Ps Decay Component Research Articles

Picosecond Time-Resolved Infrared Absorption Studies on the Photoexcited States of Poly(p-phenylenevinylene)

Picosecond time-resolved infrared absorption spectra of the photoexcited states of poly(p-phenylenevinylene) [(C6H4CHCH)n, PPV] have been observed with tunable picosecond light pulses obtained by difference-frequency generation between signal and idler waves from an optical parametric generator and amplifier. The time-resolved photoinduced infrared absorption bands due to a vibrational transition observed at 1550 cm-1 and an electronic transition at 3000 cm-1 of the photoexcited PPV have fast (≈1.7 ps) and slow (≈50 ps) decay components. By comparison of the picosecond time-resolved infrared absorption spectra of short-lived (fast decay component) and long-lived (slow decay component) transient species with the photoinduced and doping-induced infrared difference spectra of PPV, the short-lived transient species is assigned to bound polaron pairs, and the long-lived transient species is assigned to positive and negative polarons.

Dynamic process in a hexameric benzo-porphyrin studied by femtosecond transient absorption

The fast kinetics of a hexameric benzo-porphyrin compound was investigated by femtosecond transient absorption spectroscopy. Using polychromatic detection, a sequence of spectra acquired at different delays revealed complex kinetic processes on the timescale of approximately 20 ps. The samples were then investigated at selected wavelengths by using a dual-colour femtosecond pump–probe setup. The same sequence of measurements was carried out under identical conditions on a benzo-porphyrin model sample. The comparison of the transient absorption decays recorded for both compounds revealed that only in the hexamer a 20 ps decay component is present which is attributed to an intramolecular interchromophoric excited-state process.

Nature of the power-dependent ultrafast relaxation process of photoexcited charge carriers in II-VI semiconductor quantum dots: Effects of particle size, surface, and electronic structure

The power-dependent relaxation dynamics of photoexcited charge carriers in a number of II-VI semiconductor quantum dots have been studied using femtosecond laser spectroscopy. The dynamics are obtained via excitation of the quantum dots with high power 390 nm pulses of 150 fs duration, and probing of the photoexcited species by monitoring the change in absorption at 790 nm as a function of time. Particles with vastly differing surfaces, sizes, electronic structures, and solvents all show a fast 1.5–4 picosecond decay component which grows in with power, a 17 ps (CdSe) or 50 ps (CdS and Cd0.5Zn0.5S) decay component, and some transient absorption persisting beyond 600 ps. The power-dependent component for CdSe quantum dots in glass has a 1.5 ps decay time constant, while for the liquid dispersed CdS and Cd0.5Zn0.5S quantum dots it has 2–4 ps decay time constants. This variation in the time constant is due to its power dependence, the time constant decreases with increasing power. It is also shown that the power-dependent decay is only weakly dependent on surface, size, and electronic structure. With the assistance of a power-dependent nanosecond fluorescence study, we have assigned the power-dependent decay primarily to exciton–exciton annihilation. This decay mechanism becomes dominant under high intensity excitation where multiple photoexcited charge carriers are created in each quantum dot, leading to trap state saturation and an accumulation of band edge excitons. Auger recombination may also play an important role at very high excitation intensities.

Excited States and Charge Separation in Membranes of the Green Sulfur Bacterium Prosthecochloris aestuarii

AbstractTime‐resolved absorbance changes were measured in isolated membranes, depleted of chlorosomes, and in the Fenna‐Matthews‐Olson (FMO) complex of the green sulfur bacterium Prosthecochloris aestuarri. The isolated FMO complex showed a biphasic decay of excited bacteriochlorophyll a (BChl a) with time constants of about 80 and 1400 ps. Approximately the same time constants were observed upon excitation of isolated membranes together with a component of about 30 ps. It is concluded that the efficiency of energy transfer from the FMO to the core complex is very low, in agreement with earlier measurements of the efficiency of charge separation. The 30 ps decay component is ascribed to trapping of the excitation energy from the core BChl a by the reaction center.

Energy transfer and trapping of excitations in membranes of Heliobacterium chlorum at 15 K

The transfer of excitation energy in membrane fragments of Heliobacterium chlorum was studied at 15 K by picosecond transient absorption spectroscopy. Due to the high repetition rate of the laser pulses, the primary electron donor was in the oxidized state in essentially all reaction centers. The absorbance difference spectrum showed that excitations in the antenna rapidly accumulate on the long wavelength absorbing component bacteriochlorophyll (BChl) g 808. Analysis of the kinetics of excited antenna BChl g showed a lifetime of less than 1 ps for excitations on BChl g 778, whereas excited BChl g 793 showed a somewhat slower decay of about 2 ps. Time resolved anisotropy measurements showed a very rapid randomization of excitations amongst pigments with similar absorption spectrum within less than 1 ps. Kinetics as well as the absorbance difference spectrum showed that the decay of excitations accumulated on BChl g 808 was strongly inhomogeneous. About 50% of the excitations disappeared with a time constant of 4 ± 2 ps, while the others showed decays with time constants of 20 ± 5 ps and 100 ± 20 ps. The amplitude of the 20 ps component decreased with the energy density of the excitation pulses and this decay component is attributed, at least in part, to singlet-triplet quenching. The main relaxation process of excitations with a time constant of about 4 ps is most probably due to energy transfer to the oxidized reaction center. The 100 ps decay component is associated with BChls g 808 that are relatively isolated, either spatially or energetically.

Excitation energy annihilation in aggregates of chlorophyll [formula omitted] complexes

Time-resolved picosecond absorption measurements were performed on aggregates of the light-harvesting chlorophyll (Chl) a b complexes from spinach thylakoids. Variation of the intensity of the exciting laser pulse showed that efficient excitation annihilation occurs in these aggregates. From the time-integrated absorption decays, the domain size of these aggregates was calculated to be at least 300 and probably about 1000 chlorophyll a molecules. From the time-resolved chlorophyll a excited state decays, the rate of excitation annihilation per pair of excitations, γ 2, was calculated to be (2–3)·10 9s −1. It was shown that the annihilation competes with a 400 ps decay component in the mono-excitation decay, probably due to Chl a b aggregates. A second decay phase of 2–3 ns is not associated with annihilation. The calculated domain size and annihilation rate constants correspond to a nearest-neighbour transfer rate of at least 2·10 11s −1 but probably closer to 10 12s −1. Our results show that although these Chl a b complexes are dissociated from the membrane, their aggregation is such that fast energy transfer occurs over a large number of chlorophyll molecules.

Energy migration and exciton trapping in green plant photosynthesis

The possible origins of the different fluorescence decay components in green plants are discussed in terms of a random walk and Butler's bipartite model. The interaction of the excitations with the photosystem II reaction centers and, specifically, the regeneration of theses excitations by charge recombination within the reaction centers, are considered. Based on comparisons between fluorescence decay profiles, time-dependent exciton annihilation and photoelectric phenomena, it appears that the fast 200 ps decay component corresponds to primary energy transport from the antenna to the reaction centers and is dominant in filling the photosystem II reaction centers.

Picosecond spectroscopy of photosystem I reaction centers

When iron-sulfur centers A and B [l] (or P430 [2]), the stable primary electron acceptors of photosystem I reaction centers, are in the reduced state, photochemical electron transfer occurs between P700 and the earlier acceptors A1 and AZ [3]. Acceptor AZ has been identified by EPR and optic spectroscopies as an iron-sulfur center [4]. Reduced AZ has an EPR signal with g-values at 1.78,1.88 and 2.08 [5,6], and it recombines with P700’ with a halftime of 130 ms at 5 K [4]. The absorbance change at 694 nm (U6w) measured 50 ps after excitation with a 694 nm excitation pulse is-2-times that ascribable solely to WOO photooxidation; it decays in % 200 ps to the level of WOO’ alone [7]. When iron-sulfur centers A and B are reduced, the aA has the same initial amplitude, but it decays in 2 phases with lifetimes of IO ns and 3 ps [7]. The optic and EPR spectra of the 3 ps component are consistent with the assumption that photochemical charge separation occurs between P700 and a chl a dimer (electron acceptor A,) [4,7]. This electron transfer apparently occurs in < 60 ps, followed by re-oxidation of the chl a dimer by P430 in % 200 ps. This work shows that the spectrum of the 200 ps decay component of the A,4 is consistent with that produced by the formation of an anion radical of a chl a dimer whose absorption-band maximum is at 695 nm.