Search

Dynamics of the real and imaginary parts of the dielectric permittivity of a photoexcited silver layer has been investigated by means of femtosecond pump-probe spectroscopy in a surface plasmon resonance Kretschmann configuration. Both real and imaginary permittivity parts experience changes in the visible-near--IR regions under silver excitation at 400 nm. The changes are stronger, particularly those of the imaginary part, at longer wavelengths. Three excited states are formed during the relaxation process, which are attributed to the nonequilibrium and equilibrium heated electron distributions and lattice heating. Different time evolutions of the real and imaginary dielectric permittivity parts are explained by different contributions from interband and intraband transitions caused by plasma frequency and electron scattering frequency variations. OCIS codes: 300.6530, 310.6860, 240.6680.

A time-resolved reflection pump-probe method was combined with a surface plasmon resonance technique in Kretschmann geometry for the investigation of ultrafast light-induced processes in thin films. Transient changes in the gold layer's reflectivity were observed when the layer was excited by 3 ps duration pulses with photon energy exceeding the interband transition and by probing with photon energy close to the interband transition. Comparison of the experimental and modeling results has shown that the imaginary part of the dielectric function of gold increases linearly during excitation, whereas the real part remains unchanged. The decay of the light-induced changes has two components. The first component is faster than the pulse duration, and the second is much longer than 1.5 ns; they are related to cooling of the electron plasma and lattice, respectively.

A study of the influence of high intensity laser light on the optical properties and exciton relaxation dynamics in CdSe nanocrystals passivated by tri-n-octylphosphine oxide and dispersed in the optically clear poly(butamethacrylate) polymer is reported. The modification of the optical properties of the CdSe/PBMA film is observed after exposure to high intensity laser pulses.

A new study investigates the origin of nonlinear absorption in PbS-doped glass.

Research was conducted to examine the excitonic transitions in organic molecular solids built up by dipolar molecules by various absorption and luminescence spectroscopy approaches. Films and crystals of dipolar molecules of N,N-dimethylaminobenzylidene 1,3-indandione (DMABI), which are typical examples of polar molecular compounds, were studied. For DMABI crystal, the exciton band parameters and the exciton-phonon coupling constant are estimated while the potential energy diagram of exciton self-trapping process is suggested.

Novel donor-acceptor compounds consisting of singly bonded fluorene (Fl), benzothiadiazole (BT), and carbazole (Cz) functional units in the same molecule were investigated. Analysis of the optical spectra and fluorescence transients of the compounds revealed the domination of intramolecular charge transfer (ICT) states with high fluorescence quantum yield (72%-85%). A similar Cz-Fl-Cz compound exhibiting 100% fluorescence quantum yield and no ICT character was also studied as a reference to reveal the impact of electron-accepting BT groups. Thorough examination of the optical properties of the compounds in different media, i.e., dilute solution and polymer matrix, indicated their twisted conformations due to steric hindrance in the ground state and flattened geometry in the excited state for both reference and ICT compounds. Remarkable fluorescence efficiency losses (amounting to 70%) observed upon casting the molecular solutions into neat films were determined to originate from the low-fluorescent twisted conformers and migration-facilitated exciton quenching. The majority of emission efficiency losses (over 70%) were caused by the twisted conformers, whereas only less than 30% by exciton-migration-induced nonradiative deactivation. [ABSTRACT FROM AUTHOR]

The dynamics of charge carrier generation in poly(spirobifluorene-co-benzothiadiazole) was investigated by electric field-induced fluorescence quenching and differential absorption measurements. Three different time domains of carrier generation have been identified: an ultrafast phase, a subnanosecond phase, and an entire lifetime phase. The charge generation efficiencies during the first and second phases were found to be almost independent of temperature, being about 25% and 10%, respectively, at an applied electric field of 1.3×106 V/cm, while the generation efficiency during the third phase increases from 2% at 80 K to 10% at room temperature. The results of transient spectroscopy measurements and quantum chemical calculations suggest an intramolecular charge transfer for about 1 ps from the alkoxy-substituted fluorene side group to the benzothiadiazole subunit of the main chain. The formation and evolution of the resulting charge transfer states determine the way of charge carrier generation. [ABSTRACT FROM AUTHOR]

Excited state relaxation of the poly(spirobifluorene-co-benzothiadiazole) has been investigated by means of the time resolved fluorescence and femtosecond transient absorption. The experimental results and their comparison with the similar data obtained for poly(fluorene-co-benzothiadiazole) show that the long-lived excited state with the reduced dipole moment of the transition to the ground state is formed on a subpicosecond time scale due to the exciton localization on a polymer backbone and spirobifuorene side group. Excited state stabilization reduces the transition dipole moment still further, causing fluorescence relaxation faster than the relaxation of the excited state. [ABSTRACT FROM AUTHOR]

Exciton diffusion in ladder-type methyl-substituted polyparaphenylene film and solution was investigated by means of femtosecond pump-probe spectroscopy using a combined approach, analyzing exciton-exciton annihilation, and transient absorption depolarization properties. We show that the different views on the exciton dynamics offered by anisotropy decay and annihilation are required in order to obtain a correct picture of the energy transfer dynamics. Comparison of the exciton diffusion coefficient and exciton diffusion radius obtained for polymer film with the two techniques reveals that there is substantial short-range order in the film. Also in isolated chains there is considerable amount of order, as revealed from only partial anisotropy decay, which shows that only a small fraction of the excitons move to differently oriented polymer segments. It is further concluded that interchain energy transfer is faster than intrachain transfer, mainly as a result of shorter interchain distances between chromophoric units. [ABSTRACT FROM AUTHOR]

Investigates the recombination kinetics of the geminate charge pairs, generated by photoexcitation of charge transfer complexes in sensitized poly-N-epoxypropylcarbazole, by transient absorption and time-resolved fluorescence techniques and modeled by Monte Carlo method. Dynamical blue shift of the fluorescence band.

Charge carrier mobility in organic semiconductors is not a constant value unambigously characterizing some particular material, but depends on the electric field, temperature and even on time after it was generated or injected. The time dependence is particularly important for the thin-film devices where charge carriers pass the organic layer before mobility reaching its stationary value. Here we give a review of experimental techniques with ultrafast timeresolution enabling one to address the mobility kinetics and analyse properties of the time-dependent mobility in conjugated polymers and organic solar cells. We analyse kinetics during the charge carrier generation and extraction of free charge carriers. The mobility typically decreases by several orders of magnitude on a picosecond-nanosecond time scale; however, its kinetics also depends on the investigation technique. The mobility kinetics in blends for bulk heterojunction solar cells strongly depends on the stoichiometric ratio of donor and acceptor materials. [ABSTRACT FROM AUTHOR]

Discusses the dynamics of charge carrier precursor photogeneration in titanyl phthalocyanine. Use of fluorescence and photocurrent measurements; Sample excitation by two time-separated light pulses; Generation of charge carrier precursors only during the short time interval after the exciton was created; Interval that is shorter than the fluorescence state lifetime.

Copyright of Lithuanian Journal of Physics is the property of Lithuanian Academy of Sciences and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)