National Institute of Standards and Technology における超伝導研究及び生活

Occurrence, behavior and effects of nanoparticles in the environment.

In recent years, organic solar cells utilizing π-conjugated polymers have attracted widespread interest in both the academic and, increasingly, the commercial communities. These polymers are promising in terms of their electronic properties, low cost, versatility of functionalization, thin film flexibility, and ease of processing. These factors indicate that organic solar cells, although currently producing relatively low power conversion efficiencies (∼5-7%),1–3 compared to inorganic solar cells, have the potential to compete effectively with alternative solar cell technologies. However, in order for this to be feasible, the efficiencies of organic solar cells need further improvement. This is the focus of extensive studies worldwide. The backbone of a π-conjugated polymer is comprised of a linear series of overlapping pz orbitals that have formed via sp2 hybridization, thereby creating a conjugated chain of delocalized electron density. It is the interaction of these π electrons that dictates the electronic characteristics of the polymer. The energy levels become closely spaced as the delocalization length increases, resulting in a ‘band’ structure somewhat similar to that observed in inorganic solid-state semiconductors. In contrast to the latter, however, the primary photoexcitations in conjugated polymers are bound electron-hole pairs (excitons) rather than free charge carriers; this is largely due to their low dielectric constant and the presence of significant electron-lattice interactions and electron correlation effects.4 In the absence of a mechanism to dissociate the excitons into free charge carriers, the exciton will undergo radiative and nonradiative decay, with a typical exciton lifetime in the range from 100 ps to 1 ns. Achieving efficient charge photogeneration has long been recognized as a vital challenge for molecular-based solar cells. For example, the first organic solar cells were simple single-layer devices based on the pristine polymer and two electrodes of different work function. These devices, based on a Schottky diode structure, resulted in poor photocurrent efficiency.5–7 Relatively efficient photocurrent generation in an organic device was first reported by Tang in 1986,8 employing a vacuum-deposited CuPc/ perylene derivative donor/acceptor bilayer device. The differing electron affinities (and/or ionization potentials) between these two materials created an energy offset at their interface, thereby driving exciton dissociation. However, the efficiency of such bilayer devices is limited by the requirement of exciton diffusion to the donor/acceptor interface, typically requiring film thicknesses less than the optical absorption depth. Organic materials usually exhibit exciton diffusion lengths of ∼10 nm and optical absorption depths of 100 nm, although we note significant progress is now being made with organic materials with exciton diffusion lengths comparable to or exceeding their optical absorption depth.9–12 The observation of ultrafast photoinduced electron transfer13,14 from a conjugated polymer to C60 and the * To whom correspondence should be addressed. E-mail: j.durrant@ imperial.ac.uk. Chem. Rev. 2010, 110, 6736–6767 6736

Charge Photogeneration in Organic Solar Cells

Large polycyclic aromatic hydrocarbon (PAH) molecules carry the infrared (IR) emission features that dominate the spectra of most galactic and extragalactic sources. This review surveys the observed mid-IR characteristics of these emission features and summarizes laboratory and theoretical studies of the spectral characteristics of PAHs and the derived intrinsic properties of emitting interstellar PAHs. Dedicated experimental studies have provided critical input for detailed astronomical models that probe the origin and evolution of interstellar PAHs and their role in the universe. The physics and chemistry of PAHs are discussed, emphasizing the contribution of these species to the photoelectric heating and the ionization balance of the interstellar gas and to the formation of small hydrocarbon radicals and carbon chains. Together, these studies demonstrate that PAHs are abundant, ubiquitous, and a dominant force in the interstellar medium of galaxies.

Interstellar Polycyclic Aromatic Hydrocarbon Molecules

Mise au point. Theorie des effets de couplage vibronique multimodes. Probleme a 2 etats. Couplage vibronique mettant en jeu des modes et des etats degeneres. Effets du couplage vibronique multimodes en spectroscopie. Comportement statistique des niveaux d'energie vibroniques. Intersections coniques et evolution temporelle de la fluorescence

Multimode molecular dynamics beyond the Born-Oppenheimer approximation

A study was made of the solubilization of C[sub 60] in various solvents and systems of biological interest, i.e., octanols, micelles, and liposomes, using visible-UV absorption spectroscopy as a diagnostic tool. The state of incorporation of C[sub 60] molecules in micellar and colloidal liposome solutions was monitored using a number of spectroscopic criteria of solute-solvent and solute-solute interactions based on comparison with spectra obtained in alkane and octanol solvents and from thin films of C[sub 60]. Spectral red shifts and intensity modifications of C[sub 60] absorption and C[sub 60] aggregation are discussed in terms of environment-dependent physical parameters. The results indicate that C[sub 60] can be dispersed in micellar solutions of Triton X-100 and Triton X-100 R-S, the fullerene molecules being localized in the inner hydrophobic part of the micelles. C[sub 60] was shown to be incorporated, mainly as aggregates, into phosphatidylcholine liposome colloidal solutions. It is concluded that micellar and liposome solutions can be prepared which could be used to transfer individual C[sub 60] molecules, or groups of molecules, to biological cells. 38 refs., 9 figs., 2 tabs.

C60 in Model Biological Systems. A Visible-UV Absorption Study of Solvent-Dependent Parameters and Solute Aggregation

The optical limiting action of C60 in toluene solution is mainly due to reverse saturable absorption (RSA). It is shown that the formalism describing nonlinear optical response due to instantaneous two-photon absorption can be used in the case of sequential two-photon absorption, yielding effective values of the relevant parameters of optical nonlinearity due to RSA. The effective two-photon absorption parameter beta eff and the effective nonlinear refractive index parameter gamma eff', which are related respectively to the imaginary and real parts of the effective third-order susceptibility chi eff(3) were measured by the z-scan technique as a function of fullerene concentration and of incident laser intensity and wavelength over the 420-640 nm region. The concentration dependence of these parameters indicates that the solution is optically thin as far as the ground state of C60 is concerned, whereas the wavelength dependence confirms the applicability of the formalism used in the sequential two-photon absorption model. Comparisons are made with other z-scan results on C60.

http://iopscience.iop.org/article/10.1088/0953-4075/28/20/015/pdf

Concentration and wavelength dependence of the effective third-order susceptibility and optical limiting of C60 in toluene solution

Size effects on dissociation rates of polycyclic aromatic hydrocarbon cations : laboratory studies and astrophysical implications

Using synchrotron radiation as excitation source in the 6–22 eV photon energy region, a photoionization mass spectrometry study of three nucleic acid bases, adenine, thymine and uracil, revealed VUV-induced degradation pathways of these important biological molecules. The fragmentation patterns, ionization energies and ion appearance energies (AE) are reported, many for the first time, and are compared with results of electron impact and other studies. AE values enabled heats of formation of parent and some fragment ions to be revised or determined for the first time. Thermochemical data, coupled with the observed AEs, were also useful in clarifying dissociative photoionization pathways. The main neutral loss species are HCN for adenine, HNCO and CO for thymine and uracil, but many subsequent and other fragmentation pathways, including some not suggested previously, are observed and discussed. The hyperconjugation properties of the methyl group make CO loss easier in thymine than in uracil. The astrophysically important fragment ion HCNH + is shown to be formed by several fragmentation pathways in all three nucleobases. The relative importance of competitive fragmentation processes was determined in some cases. Some astrophysical implications concerning the prospects for observation and survival of these nucleic acid bases in the interstellar medium and in meteorites are briefly discussed.

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Photoion mass spectrometry of adenine, thymine and uracil in the 6–22 eV photon energy range

A photoionization mass spectrometry study in the 6–22 eV photon energy region of five amino acids, glycine-h5 and its -d5 isotopologue, a-alanine, b-alanine, a-aminoisobutyric acid and a-valine, revealed VUV-induced degradation pathways of these important biological molecules. The fragmentation patterns, ionization energies and ion appearance energies are reported, many for the first time, and are compared with results of electron impact and other studies. Assignment of ion peaks and determination of fragment ion formation channels were assisted by mass spectral data on deuterated isotopologues of the three proteinaceous amino acids studied. Thermochemical data, coupled with the observed ion appearance energies, was also useful in clarifying dissociative photoionization pathways. Ion pair formation appears to occur in certain low energy dissociation processes. Isomeric interconversion between a-alanine and b-alanine cations does not occur up to 20 eV excitation energy. Some astrophysical implications concerning the prospects for amino acid observation and survival in the interstellar medium and in meteorites are briefly discussed. 2003 Elsevier B.V. All rights reserved.

Sydney Leach

Papers

C60 in Model Biological Systems. A Visible-UV Absorption Study of Solvent-Dependent Parameters and Solute Aggregation

Concentration and wavelength dependence of the effective third-order susceptibility and optical limiting of C60 in toluene solution

Size effects on dissociation rates of polycyclic aromatic hydrocarbon cations : laboratory studies and astrophysical implications

Photoion mass spectrometry of adenine, thymine and uracil in the 6–22 eV photon energy range

Photoion mass spectrometry of five amino acids in the 6-22 eV photon energy range