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Halogen atoms, particularly chlorine atoms, are well known to be highly reactive and to play a central role in the chemistry of the upper atmosphere. A large potential source of these halogens in the lower atmosphere (troposphere) exists in the form of sea salt particles. A variety of laboratory, field and modelling studies strongly suggests that there are heterogeneous reactions of sea salt particles which generate photochemically active halogen species such as Cl in marine areas. In addition, there is increasing evidence for a contribution of bromine atoms to tropospheric chemistry in marine regions at high latitudes. We review here briefly the potential importance of such halogen reactions and evidence for their role in the chemistry of the troposphere. Studies carried out in this laboratory to elucidate, at a molecular level, the mechanisms of reaction of synthetic sea salt and its components with gases of tropospheric interest are reviewed. Initial results obtained using a new aerosol apparatus recently constructed in this laboratory to study the reactions of aerosol particles above and below the deliquescence point of the salts are also discussed. © 1999 Taylor and Francis Group, LLC.

Organic–inorganic halocuprates based on monovalent copper are promising luminescent compounds for optoelectronic applications; however, their relaxation processes in the excited electronic state are severely underexplored. In this contribution, we prepare thin films of bis(tetraethylammonium) di-µ-bromo-dibromodicuprate(I) [N(C2H5)4]2[Cu2Br4], abbreviated (TEA)2Cu2Br4, which features a "molecular salt" structure containing discrete [Cu2Br4]2− anions. This compound, which has an absorption peak at 283 nm, displays a blue, strongly Stokes-shifted emission with a peak at 467 nm. Transient photoluminescence (PL) experiments using broadband emission detection and time-correlated single-photon counting (TCSPC) both find an excited-state lifetime of 57 μs at 296 K. UV–Vis transient absorption experiments at 296 K covering time scales from femto- to microseconds provide evidence for the formation of the T1 state through intersystem crossing from S1 with a time constant of 184 ps. The triplet state subsequently decays to S0 predominantly by phosphorescence. In addition, the time constants for carrier–optical phonon scattering (1.8 ps) and acoustic phonon relaxation (8.3 ns and 465 ns) of (TEA)2Cu2Br4 are provided. [ABSTRACT FROM AUTHOR]

The ultrafast internal conversion (IC) dynamics of aldehyde-substituted apocarotenoids (n'-apo-[beta]-caroten-n'-als with n = 4, 8 and 12) are examined in a systematic variation of conjugation length and solvent polarity by using time-resolved femtosecond transient absorption spectroscopy. In highly polar solvents, the IC to [S.sub.0] is very fast, irrespective of the conjugation length, which could be due to the stabilization of an intramolecular charge transfer (ICT) state in 12'-apo-[beta]-caroten-12'-al and 8'-apo-[beta]-caroten-8'-al.

The rate constants of the recombination reaction of p-fluorobenzyl radicals are measured over a pressure and temperature range with helium, argon, and CO2 employed as bath gases. Results confirm the significant role of radical complexes in the recombination kinetics of benzyl-type radicals in the gas-liquid transition range, while the kinetic contribution of a 'radical-complex' mechanism, solely based on radial-bath gas interactions, explain a rate enhancement.

Transient UV absorption spectra and kinetics of the CH2I radical in the gas phase are investigated at 313 K. The spectra are found to be a mixture of hot and cold CH2I radicals and parent CH2I2 in the early time range up to a few microseconds.

We present a femtosecond pump-probe UV-Vis broadband transient absorption spectroscopy study of two styryl-substituted BODIPY chromophores with different position of the substituent. The α-substituted isomer shows typical BODIPY-type spectral features, such as sharp absorption and emission bands, a small Stokes shift and an excited-state lifetime in the 4 ns range, which only weakly depends on the solvent. In contrast, β-styryl-BODIPY features much broader steady-state absorption and emission spectra and a larger Stokes shift, particularly in polar solvents. Transient absorption spectroscopy including a complete global kinetic analysis reports a substantial decrease in S1 lifetime to 300 ps for polar solvents upon change from α- to β-substitution. In the case of the α-isomer, TD-DFT calculations identify a typical "cyanine-type" electron rearrangement upon S0 → S1 excitation accompanied by a slight reduction in dipole moment. In contrast, the same transition in the β-isomer shows strong intramolecular charge transfer (ICT) character involving a substantial increase in dipole moment. Assuming a simple energy-gap-law argument, the accelerated nonradiative decay of the β-isomer in polar solvents may be linked to the decrease of the S1(ICT)-S0 energy difference. BODIPY dyes with a conjugated substituent in β-position therefore suffer a substantial loss in fluorescence brightness in polar environments compared with their α-substituted counterparts. This might limit their applicability in fluorescence imaging. [ABSTRACT FROM AUTHOR]

The microscopic environment in various ionic liquids (ILs) were probed by using the ultrafast excited-state dynamics of the carbonyl-containing carotenoid 12'-apo-[beta]-carotenoic-12'-acid (12'CA). The results suggested that the dynamics of 12'CA in ILs is influenced by the local electrostatic interactions between the cation charge (monopole) and the probe's dipole.

An attempt is made to study the ultrafast internal conversion (IC) dynamics of the apocarotenoid citranaxanthin for the first time by means of two-color transient lens (TL) pump-probe spectroscopy. It is suggested that the S1 lifetime [tau.sub.1] varies slightly between solvents 10-12 ps and the only clear decrease is observed for methanol.

The combination reactions CCl3 +CCl3(+M) --> C2Cl6(+M) and CCl3+ Br(+M) --> CCl3Br(+M) were studied at 250 and 300K over the pressure range of 0.01-1000 bar in the bath gases helium, argon, xenon, N2, CO2 and SF6. It was observed that in the helium, N2 and argon bath gases, at pressures above approximately 40 bar, the reactions became increasingly faster when pressure was further raised until they finally started to slow at densities where diffusion-controlled kinetics dominates.

Coherent phonon oscillations in the UV-Vis transient absorption and circular dichroism response of two chiral polyfluorene-based copolymer thin films are investigated. A slow oscillation in the hundred picosecond regime indicates the propagation of a longitudinal acoustic phonon with a frequency in the gigahertz range through cholesteric films of PFPh and PFBT, which allow for the optical determination of the longitudinal sound velocity in these polymers, with values of (2550 ± 140) and (2490 ± 150) m s−1, respectively. The oscillation is induced by a strain wave, resulting in a pressure-induced periodic shift of the electronic absorption bands, as extracted from a Fourier analysis of the transient spectra. The acoustic phonon oscillation is also clearly detected in the transient circular dichroism (TrCD) response of PFPh, indicating a transient pressure-induced shift of the CD spectrum and possibly also phonon-induced chirality changes via pitch length modulation of the cholesteric helical polymer stack. [ABSTRACT FROM AUTHOR]

An experimental study to derive the falloff curve and rate constants for the reaction CCl3 + O2(+M) - -> CCl3O2 (+M) at 260, 300 and 346K over the pressure range of 2-900bar is presented. An onset of diffusion-controlled kinetics at pressures above about 300 bar of N2 was observed, in agreement with predictions from simple diffusion models.