Subpicosecond Relaxation Research Articles

Exciton Modeling of Energy-Transfer Dynamics in the LHCII Complex of Higher Plants: A Redfield Theory Approach

We propose an exciton model for the peripheral plant light-harvesting complex LHCII that allows us to explain the absorption (OD) and linear dichroism (LD) spectra, the superradiance (SR), the pump−probe transient absorption (TA), the three-pulse photon echo peak shift (3PEPS), and transient grating (TG) kinetics at different excitation wavelengths. To calculate the nonlinear response we used the Liouville equation for the density matrix, expanded up to the third order with respect to the external field, with the Redfield relaxation superoperator in the exciton eigenstate basis. We found a few configurations of the antenna, with specific chlorophyll (Chl) a/b identities, orientations, and site energies, that allowed a simultaneous fit of these data while taking into account the excitonic interactions, the static disorder, and weak exciton−phonon coupling which induced dephasing and relaxation (energy transfer) between the exciton states. The spectral density of the exciton−phonon coupling adjusted from the fit allowed us to determine the time scales and pathways of energy transfer in LHCII. We find that the intraband (Chl b→Chl b and Chl a→Chl a) energy-transfer dynamics includes sub-picosecond (250−600 fs) exciton relaxation within dimeric or, in the Chl a band, more complicated clusters, sub-picosecond (600−800 fs) hopping between spatially separated clusters (in the a band), and “slow” (picosecond) migration between localized states. The interband (Chl b→Chl a) transfer is characterized by the presence of very fast channels, the fastest taking only 120 fs, which connect both localized and dimeric b states with the a band. The overall relaxation/migration dynamics can be directly viewed by means of the time-dependent density matrix in the exciton and site representation. The latter representation allows us to visualize the time-dependent degree of delocalization. While the individual exciton states can be delocalized over 2−2.5 molecules in the Chl a region, thermal mixing results in a coherence size of 1.4−1.8 for the steady-state wave packet at room temperature. Altogether, we conclude that the experimentally determined dynamic and static properties of LHCII can be simulated very well on the basis of the proposed excitonic model.

Femtosecond relaxation dynamics in dialkoxy-substituted poly-(p-phenylenevinylene) derivatives

We investigated the sub-picosecond relaxation dynamics of two alkoxy-substitued poly-p-phenylenevinylenes that differ only for the structure of the alkoxy side chains. Photoinduced absorption, bleaching, and stimulated emission were observed in distinct spectral regions. Our results show that the temporal dynamics of stimulated emission strongly depends on excitation density. We ascribe such behavior to on-chain ultrafast nonlinear processes, which are almost independent on the substituents grafted onto the polymeric backbone.

Sub-picosecond electron relaxation of near-infrared intersubband transitions in n-doped (CdS/ZnSe)/BeTe quantum wells

We report on intersubband (ISB) absorption and ultrafast ISB energy relaxation of carriers in n-type doped (CdS/ZnSe)/BeTe quantum wells (QWs), grown by molecular-beam epitaxy. The highly n-type doped QW samples were obtained by introducing a few monolayer CdS into a ZnSe/BeTe QW, and ISB absorption with a peak wavelength as short as 1.62 μm, covering 1.55 μm within its absorption bandwidth, was achieved. The ISB carrier relaxation was investigated by means of femtosecond (∼150 fs) one-color pump and probe technique at the ISB absorption peak. The ISB carrier relaxation time of 270 fs was observed in the sample with the absorption peak at 1.82 μm. The slow decay component with a time constant of a few ps, which has been observed in ZnSe/BeTe QWs, was not observed in the (CdS/ZnSe)/BeTe QWs, indicating that the Γ(ZnSe)–X(BeTe) electron transfer is suppressed, as expected from the band alignment.

An Optical and Theoretical Investigation of the Ultrafast Dynamics of a Bisthienylethene-Based Photochromic Switch

The switching behavior of 1,2-bis(5-phenyl-2-methylthien-3-yl)cyclopentene is studied by means of polarization selective nonlinear optical spectroscopy and time-dependent density functional theory. The combined information from the observed population and orientational dynamics together with the results of theoretical calculations show that on a subpicosecond time scale rapid mixing and relaxation of electronic states occur, before switching takes place. Such preswitching dynamics was not studied in detail in these systems before. Then, the switching process itself occurs by the formation of a C-C bond in the central cyclopentene ring with a time constant of 4.2 ps. Driven by the ring closure, the side groups of the switch molecules rotate to a nearly coplanar conformation with a time constant of about 8 ps. The switching process is completed by relaxation of the vibrationally hot ground state of the closed form of the molecule to thermal equilibrium.

Electron transfer and solvation in 9,9 ′-bianthryl and derivatives: a sub-ps fluorescence upconversion study

The time-evolution of the fluorescence spectra of 9,9 ′-bianthryl and 9-carbazolyl-anthracene in ethanol and butanol have been studied with subpicosecond time-resolution by the fluorescence upconversion technique. For both compounds, an initial phase (<1 ps) with a structured anthracene-type fluorescence spectrum could be observed which transforms into an unstructured redshifted emission spectrum further shifting to the red on the time scale of the slow component of solvent relaxation. The results are compared to recent transient absorption studies and are interpreted as evidence for a subpicosecond relaxation process (ca 0.4 ps) occurring prior to electron transfer in these compounds.

Solvation dynamics of DCM in micelles

The solvation dynamics of 4-(dicyanomethylene)-2-methyl-6( p-dimethylaminostyryl) 4H-pyran (DCM) have been studied in neutral (Triton X-100, TX), cationic (cetyl trimethyl ammonium bromide, CTAB) and anionic (sodium dodecyl sulfate, SDS) micelles using picosecond time-resolved Stokes shift. Above the critical micellar concentration for all three micelles, DCM exhibits wavelength-dependent fluorescence decays. At short wavelengths, a fast decay is observed while, at long wavelengths, a distinct growth precedes the decay. The time-dependent Stokes shift indicates that the water molecules in the Stern layer of the micelles relax on a timescale which is markedly slower than the sub-picosecond relaxation dynamics in pure water.

Solvation dynamics in organized assemblies, 4-aminophthalimide in micelles

Solvation dynamics of 4-aminophthalimide (4-AP) has been studied in neutral (triton X-100, TX), cationic (cetyl trimethyl ammonium bromide, CTAB) and anionic (sodium dodecyl sulfate, SDS) micelles using picosecond time resolved Stokes shift. Above critical micellar concentration (CMC), for all the three micelles, 4-AP exhibits wavelength dependent fluorescence decays with a fast decay at the blue end and a slow decay accompanied by a distinct growth at the red end of the emission spectra. The time dependent Stokes shift indicates that the water molecules in the Stern layer of the micelles relax in ≈100 ps time scale which is markedly slower than the subpicosecond relaxation dynamics observed in ordinary bulk water.

Solvation Dynamics of Coumarin 480 in Micelles

The picosecond time-resolved Stokes shift of the laser dye, coumarin 480 (I) is studied in neutral (Triton X-100, TX), cationic (cetyltrimethylammonium bromide, CTAB), and anionic (sodium dodecyl sulfate, SDS) micelles. Above critical micellar concentration (cmc) for all three micelles I exhibits wavelength dependent fluorescence decays and a distinct growth at the long wavelengths. The time dependent Stokes shift studies indicate that the water molecules in the Stern layer of the micelles relax on the time scale 180−550 ps, which is slower than the subpicosecond relaxation dynamics observed in ordinary bulk water.

Electron relaxation in the quantum-Hall-effect geometry: One- and two-phonon processes.

The longitudinal-optical (LO) phonon-assisted electron relaxation has been investigated in a two-dimensional electron system in the quantum-Hall-effect geometry. The phonon emission rates versus inter-Landau-level separations are calculated. One LO- and two LO+LA-phonon emission processes via polar optical and deformation acoustical itneractions are considered. To obtain a finite relaxation rate associated with one-phonon emission, the allowance for the Landau-level broadening or for the LO-phonon dispesion is made. Below the LO-phonon energy ${\mathrm{\ensuremath{\omega}}}_{\mathrm{LO}}$ within an energy range of the order of \ensuremath{\Elzxh} \ensuremath{\surd}${\mathrm{\ensuremath{\omega}}}_{\mathit{B}}$/\ensuremath{\tau} the one-phonon relaxation rate exceeds 1 ${\mathrm{ps}}^{\mathrm{\ensuremath{-}}1}$. (Here \ensuremath{\tau} is the relaxation time deduced from the mobility, and ${\mathrm{\ensuremath{\omega}}}_{\mathit{B}}$ is the cyclotron fjrequency. In GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterostructure with the mobility \ensuremath{\mu}=25 ${\mathrm{V}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{m}}^{2}$ this range makes up 0.7 MeV.) The two-phonon emission has a significant contribution to the relaxation above ${\mathrm{\ensuremath{\omega}}}_{\mathrm{LO}}$. At energy separations of the order of ${\mathit{ca}}_{\mathit{B}}^{\mathrm{\ensuremath{-}}1}$ (c is the sound velocity, ${\mathit{a}}_{\mathit{B}}$ is the magnetic length) the LO+LA-phonon emission provides a mechanism of subpicosecond relaxation while in a wide energy range of the order of \ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{\mathit{B}}$ the subnanosecond relaxation can be achieved.

Solvation dynamics studied by ultrafast transient hole burning

The time-resolved solvation of s-tetrazine in propylene carbonate is studied by ultrafast transient hole burning. In agreement with mode-coupling theory, the temperature dependence of the average relaxation time follows a power law in which the critical temperature and exponent are the same as in other relaxation experiments. Our recent theory for solvation by mechanical relaxation provides a unified and quantitative explanation of both the subpicosecond phonon-induced relaxation and the slower structural relaxation.

Measurements of the subpicosecond relaxation rates of the first excited singlet states of some pseudoazulenes in solution

The subpicosecond population decay times of the S1 states of a number of pseudoazulenes have been measured in solvents of different polarity, using a two-photon, two-color, pump-probe method. The nonradiative decay rates of the S1 states vary exponentially with the S1–S0 energy gap, and the results are interpreted in terms of the energy gap law formalism of radiationless transition theory. The behavior of the excited states of the pseudoazulenes is compared with that of azulene and its derivatives.

Ultrafast dynamics of intersubband relaxation in GaAs quantum wells: hot carrier and phonon populations effects

The dynamics of intersubband relaxation in GaAs quantum wells and the role of hot carriers and the phonon distributions have been investigated using two different optical techniques with femtosecond resolution: 1) time-resolved photoluminescence and 2) pump and probe experiments. The (2→1) intersubband relaxation times have been measured as functions of well widths (100Å < L well < 220Å), under different experimental conditions (15K < T lattice < 300K, and 1×10 10 cm -2 < excitation densities < 1×10 12 cm -2). The electron intersubband relaxation time is deduced from the decay time of the n=2 well luminescence (or differential transmission) intensity. For thin wells (<150Å), a fast intersubband (2→1) relaxation time ≤ 3 ps has been measured. For thicker wells, the measured decay times are found to be critically dependent on the excitation conditions (vary from 5 ps to 40 ps). The well width dependence of the intersubband relaxation time does not show the strong dependence (2 orders of magnitude) predicted theoretically for electron-LO phonon scattering. Our results show that the hot phonon populations and the slow carrier cooling rate limit the observation of subpicosecond relaxation time. For thick well widths, our results also suggest that hot carriers effects play an important role in the intersubband relaxation mechanisms.

Subpicosecond thermalization and relaxation of highly photoexcited electrons and holes in intrinsic and p-type GaAs and InP.

A combined experimental and theoretical investigation of the thermalization and relaxation of optically excited electrons and holes in intrinsic and p-type bulk GaAs and InP is presented. Using 50-fs and 2-eV laser-excitation pulses the hot-carrier dynamics was studied by the transient-absorption changes at 2 eV and by time-resolved luminescence spectroscopy, with a time resolution of 50--80 fs. The materials and doping levels were chosen to obtain, in combination with extensive ensemble Monte Carlo simulations, detailed information on intervalley and interband transfers of electrons and holes and on the effects of nonequilibrium optic phonons, of ionizing carrier--neutral-acceptor scatterings, and of dynamical screening of the various carrier-carrier interactions. Very good agreement between theory and experiment is found for the transient-absorption bleachings and the band-gap luminescence as functions of time and doping. For times beyond 500 fs the effective plasma-temperatures ${\mathit{T}}_{\mathit{e}\mathit{f}\mathit{f}}$, defined from the measured luminescence spectra, are well reproduced by theory. For shorter times, the calculated ${\mathit{T}}_{\mathit{e}\mathit{f}\mathit{f}}$ are systematically too high. This deviation is tentatively ascribed to effects of collisional broadening and band nonparabolicities.

Quantum well intersubband relaxation in resonant tunneling diodes and transistors

The intersubband relaxation rate due to electron-electron interaction is calculated in analytical form and compared to the intersubband relaxation rates for polar optical phonon emission and for impurity scattering. It is found that for resonant tunneling diodes biased into the second resonance, the intersubband relaxation rate due to electron-electron scattering can become comparable to that due to phonon emission and can involve subpicosecond relaxation times. Taken together, these intersubband relaxation rates are sufficiently strong to significantly reduce the gain of a resonant tunneling transistor, i.e., a device in which an ohmic contact is made to the quantum well of a resonant tunneling diode. This leads us to propose a novel device structure similar to the resonant tunneling transistor, but where the subband for base potential modulation and the subband for hot charge transfer are located at different points in the Brillouin zone. This structure has the advantage that the intersubband relaxation rate is significantly reduced due to the increased momentum exchange on intersubband scattering. A possible material system that has all the needed band lineups to implement this device structure is the GaAs/AlAs/Ge material system.

Solvent-electronic state interactions measured from the glassy to the liquid state. I. Ultrafast transient and permanent hole burning in glycerol

Picosecond transient and permanent hole burning spectroscopies are used to study the interaction between glycerol and the electronic states of the nonpolar solute dimethyl-s-tetrazine. Variable temperature measurements from the low temperature glass into the fluid region have identified a phonon-modulated interaction in agreement with a previous hypothesis [J. Chem. Phys. 94, 5787 (1991)]. However, this mechanism alone cannot account for the magnitude of the subpicosecond line broadening seen at room temperature. A second relaxation mechanism is identified, which is connected to the structural coordinates involved in the glass transition and reaches subpicosecond relaxation times at room temperature.

Subpicosecond relaxation of solvent perturbations of nonpolar electronic states measured by transient hole burning

The results of transient hole burning of the nonpolar solute dimethyl-s-tetrazine (DMST) in glycerol, glycerol triacetate, sorbitol hexaacetate, squalane, and hexane solutions are reported. Despite the high viscosity and diversity of chemical types of the solvents, the solvent-induced perturbations of the electronic states of DMST relax in less than 1 ps in all cases. This contrasts strongly with the relaxation of polar electronic states, which are typically slow in viscous solvents. A new type of solvent-interaction mechanism appears to be important. We suggest that short-range repulsions modulated by phonon-like solvent modes are responsible.

Femtosecond time-resolved coherent Raman scattering from β-carotene in solution. Ultrahigh frequency (11 THz) beating phenomenon and sub-picosecond vibrational relaxation

Femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) has been observed for all-trans β-carotene in various solvents. A regular oscillating pattern (11 THz) owing to the beat among the two strong Raman transitions (≈ 1520 and ≈ 1150 cm −1) is observed when the frequency difference between the pump and Stokes radiation is set in-between the two strong Raman bands. The vibrational relaxation rate has been measured for the “in-phase” C=C stretching band. The relaxation rate is found to be T 2 ≈ 0.6 ps and independent of solvent. This rate is due to intramolecular vibrational relaxation.

Monte Carlo modeling of femtosecond relaxation processes in AlGaAs/GaAs quantum wells

Results are presented from Monte Carlo simulations of the femtosecond relaxation of photoexcited electrons in AlGaAs/GaAs quantum wells. Two experiments are simulated: in one electrons are initially excited at high energies far from equilibrium, and in other electrons are excited at low energies close to the bottom of the band. The effects of electron-electron, polar optical phonon, and intervalley deformation potential scattering are studied. For comparison, subpicosecond relaxation in bulk GaAs is also discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ultrafast conformation equilibration in triphenyl methane dyes analyzed by time resolved induced photoabsorption

Dual frequency pump–probe induced photoabsorption recovery is studied in two triphenylmethane dyes, malachite green and crystal violet, using femtosecond laser pulses. Comparison of the decays observed for different polarization conditions allows us to analyze the symmetry of electronic states. We observe that the simplest C2 or D3 symmetries cannot account for all our observations. In addition to the previously reported electronic relaxation decays, we observe two additional subpicosecond relaxation components. A first rising component is attributed to the conformational relaxation of torsional modes and a faster decay can be interpreted as originating from solvent limited vibrational relaxation. We attribute the solvent dependence of these relaxations to the nonrigid nature of the triphenylmethanes.

Subpicosecond molecular dynamics studied by degenerate four-wave mixing with incoherent light.

A time-resolved study of the relaxation times of the third-order susceptibility for the optical Kerr effect in ${\mathrm{CS}}_{2}$ and nitrobenzene was made by degenerate four-wave mixing using 5-ns pulses from a broadband laser. In the case of ${\mathrm{CS}}_{2}$ we observed both subpicosecond (\ensuremath{\sim}0.2 ps) and picosecond (\ensuremath{\sim}2 ps) relaxation times for the susceptibility tensor element of ${\ensuremath{\chi}}_{\mathrm{xyyx}}^{(3)}$ and ${\ensuremath{\chi}}_{\mathrm{xxxx}}^{(3)}$ both at 553 and 623 nm. The observed relaxation times agree with those obtained utilizing subpicosecond pulses. The relaxation times of nitrobenzene could not be measured because the shorter relaxation time is shorter than the resolution time and the longer one is longer than that of ${\mathrm{CS}}_{2}$.