of recombination enhancement by intragap states, and relaxation kinetics is described by a theoretical model of two concurrent relaxation channels involving deep and shallow impurity levels. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrafast Electron−Hole Dynamics in Core/Shell CdSe/CdS Dot/Rod Nanocrystals

We investigated the transient bleaching and absorption of the asymmetric core/shell CdSe/CdS nanorods using the pump-probe technique. We observed ultrafast carrier relaxation and identified hole localization dynamics with 650 +/- 80 fs time constant. Upon pumping the CdSe core, we found an intense bleaching signal in the CdS spectral region, which we assigned to the delocalization of the electronic wave function on the basis of envelope-function theoretical calculations.

Transient bleaching pulse shapes in electrochromic polysiloxane thin films

Transient bleaching currents are examined for a series of electrochromic carbazole-modified polysiloxane thin films. The shape of the bleaching currents is found to be sensitive to the type of counter-ion employed and the initial density of counter-ion charge that is deposited in the films during the colouration stage. Pure space-charge-limited (SCL) transport of counter-ions that posses a well-defined mobility is inconsistent with the observed pulse shapes and a modified SCL model that involves fast and slow mobility sites is proposed. The modified SCL model may describe a variety of transient bleaching pulse shapes (including current overshoots) as charge is removed from the electrochromic films and the counter-ions are constrained to transfer from slow to fast discharge sites.

Photochemistry of a Retinal Protonated Schiff-Base Analogue Mimicking the Opsin Shift of Bacteriorhodopsin

A retinal Schiff base analogue which artificially mimics the protein-induced red shifting of absorption in bacteriorhodopsin (BR) has been investigated with femtosecond multichannel pump probe spectroscopy. The objective is to determine if the catalysis of retinal internal conversion in the native protein BR, which absorbs at 570 nm, is directly correlated with the protein-induced Stokes shifting of this absorption band otherwise known as the "opsin shift". Results demonstrate that the red shift afforded in the model system does not hasten internal conversion relative to that taking place in a free retinal-protonated Schiff base (RPSB) in methanol solution, and stimulated emission takes place with biexponential kinetics and characteristic timescales of approximately 2 and 10.5 ps. This shows that interactions between the prosthetic group and the protein that lead to the opsin shift in BR are not directly involved in reducing the excited-state lifetime by nearly an order of magnitude. A sub-picosecond phase of spectral evolution, analogues of which are detected in photoexcited retinal proteins and RPSBs in solution, is observed after excitation anywhere within the intense visible absorption band. It consists of a large and discontinuous spectral shift in excited-state absorption and is assigned to electronic relaxation between excited states, a scenario which might also be relevant to those systems as well. Finally, a transient excess bleach component that tunes with the excitation wavelength is detected in the data and tentatively assigned to inhomogeneous broadening in the ground state absorption band. Possible sources of such inhomogeneity and its relevance to native RPSB photochemistry are discussed.

Ultrafast Optical Responsive SWNT/Polymer Thin Films in Guided Wave Mode Geometry

AbstractSingle walled carbon nanotubes (SWNT) having good optical and electrochemical properties were solublized by aromatic polymers. The polymer‐wrapped SWNT film cast from solution showed transient bleaching with a time constant of recovery less than 1 ps in both the visible and optical telecommunication wavelength region. Polymer thin film containing SWNT was assembled into the guided wave mode geometry with a low refractive index polymer.

Effect of Mutation on the Dissociation and Recombination Dynamics of CO in Transcriptional Regulator CooA: A Picosecond Infrared Transient Absorption Study

The CO ligation process in a mutant (H77G) of CooA, the CO-sensing transcriptional regulator in Rhodospirillum rubrum, is studied with femtosecond time-resolved transient absorption spectroscopy in the mid-infrared region. Following photolyzing excitation, a transient bleach in the vibrational region of bound CO due to the CO photodissociation is detected. In contrast to the spectra of the wild-type (WT) CooA, the transient bleach spectra of H77G CooA show a bimodal shape with peaks shifting to the lower frequency during spectral evolution. The CO recombination dynamics show single-exponential behavior, and the CO escaping yield is higher than that of the WT CooA. A reorientation process of CO relative to the heme plane during recombination is revealed by anisotropy measurements. These phenomena indicate changes in the protein response to the CO ligation and suggest an alteration to the CO environment caused by the mutation. On the basis of these results, the role of His77 in the CO-dependent activation of CooA and a possible activation mechanism involving collaborative movement of the heme and the amino residues at both sides of the heme plane are discussed.

Nonlinear optical properties and Q-switch performance of silica glasses doped with Cu_xSe nanoparticles

Glasses containing copper selenide nanoparticles (CuxSe) reveal an intense absorption band peaking at 1 μm (1.24 eV). The transient bleaching and intensity-dependent transmission of silica glasses with CuxSe nanoparticles of different stoichiometry are studied with 1.08 μm (1.15 eV) picosecond pulses. The bleaching relaxation time decreases with a shift in the absorption band maximum to the lower photon energies. The dependence of absorption on the input energy of the pulses is saturationlike at the beginning of the plateau at ~40 mJ/cm2. Passive Q-switching of the Nd3+:KGd(WO4)2 laser at 1.067 μm is realized with the CuxSe-doped glasses for different x.

Vibrational coherence in bacterial reaction centers with genetically modified B-branch pigment composition

Femtosecond absorption difference spectroscopy was applied to study the time and spectral evolution of low-temperature (90 K) absorbance changes in isolated reaction centers (RCs) of the HM182L mutant of Rhodobacter (Rb.) sphaeroides. In this mutant, the composition of the B-branch RC cofactors is modified with respect to that of wild-type RCs by replacing the photochemically inactive B B accessory bacteriochlorophyll (BChl) by a photoreducible bacteriopheophytin molecule (referred to as Φ B). We have examined vibrational coherence within the first 400 fs after excitation of the primary electron donor P with 20-fs pulses at 870 nm by studying the kinetics of absorbance changes at 785 nm (Φ B absorption band), 940 nm (P*-stimulated emission), and 1020 nm (B A − absorption band). The results of the femtosecond measurements are compared with those recently reported for native Rb. sphaeroides R-26 RCs containing an intact B B BChl. At delay times longer than ∼50 fs (maximum at 120 fs), the mutant RCs exhibit a pronounced BChl radical anion (B A −) absorption band at 1020 nm, which is similar to that observed for Rb. sphaeroides R-26 RCs and represents the formation of the intermediate charge-separated state P +B A −. Femtosecond oscillations are revealed in the kinetics of the absorption development at 1020 nm and of decay of the P*-stimulated emission at 940 nm, with the oscillatory components of both kinetics displaying a generally synchronous behavior. These data are interpreted in terms of coupling of wave packet-like nuclear motions on the potential energy surface of the P* excited state to the primary electron-transfer reaction P* → P +B A − in the A-branch of the RC cofactors. At very early delay times (up to 80 fs), the mutant RCs exhibit a weak absorption decrease around 785 nm that is not observed for Rb. sphaeroides R-26 RCs and can be assigned to a transient bleaching of the Q y ground-state absorption band of the Φ B molecule. In the range of 740–795 nm, encompassing the Q y optical transitions of bacteriopheophytins H A, H B, and Φ B, the absorption difference spectra collected for mutant RCs at 30–50 fs resemble the difference spectrum of the P +Φ B − charge-separated state previously detected for this mutant in the picosecond time domain (E. Katilius, Z. Katiliene, S. Lin, A.K.W. Taguchi, N.W. Woodbury, J. Phys. Chem., B 106 (2002) 1471–1475). The dynamics of bleaching at 785 nm has a non-monotonous character, showing a single peak with a maximum at 40 fs. Based on these observations, the 785-nm bleaching is speculated to reflect reduction of 1% of Φ B in the B-branch within about 40 fs, which is earlier by ∼80 fs than the reduction process in the A-branch, both being possibly linked to nuclear wave packet motion in the P* state.

Nonadiabatic Molecular Dynamics Simulations of Correlated Electrons in Solution. 2. A Prediction for the Observation of Hydrated Dielectrons with Pump−Probe Spectroscopy

The hydrated dielectron is a highly correlated, two-electron, solvent-supported state consisting of two spin-paired electrons confined to a single cavity in liquid water. Although dielectrons have been predicted to exist theoretically and have been used to explain the lack of ionic strength effect in the bimolecular reaction kinetics of hydrated electrons, they have not yet been observed directly. In this paper, we use the extensive nonadiabatic mixed quantum/classical excited-state molecular dynamics simulations from the previous paper to calculate the transient spectroscopy of hydrated dielectrons. Because our simulations use full configuration interaction (CI) to determine the ground and excited state two-electron wave functions at every instant, our nonequilibrium simulations allow us to compute the absorption, stimulated emission (SE), and bleach spectroscopic signals of both singlet and triplet dielectrons following excitation by ultraviolet light. Excited singlet dielectrons are predicted to display strong SE in the mid infrared and a transient absorption in the near-infrared. The near-infrared transient absorption of the singlet dielectron, which occurs near the peak of the (single) hydrated electron's equilibrium absorption, arises because the two electrons tend to separate in the excited state. In contrast, excitation of the hydrated electron gives a bleach signal in this wavelength region. Thus, our calculations suggest a clear pump-probe spectroscopic signature that may be used in the laboratory to distinguish hydrated singlet dielectrons from hydrated electrons: By choosing an excitation energy that is to the blue of the peak of the hydrated electron's absorption spectrum and probing near the maximum of the single electron's absorption, the single electron's transient bleach signal should shrink or even turn into a net absorption as sample conditions are varied to produce more dielectrons.

Ultrafast Responses of Doubly N-Confused Hexaphyrin Derivatives

ABSTRACT Confused hexaphyrin derivatives, which are remarkable as the organic optical materials for information processing technology, have been synthesized. Hexaphyrin derivatives, free base type, substituted one and metal complex were studied in view of photophysical properties. The effects of π-conjugation have been studied by measuring of absorption and fluorescence spectra. They were excited by femtosecond pulsed laser to see the photoresponsibility and dynamics. Instantaneous changes, transient bleaching or transient absorption, will be discussed in the visible region.

Observation of Triplet Intraligand Excited States through Nanosecond Step-Scan Fourier Transform Infrared Spectroscopy

Nanosecond step-scan Fourier transform infrared spectroscopy permits the observation of triplet intraligand ((3)IL) character in the excited states of [Ru(bpy)2(PNI-phen)]2+ and [Ru(PNI-phen)3]2+ where PNI is 4-piperidinyl-1,8-naphthalimide. After pulsed 355-nm laser excitation, the two ground-state imide C=O bands in each compound are bleached and two substantially lower energy vibrations are produced; the lower energy feature appears as two distinct bands split by an overlapping transient bleach. Model studies confirm that the time-resolved vibrational data are consistent with photoinduced sensitization of the 3IL excited state. Density functional theory calculations also support these assignments because localization of triplet electron density on the PNI moiety is expected to lead to red-shifted C=O vibrations of magnitude similar to those measured experimentally. The current results illustrate that triplet electron density can be directly tracked by time-resolved infrared measurements in metal-organic chromophores and that frequency shifts comparable to those observed in charge-transfer systems can be realized.

Vibrational Lifetimes and Frequency-Gap Law of Hydrogen Bending Modes in Semiconductors

Vibrational lifetimes of hydrogen and deuterium related bending modes in semiconductors are measured by transient bleaching spectroscopy and high-resolution infrared absorption spectroscopy. We find that the vibrational lifetimes follow a universal frequency-gap law; i.e., the decay time increases exponentially with increasing decay order, with values ranging from 1 ps for a one-phonon process to 265 ps for a four-phonon process. The temperature dependence of the lifetime shows that the bending mode decays by lowest-order multiphonon process. Our results provide new insights into vibrational decay and the giant isotope effect of hydrogen in semiconductor systems.

Ultrafast Optical Responses of Doubly N-Confused Hexaphyrin Derivatives with Femtosecond Laser Excitation

Confused hexaphyrin derivatives were excited by femtosecond pulsed laser to study transient responses. An extended ?-conjugated system showed characteristic absorption changes with relatively weak power of laser irradiation. Recovery of transient bleaching was observed with time constants less than 100ps by both 400nm and 800nm excitation. Copper complex derivative showed the fastest response of 21 ps. Optical responses in the visible region and its mechanism are discussed.

Ultrafast Electron Relaxation Dynamics in Coupled Metal Nanoparticles in Aggregates

We report the effect of aggregation in gold nanoparticles on their ultrafast electron-phonon relaxation dynamics measured by femtosecond transient absorption pump-probe spectroscopy. UV-visible extinction and transient absorption of the solution-stable aggregates of gold nanoparticles show a broad absorption in the 550-700-nm region in addition to the isolated gold nanoparticle plasmon resonance. This broad red-shifted absorption can be attributed to contributions from gold nanoparticle aggregates with different sizes and/or different fractal structures. The electron-phonon relaxation, reflected as a fast decay component of the transient bleach, is found to depend on the probe wavelength, suggesting that each wavelength interrogates one particular subset of the aggregates. As the probe wavelength is changed from 520 to 635 nm across the broad aggregate absorption, the rate of electron-phonon relaxation increases. The observed trend in the hot electron lifetimes can be explained on the basis of an increased overlap of the electron oscillation frequency with the phonon spectrum and enhanced interfacial electron scattering, with increasing extent of aggregation. The experimental results strongly suggest the presence of intercolloid electronic coupling within the nanoparticle aggregates, besides the well-known dipolar plasmon coupling.