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Femtosecond studies of exciton dynamics in a novel main chain chiral conjugated poly(arylenevinylene) (1997)

Zhang, J. Z. ; Kreger, M. A.

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 106 (1997), S. 3710-3720

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The formation and decay dynamics of photogenerated excitons in an optically active poly(arylenevinylene), PAV, in solution have been studied using femtosecond transient absorption spectroscopy. Photoexcitation initially creates hot excitons which quickly (〈200 fs) relax geometrically towards the equilibrium position in the excited state. The exciton subsequently decays following a double exponential with time constants of 6.5 and 420 ps in toluene. The decays become faster (5 and 250 ps) in pyridine, indicating a dependence of the relaxation process on the solvent environment. The fast decay is attributed to vibrational relaxation and internal conversion (recombination) of the exciton from the excited to the ground electronic state through tunneling or thermal-activated barrier crossing before thermalization. The slow decay is assigned to conversion of the thermalized exciton to the ground state through both radiative and nonradiative pathways. Anisotropy decay shows a fast component (6 ps in toluene and 10 ps in pyridine) and an offset which persists up to 650 ps. Possible explanations for the fast decay include internal conversion, vibrational relaxation, conformational change, and exciton migration. The offset may decay on a longer time scale through local reorientation of the conjugation segments, exciton migration, or rotational diffusion of the polymer. Comparison to a well-studied system, MEH-PPV [poly(2-methoxy, 5-(2-ethylhexoxy)-p-phenylenevinylene], provides further insight into the relaxation mechanism of photoexcitations in this PAV polymer. © 1997 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.473423

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Preparation and ultrafast optical characterization of metal and semiconductor colloidal nano-particles (1997)

Smith, B. A. ; Waters, D. M. ; Faulhaber, A. E. ; [et al.]

Springer

Journal of sol gel science and technology 9 (1997), S. 125-137

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ISSN: 1573-4846

Keywords: electron dynamics ; colloids ; metal and semiconductor nano-particles ; interfaces ; femtosecond laser spectroscopy

Source: Springer Online Journal Archives 1860-2000

Topics: Chemistry and Pharmacology

Notes: Abstract The ultrafast dynamics of photoinduced electrons in several metal and semiconductor colloidal nanoparticle systems are characterized using femtosecond laser spectroscopy. Various preparation methods are used and, in several cases, modified for making particles with long-term stability and narrow and controllable size distributions. The particle size and size distribution are determined using transmission electron microscopy and electronic absorption spectroscopy. For aqueous gold and silver colloids, spatial size confinement is found to cause substantially slower electronic relaxation due to reduction of non-equilibrium electron transport and weaker electron-phonon coupling. In gold colloids, photoejection of electrons into the liquid is observed, which is attributed to a two-photon enhanced ionization process. The effect of surfactant on the electron dynamics in CdS colloids is examined and found to be significant, substantiating the notion that electrons are dominantly trapped at the liquid-solid interface. In Ru3+-doped TiO2 colloids, the electronic decay is found to be as fast as or even faster than in undoped TiO2 and other semiconductor colloids such as CdS, suggesting that ion doping of large bandgap semiconductor colloids is not necessarily effective in lengthening the electron lifetime. In almost all cases studied, the majority of the photoinduced electrons are found to decay within a few tens of picoseconds due to non-radiative relaxation. The results are discussed in the context of the potential applications of metal and semiconductor nano-particles in areas including photocatalysis and photoelectrochemistry.