Relaxation Time Of Excited State Research Articles

Quasi-classical simulations of resonance Raman spectra based on path integral linearization.

Based on a linearization approximation coupled with path integral formalism, we propose a method derived from the propagation of quasi-classical trajectories to simulate resonance Raman spectra. This method is based on ground state sampling followed by an ensemble of trajectories on the mean surface between the ground and excited states. The method was tested on three models and compared to a quantum mechanics solution based on a sum-over-states approach: harmonic and anharmonic oscillators and the HOCl molecule (hypochlorous acid). The method proposed is able to correctly characterize resonance Raman scattering and enhancement, including the description of overtones and combination bands. The absorption spectrum is obtained at the same time, and the vibrational fine structure can be reproduced for long excited state relaxation times. The method can also be applied to dissociating excited states (as is the case for HOCl).

Universal anomaly of dynamics at phase transition points induced by Pancharatnam-Berry phase

Dynamical anomalies are often observed near both the continuous and first-order phase transition points. We propose that the universal anomalies could originate from the geometric phase effects. A Pancharatnam-Berry phase is accumulated continuously in quantum states with the variation of tuning parameters. Phase transitions are supposed to induce an abrupt shift of the geometric phase. In our multi-level quantum model, the quantum interference induced by the geometric phase could prolong or shorten the relaxation times of excited states at phase transition points, which agrees with the experiments, models under sudden quenches and our semi-classical model. Furthermore, we find that by setting a phase shift of π, the excited state could be decoupled from the ground state by quantum cancellation so that the relaxation time even could diverge to infinity. Our work introduces the geometric phase to the study of conventional phase transitions as well as quantum phase transition, and could substantially extend the dephasing time of qubits for quantum computing.

Relaxation Times and Population Inversion of Excited States of Arsenic Donors in Germanium

The relaxation times of excited states of arsenic dopant in germanium at cryogenic temperatures T < 15 K have been experimentally studied by the optical pump-probe method using radiation of a free-electron laser. Two variants of the excitation of impurity centers have been used in the experiment: (i) from the 1s(A1) ground state of the dopant and (ii) from the 1s(T2) first excited state having a finite thermal population. In the former variant, it has been shown that the decay times of the 2p0 and 3p± states are about 0.8 and 0.6 ns, respectively. In the latter variant, a single measurement can simultaneously provide the relaxation times of two 2p± and 1s(T2) states about 0.6 and no more than 0.16 ns, respectively. The data obtained have indicated the possibility of forming population inversion and the gain of terahertz radiation at the 2p± → 1s(T2) and 2p0 → 1s(T2) transitions at the optical excitation of the mentioned impurity centers.

Laser-induced persistent photovoltage on the surface of a ternary topological insulator at room temperature

Using time- and angle-resolved photoemission, we investigate the ultrafast response of excited electrons in the ternary topological insulator (Bi1-xSbx)2Te3 to fs-infrared pulses. We demonstrate that at the critical concentration x = 0.55, where the system becomes bulk insulating, a surface voltage can be driven at room temperature through the topological surface state solely by optical means. We further show that such a photovoltage persists over a time scale that exceeds ∼6 μs, i.e, much longer than the characteristic relaxation times of bulk states. We attribute the origin of the photovoltage to a laser-induced band-bending effect which emerges near the surface region on ultrafast time scales. The photovoltage is also accompanied by a remarkable increase in the relaxation times of excited states as compared to undoped topological insulators. Our findings are relevant in the context of applications of topological surface states in future optical devices.

Terahertz-range spontaneous emission under the optical excitation of donors in uniaxially stressed bulk silicon and SiGe/Si heterostructures

The results of measurements of the total terahertz-range photoluminescence of Group-V donors (phosphorus, antimony, bismuth, arsenic) in bulk silicon and SiGe/Si heterostructures depending on the excitation intensity are presented. The signal of bulk silicon was also measured as a function of uniaxial stress. The results of measurement of the dependence of the spontaneous emission intensity on the uniaxial stress is in rather good agreement with theoretical calculations of the relaxation times of excited states of donors in bulk silicon. Comparative measurements of the spontaneous emission from various strained heterostructures showed that the photoluminescence signal is caused by donor-doped silicon regions.

Ultrafast electron and energy transfer in dye-sensitized iron oxide and oxyhydroxide nanoparticles

An emerging area in chemical science is the study of solid-phase redox reactions using ultrafast time-resolved spectroscopy. We have used molecules of the photoactive dye 2',7'-dichlorofluorescein (DCF) anchored to the surface of iron(III) oxide nanoparticles to create iron(II) surface atoms via photo-initiated interfacial electron transfer. This approach enables time-resolved study of the fate and mobility of electrons within the solid phase. However, complete analysis of the ultrafast processes following dye photoexcitation of the sensitized iron(III) oxide nanoparticles has not been reported. We addressed this topic by performing femtosecond transient absorption (TA) measurements of aqueous suspensions of uncoated and DCF-sensitized iron oxide and oxyhydroxide nanoparticles, and an aqueous iron(III)-dye complex. Following light absorption, excited state relaxation times of the dye of 115-310 fs were found for all samples. Comparison between TA dynamics on uncoated and dye-sensitized hematite nanoparticles revealed the dye de-excitation pathway to consist of a competition between electron and energy transfer to the nanoparticles. We analyzed the TA data for hematite nanoparticles using a four-state model of the dye-sensitized system, finding electron and energy transfer to occur on the same ultrafast timescale. The interfacial electron transfer rates for iron oxides are very close to those previously reported for DCF-sensitized titanium dioxide (for which dye-oxide energy transfer is energetically forbidden) even though the acceptor states are different. Comparison of the alignment of the excited states of the dye and the unoccupied states of these oxides showed that the dye injects into acceptor states of different symmetry (Ti t2gvs. Fe eg).

Optical nonlinearity of organic dyes as studied by Z-scan and transient grating techniques

The excited state absorption cross-section of 5,5′-dichloro-11-diphenyl-amino-3,3′-diethyl-10, 12-ethylinethiatricarbocyanine perchlorate (IR140) have been measured by using a single beam transmission technique. Z-scan experiments have been used to find out a few nonlinear parameters. The excited state relaxation times have also been measured by using laser induced transient grating (LITG) technique.

Ultrafast relaxation times of metalloporphyrins by time-resolved degenerate four-wave mixing with incoherent light

We present experimental results on the measurement of the population-relaxation times of the excited states of some metalloporphyrins such as zinc meso-tetra-phenylporphyrin (ZnmTPP), zinc meso-tetra (p-methoxyphenyl) tetrabenzporphyrin (ZnmpTBP), tetratollylporphyrin (TTP), cobalt tetratollylporphyrin (CoTTP) and nickel tetratollylporphyrin (NiTTP) with time-resolved degenerate four-wave mixing with the incoherent emission of a nanosecond pumped-dye laser. The experiments are carried out in the counterpropagating geometry (k4=k1+k2-k3), where k1 and k2 are counterpropagating to one another and k3 is at angle θ (<90° with respect to k1). Excited-state and ground-state relaxation times are calculated based on the theoretical formalism developed by Hiromi Okamoto [J. Opt. Soc. Am. B, 10, 2353, 1993]. The signals are recorded by delaying the probe beam (k3) for different fixed delays of the backward pump beam (k2). We observe strong signal at a second peak owing to the coherence of beams k2 and k3 for samples exhibiting excited-state absorption and a narrow peak corresponding to the coherence of beams k1 and k3 for samples that do not exhibit excited-state or reverse-saturable absorption. The nonlinear absorption is studied with the Z-scan technique. From the experimental data, we estimate the relaxation times of excited states.

Polar enhancement of the nonlinear optical properties of Fe2O3 microcrystallites

The dependence on surface environment of laser-induced luminescence and the nonlinear optical response of Fe2O3 microcrystallites is studied. The luminescence is decreased, the third-order susceptibility enhanced, the excited state relaxation time is decreased and the reverse saturable absorption enhanced. These phenomena are analyzed in terms of the surface effect.

Fast energy transfer in green photosynthetic bacteria Chlorobium limicola studied by spectral hole burning

Results of persistent hole burning fluorescence spectroscopy of green sulphur photosynthetic bacterium Chlorobium limicola are presented. Homogeneous spectral linewidths were determined from holewidths burned into inhomogeneously broadened NIR fluorescence spectra of chlorosomal bacteriochlorophyll-c at 1.8-4.2 K. The determined excited state relaxation time is strongly influenced by ultrafast energy transfer.

RELAXATION PROCESS MEASUREMENTS OF EXCITED STA-TE IN TETRAPHENYL PERPHYRINS BY LASER FREQUENCY DOMAIN TECHNIQUE

The relaxation time of excited state of FeTTP in CHCl3 has been measured by laser frequency domain technique. The relaxation time of the first excited state is obtained to be 0.82 ps by the numerical fitting for the experimental curve.

Relaxation time of excited state of cationic-anionic polymethine dyes

Relaxation time of excited state of cationic-anionic polymethine dyes

Studies on chromophore coupling in isolated phycobiliproteins: III. Picosecond excited state kinetics and time-resolved fluorescence spectra of different allophycocyanins from Mastigocladus laminosus

The excited state kinetics of three different allophycocyanin (AP) complexes has been studied by picosecond fluorescence spectroscopy. Both the fluorescence kinetics and the decay-associated fluorescence spectra of the different complexes can be understood on the basis of a structural model for AP which uses (a) an analogy to the known x-ray determined structure of C-phycocyanin, (b) the biochemical analogies of AP and C-phycocyanin, and (c) the biochemical composition of AP-B (AP-681). A model is developed that describes the excited state kinetics as a mixture of internal conversion processes within a coupled exciton pair and energy transfer processes between exciton pairs. We found excited state relaxation times in the range of 13 ps (AP with linker peptide) up to 66 ps (AP-B). The trimeric aggregates AP 660 and AP 665 show one fast relaxation component each, as was expected on the basis of their symmetry properties. The lower symmetry of AP-B (AP-681) gives rise to two fast lifetime components (tau(1) = 23 ps and tau(2) = 66 ps) which are attributed to internal conversion and/or energy transfer between excitonic states formed by the coupling of symmetrically and spectrally nonequivalent chromophores. It is proposed that the internal conversion between exciton states of strongly coupled chromophores fulfills the requirements of the small energy gap limit. Thus, internal conversion rates in the order of tens of picoseconds are feasible. The influence of the interaction of the linker peptide on the properties of the AP trimer are manifested in the fluorescence kinetics. Lack of the linker peptide in AP 660 gives rise to a heterogeneity in the chromophore conformations and chromophore-chromophore interactions.

Comparison of experiment and theory for the resonance Raman spectrum of I2 in solution. II. The Raman excitation and depolarization profiles in n-hexane

The calculations for I2 in n-hexane, which were started in paper I [J. Chem. Phys. 85, 3791 (1986)] are improved by considering the effect of the D state and of charge–tranfer states. Changes in intensities and positions of the B states and the effect of the rotation of I2 in the excited states are also considered. The experimental excitation profiles are recalibrated and compared to calculations. The depolarization profiles are measured and are also compared to calculations. The measured Raman cross sections are higher than calculated in the high frequency wing of the profile—a discrepancy for which we have not found an explanation. The depolarization ratios of the overtones may be explained by shifting the B and B″ states about 200 cm−1 further apart than in the gas phase and by taking into account the effect of rotation of the I2. The excited state relaxation time, T2, is still determined as about 0.3 ps.

Equal-pulse correlation technique for measuring femtosecond excited state relaxation times

A new technique for measuring extremely fast excited state relaxation times, on the order of or less than the laser pulse width, in the presence of a longer relaxation time is described. The difficulties involved in using the conventional pump-and-probe technique and the corresponding advantages of the new technique are illustrated with numerical examples for specific three-level systems. Qualitative experimental results on semiconductors and organic dye molecules substantiating the results are shown.