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Phase-pure powders of the vacancy-ordered, metal-deficient perovskite cesium hexabromopalladate(IV), Cs2PdBr6, were synthesized and deposited as a dense nanocrystalline thin film by spin-coating a solution in DMSO on glass substrates. The thin films featured a panchromatic absorption spectrum with an extended wing reaching into the near-infrared region. Ultrafast broadband UV–vis–NIR pump–probe spectroscopy provided time scales for fast carrier trapping with a time constant of 1.4 ps followed by a biphasic recombination with a time constant of 31 ps and a much slower time constant, which was beyond our observation window of 1.5 ns. The recombination time constants were independent of the initial carrier number density over the range of 2.4 × 1017–2.9 × 1018cm–3, suggesting a trap-mediated recombination mechanism with a fast and slow channel. The transient absorption kinetics identified pronounced coherent oscillations, and a Fourier transform analysis revealed a dominant wavenumber of 187 cm–1, which─according to accompanying DFT calculations─arose from a strongly Raman-active A1gphonon mode. This finding suggested considerable electron–phonon coupling in this material. Electrical measurements employing step-scan Fourier transform photocurrent spectroscopy (FTPS) of Cs2PdBr6thin films deposited on interdigitated ITO electrodes found an extended electrical response of the film with a tail reaching 1400 nm, which is also in agreement with a substantial concentration of deep traps. Summarizing the above findings, although Cs2PdBr6appears to be an appealing material in terms of its extended absorption spectrum, its indirect band gap, the presence of deep traps, and its strong electron–phonon coupling render it a far-from-ideal absorber material for photovoltaic applications.

We present a detailed spectroscopic study of 1,3-bis(N-carbazolyl)benzene (mCP), a prototypical host material for blue emitters in organic light-emitting diodes, using time-resolved emission and ultrafast transient absorption spectroscopy. Upon photoexcitation, mCP dissolved in tetrahydrofuran and mCP dispersed at low number density in poly(methyl methacrylate) (PMMA) films show a monoexponential emission decay in the range 6–7 ns. Transient absorption experiments detect the formation of the T1triplet state from S1with a quantum yield of ca. 20%, a band maximum at 450 nm, and a lifetime on the microsecond time scale. The strong spectral overlap of S1stimulated emission and S1excited state absorption suggests that S1–S1singlet–singlet annihilation (SSA) based on Förster resonance energy transfer (FRET) should be feasible at high mCP concentrations in mCP:PMMA and neat mCP films. At these concentrations, the intensity of the mCP emission is strongly quenched and the spectra show a dramatic change exhibiting monomer, aggregate, and excimer emission bands. The small Stokes shift and thus the good overlap of the absorption and emission spectra of mCP open up the possibility for efficient diffusive S1singlet hopping in neat mCP films by means of a multi-step homo-FRET mechanism involving S1and the respective S0nearest neighbor. Fluence-dependent transient absorption measurements find pronounced S1–S1SSA. Kinetic modeling suggests a bimolecular diffusive SSA mechanism with a rate constant kdiffof 1.40 × 10–8cm3s–1. In contrast, modeling based on direct S1–S1FRET results in an unrealistically large Förster radius of 17 nm. These experiments therefore suggest that mCP molecules in S1efficiently diffuse through the mCP film by multi-step homo-FRET and annihilate upon approaching a critical distance to another mCP molecule in S1. Finally, we find clear spectral evidence for vibrationally hot mCP molecules (S0*) in the transient absorption spectra of neat mCP thin films, which cool down on a time scale from nanoseconds to 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 S1lifetime 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→ S1excitation 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)-S0energy 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.

Ultrafast broadband UV–vis transient circular dichroism (TrCD) and transient absorption (TA) spectroscopy are employed to study the dynamics of chiral polymer thin films. Helical cholesteric arrangements are formed by mixing chiral binaphthalene derivatives with the intrinsically achiral polyfluorene-based copolymer F8BT. The induced circular dichroism (ICD) response of F8BT shows a mirror-image-like shape when the chirality of the additive is changed. Peaks in the CD spectra reach large absolute values of about 800 mdeg. Photoexcitation by ultrashort laser pulses results in TrCD spectra of opposite sign for cholesteric polymer films of different helicity, demonstrating transient chiral discrimination on the femtosecond time scale over the wavelength range 260–700 nm. Differences between the excited-state and ground-state CD spectra considerably depend on the electronic transition. The excited-state processes in these systems are evaluated by a detailed kinetic analysis providing rate constants for the relaxation of the different singlet exciton states as well as singlet–singlet annihilation processes of F8BT involving diffusive bimolecular recombination and Förster resonance energy transfer.

The C40carotenoid capsanthin is of photophysical interest because it belongs to the family of terminally carbonyl-substituted apocarotenes. These have the potential to exhibit intramolecular charge transfer (ICT) character in the excited state. We studied its ultrafast dynamics in different solvents using broadband transient absorption spectroscopy in the 260–1600 nm range. Photoexcitation initially populated the S2(11Bu+) state which decayed to the S1(21Ag–) state in 120–150 fs. The lifetime of the S1state decreased from 10.3 ps (isooctane) to 9.1 ps (methanol). Together with the absence of stimulated emission this suggested a weak ICT character of S1. A steric influence of the five-membered ring in capsanthin was identified based on a comparison with the sister apocarotenone citranaxanthin which features methyl substitution at the keto group. While both apocarotenones exhibit the same S1lifetime in isooctane, the decrease in lifetime in polar solvents is weaker for capsanthin because presumably the five-membered ring sterically perturbs stabilization of ICT character by the solvent. For the S1state of capsanthin we further observed intramolecular vibrational redistribution and collisional energy transfer to the solvent with time constants of 340–420 fs and 8.5–9.1 ps, respectively. No evidence for excited-state photoisomerization of capsanthin was found.

Lead-free double perovskites have been proposed as promising nontoxic photovoltaic materials for the replacement of lead perovskites. While the latter ones reach remarkably high power conversion efficiencies (PCEs) above 23% in small lab devices, the lead-free double perovskites so far have severely underperformed, with PCEs below 3% for the prototypical system Cs2AgBiBr6, in spite of considerable optimization efforts by several groups. Here, we present a detailed study of Cs2AgBiBr6thin films deposited on poly(methyl methacrylate) and mesoporous TiO2. Femtosecond UV–vis–NIR transient absorption experiments clearly identify the presence of excitons. In addition, strong electron–phonon coupling via Fröhlich interactions is observed in terms of pronounced coherent oscillation of a strong A1goptical phonon mode of the double perovskite at 177 cm–1. Similar behavior is also found for the related vacancy-ordered perovskite Cs3Bi2Br9and the parent compound BiBr3. Excitonic effects and electron–phonon coupling are known to induce unwanted electron–hole recombination and hamper carrier transport. New strategies will thus be required for efficient carrier extraction at the interfaces of the double perovskite with electron and hole transport layers.

The ultrafast charge carrier dynamics of the metal-deficient lead-free perovskite derivative methylammonium antimony iodide ((CH3NH3)3Sb2I9, MASbI) is studied using femtosecond UV–vis–NIR transient absorption spectroscopy. The transient and steady-state absorption spectra show strong excitonic features. This is confirmed by an analysis of the near-band-gap absorption using Elliott’s theory and its reformulation by Tanguy with subsequent modeling of the free-carrier and exciton contributions using the Saha equation. Time scales of the different carrier scattering processes are identified. The kinetics in the UV–vis range is largely independent of pump laser fluence suggesting that the dynamics involves localized excitons, with recombination time constants of 190 ps and >5 ns. Damped oscillations in the kinetics indicate coherent phonon dynamics and strong electron-phonon coupling. Fourier transformation of the time-domain data provides the steady-state Raman spectrum of MASbI which is dominated by vibrations of the [Sb2I9]3–anion. Exciton dissociation at the interfaces with mesoporous TiO2and a triarylamine-based hole transport material is found to be negligible. Our experiments suggest that it will be challenging to fabricate efficient photovoltaic devices from such lead-free antimony- and also bismuth-based zero-dimensional perovskite derivatives, because nonradiative exciton recombination largely governs their carrier dynamics.

We present an investigation of the ultrafast charge carrier dynamics for the one-third metal-deficient lead-free perovskite (CH3NH3)3Bi2I9on mesoporous TiO2. Excitation of the perovskite at 400 or 505 nm leads to characteristic second-derivative-type spectral features in the transient absorption spectra, suggesting substantial contributions of bound excitons, in contrast to the widely used lead-based perovskites. The immediate appearance of broad NIR absorption is assigned to TiO2conduction band electrons formed by instantaneous dissociation of a subpopulation of excitons at the perovskite/TiO2interface. Excitation with excess energy above the perovskite’s band gap opens up an additional fast (70 fs) exciton dissociation channel with about 26% amplitude. Antisolvent-assisted synthesis of (CH3NH3)3Bi2I9reduces the crystallite size to about 500 nm but has only a minor effect on the carrier dynamics. The results suggest that photovoltaic applications of this material will likely require bulk-heterojunction architectures to efficiently split the excitons into free carriers.

Microcrystalline powders and thin films of the copper(II)-based layered hybrid organic–inorganic methylammonium (MA) and 2-phenylethylammonium (PEA) perovskite-inspired compounds (MA)2CuCl4, (MA)2CuCl2Br2, (MA)2CuBr4, (PEA)2CuCl4, (PEA)2CuCl2Br2, and (PEA)2CuBr4were synthesized and structurally characterized by X-ray diffraction. They are classified by monoclinic or orthorhombic space groups. Central structural elements are two-dimensional networks of distorted corner-sharing CuX6(X = halide) octahedra. The thin films showed pronounced texture effects, with the octahedral layers arranged parallel to the substrate’s surface. The steady-state and transient optical response in the UV–vis range was investigated in detail. All compounds showed pronounced ligand-to-metal charge-transfer (LMCT) bands in the visible range and weak ligand-field (LF) absorption bands in the red to near-infrared. With increasing bromide content, the LMCT absorption bands progressively shifted to longer wavelengths, whereas the LF bands were much less affected. All of these copper(II) compounds did not show photoluminescence in the visible range. Upon excitation into the LMCT bands, transient absorption measurements found an ultrafast nonradiative relaxation channel from the initially populated LMCT to the LF states on a picosecond timescale, with the resulting excess energy being dissipated as heat. This photophysical property will be highly unfavorable for solar-cell applications as it will strongly limit the achievable open-circuit voltage. Internal conversion from the LF states to the electronic ground state occurred on a nanosecond timescale for all systems.

Silver iodobismuthate thin films obtained from mixtures of AgI and BiI3have been frequently suggested as promising alternatives for lead-based perovskite materials in photovoltaic applications. Here, we investigated (AgI)x(BiI3)ythin films with stoichiometric ratios (x/y) of 3:1, 2:1, 1:1, and 1:2 produced via a low-temperature (<100 °C) spin coating process from AgI/BiI3solutions in dimethyl sulfoxide. Several critical observations on the basis of ultrafast broadband UV–vis transient absorption spectroscopy and photocurrent spectroscopy were made for these Ag–Bi–I materials, which will have a considerable impact on their photovoltaic performance: Their carrier recombination kinetics were independent of the initial carrier number density over the range of 1.3–6.6 × 1017cm–3and well approximated by a monoexponential decay with a rate constant krecin the range of 2.0–4.3 × 108s–1, which is consistent with trap-mediated charge-carrier recombination. Moreover, pronounced coherent phonon dynamics was observed for all of these (AgI)x(BiI3)ycompounds, thereby suggesting strong electron–phonon coupling, which will favor charge-carrier localization and nonradiative carrier recombination, in agreement with the virtually absent photoluminescence of these materials. In addition, Fourier-transform step-scan photocurrent spectroscopy (FTPS) on AgBi2I7provided an Urbach energy of 70 meV and found indications for deep defects (0.6 eV below the band gap), which was also consistent with a trap-mediated recombination mechanism. A combination of all of these effects is likely responsible for the still quite low light-harvesting performance of this class of materials, which has been reported to show photovoltaic conversion efficiencies (PCEs) below 5%. Finally, for “AgI-rich” compounds (3:1, 2:1), we found a substantial separate contribution of carrier loss processes through ultrafast relaxation in AgI domains with time constants of 0.73 and 2.4 ps.

Ultrafast photoinduced electron injection and subsequent relaxation steps of the indoline dye D149 were investigated for mesoporous thin films using pump–supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370–770 nm. Three types of mesoporous ZnO layers were compared: Electrodeposited ZnO thin films prepared by using the structure-directing agents (1) Eosin Y (EY) and (2) Coumarin 343 (C343), which are known to produce ZnO layers of different morphology; and (3) mesoporous ZnO thin films consisting of sintered nanoparticles with a diameter of 20 nm. As a reference system, we used a “non-injecting” mesoporous thin film with large band-gap, consisting of Al2O3nanoparticles with a diameter of 50 nm. On all ZnO thin films, D149 efficiently injects electrons. We observed two components, a dominant one with τ≤ 70 fs (time-resolution-limited) and a slower one with a time constant of 250–350 fs. Fast initial charge separation is also consistent with the immediate appearance of oscillatory structure due to a change in refractive index of ZnO upon electron injection. For all ZnO thin films, we observe a transient shift of the spectra with a time constant of ca. 20 ps which is assigned to a transient Stark effect (= electrochromism). The S0→ S1absorption band of D149 is shifted due to the build-up of a local electric field between dye radical cations and ZnO conduction band electrons. On longer timescales, step-scan FTIR spectroscopy revealed a slightly faster cation-electron recombination on ZnO/C343 than on ZnO/EY thin films. On Al2O3, metastable close cation-electron pairs are formed initially which recombine afterwards, and no transient Stark effect is observed. Complementary PSCP experiments on D149 in the ionic liquid [C2mim]+[N(CN)2]–show that D149 is ideally suited for solar cell applications because of its long S1lifetime of 590 ps. In addition, biphasic solvation dynamics are observed in this IL with a subpicosecond component and a much slower component related to the viscosity of the IL which is well described by a stretched exponential.

We investigated the excited state dynamics of selected all-transC40xanthophyll carotenoids by ultrafast pump-supercontinuum probe (PSCP) spectroscopy in the spectral range 350–770 nm upon photoexcitation by a femtosecond laser (ca.80 fs pulse length) near 500 nm in acetone. The following carotenoids were selected considering a systematic variation of the number and position of carbonyl (CO) and hydroxyl (OH) functional groups on the β-ionone rings and the change of the effective conjugation length of the polyene system: β-cryptoxanthin (1), echinenone (2), 3-hydroxyechinenone (3), 3′-hydroxyechinenone (4), canthaxanthin (5), adonirubin (6) and astaxanthin (7). The carotenoids featuring an increasing conjugation length, for example, (1), (3) and (6), showed a systematically shorter S1lifetime of 8.7, 6.2 and 4.7 ps, respectively. Carotenoids with OH groups adjacent to a CO group showed slightly broader steady-state and transient absorption bands, but the influence on the S1lifetime was minor compared to the case of CO substitution. Moreover, in all cases we observed clearly visible “S*signals”, namely a longer-lived characteristic S0→ S2red-edge absorption and a negative signal in the S0→ S2bleach region. This spectral signature is assigned to highly vibrationally excited molecules in the ground electronic state S0*which are generated by internal conversion from S1. The S0*species cool with a time constant of 9.2–9.6 ps in the case of the keto-substituted carotenoids (2)–(7), whereas we obtain a larger value of 13.2 ps for (1) which does not have a keto group. Our findings are supported by results from a global kinetic analysis procedure.

We investigated the ultrafast carrier dynamics and phonon relaxation of CVD-grown monolayer and 9-layer graphene on a quartz substrate. Excitation was performed at 400 and 800 nm. The normalized change in optical density ΔOD was probed over the range 260–640 nm (1.94–4.77 eV), reaching down into the region of graphene’s Fano resonance, previously not investigated in femtosecond broadband pump–probe experiments. Time constants of 160 fs and 4 ps were found and assigned to carrier–optical phonon scattering and slower phonon relaxation processes, respectively. The carrier distribution at early times was clearly hotter for 400 nm excitation than for 800 nm excitation. A pronounced spectral bleach feature was observed below 300 nm. It immediately formed after photoexcitation and recovered slowly, with a time constant of 35 ps for monolayer and time constants of 120 and 970 ps for 9-layer graphene. The same dynamics were found for weak transient absorption features above 300 nm, which emerged after ca. 0.5 ps. The slow dynamics were assigned to interfacial heat flow from graphene to the quartz substrate. The bleach and absorption features were well described by a simple model assuming a red-shift of the Fano resonance. This red-shift disappeared with progressive cooling of graphene. We therefore suggest that the red-shift is induced by shrinking of the band separation due to lattice heating.

Molecular switches based on helical tetrasubstituted alkenes, substituted with either electron‐withdrawing (CF3, F, CN; 2 a–c, 3 a,c) or ‐donating substituents (Me, OMe; 2 d,e), have been synthesized from acyclic precursors 4and 5in a domino carbopalladation/Stille reaction. This palladium‐catalyzed process allowed the rapid assembly of two CC bonds, two six‐membered rings, and the tetrasubstituted double bond in a completely diastereoselective fashion. The electronic effects of the substituents on the overall switching process were investigated by alternating irradiation of two different wavelength regions. Although the substituents had only a small influence on the absorption maxima, drastic differences in the switching behavior were observed.

Auflösen und spionieren lassen: Durch Einführung einer Aldehyd‐Funktion in ein einfaches Carotinoid erhält man eine leistungsfähige laserspektroskopische Sonde für die Untersuchung der Solvatationsdynamik und der Polarität ionischer Flüssigkeiten durch ultraschnelle PSCP‐Breitbandabsorptionsspektroskopie (PSCP=pump supercontinuum probe).

The ultrafast internal conversion (IC) dynamics of the carbonyl carotenoid 12′-apo-β-caroten-12′-al has been investigated in solvents of varying polarity using time-resolved femtosecond transient absorption spectroscopy. The molecules were excited to the S2state by a pump beam of either 390 or 470 nm. The subsequent intramolecular dynamics were detected at several probe wavelengths covering the S0→ S2and S1→ Snbands. For the S1→ S0internal conversion process, a remarkably strong acceleration with increasing polarity was found, e.g., lifetimes of τ1= 220 ps (n-hexane), 91 ps (tetrahydrofuran) and 8.0 ps (methanol) after excitation at 390 nm. The observation can be rationalized by the formation of a combined S1/ICT (intramolecular charge transfer) state in the more polar solvents. The effect is even stronger than the strongest one reported so far in the literature for peridinin. Addition of lithium salts to a solution of 12′-apo-β-caroten-12′-al in ethanol leads only to small changes of the IC time constant τ1. In addition, we estimate an upper limit for the time constant τ2of the S2→ S1internal conversion process of 300 fs in all solvents.

The ultrafast internal conversion IC dynamics of seven C40carotenoids have been investigated at room temperature in a variety of solvents using two-color transient lens TL pump–probe spectroscopy. We provide comprehensive data sets for the carbonyl carotenoids canthaxanthin, astaxanthin, and—for the first time—echinenone, as well as new data for lycopene, β-carotene, 3R,3′R-zeaxanthin and 3R,3′R,6′R-lutein in solvents which have not yet been investigated in the literature. Measurements were carried out to determine, how the IC processes are influenced by the conjugation length of the carotenoids, additional substituents and the polarity of the solvent. TL signals were recorded at 800 nm following excitation into the high energy edge of the carotenoid S2band at 400 nm. For the S2lifetime solvent-independent upper limits on the order of 100–200 fs are estimated for all carotenoids studied. The S1lifetimes are in the picosecond range and increase systematically with decreasing conjugation length. For instance, in the sequence canthaxanthinechinenoneβ-carotene 131211 double bonds one finds τ1 5, 7.7 and 9 ps for the S1→ S0IC process, respectively. Hydroxyl groups not attached to the conjugated system have no apparent influence on τ1, as observed for canthaxanthinastaxanthin τ1 5 ps in both cases. For all carotenoids studied, τ1is found to be insensitive to the solvent polarity. This is particularly interesting in the case of echinenone, canthaxanthin and astaxanthin, because earlier measurements for other carbonyl carotenoids like, e.g., peridinin partly showed dramatic differences. The likely presence of an intramolecular charge transfer state in the excited state manifold of C40carbonyl carotenoids, which is stabilized in polar solvents, has obviously no influence on the measured τ1.

The data bases for low-pressure rate coefficients of the dissociation of O3and the reverse recombination of O with O2in the bath gases M He, Ar, N2, CO2and SF6are carefully analyzed. At very high temperatures, the rate constants have to correspond solely to the energy transfer ET mechanism. On condition that this holds for Ar and N2near 800 K, average energies transferred per collision of −〈ΔE〉hc 18 and 25 cm−1are derived, respectively. Assuming an only weak temperature dependence of 〈ΔE〉 as known in similar systems, rate coefficients for the ET-mechanism are extrapolated to lower temperatures and compared with the experiments. The difference between measured and extrapolated rate coefficients is attributed to the radical complex RC mechanism. The derived rate coefficients for the RC-mechanism are rationalized in terms of equilibrium constants for equilibria of van der Waals complexes of O or O2 with the bath gases and with rate coefficients for oxygen abstraction from these complexes. The latter are of similar magnitude as rate coefficients for oxygen isotope exchange which provides support for the present interpretation of the reaction in terms of a superposition of RC- and ET-mechanisms. We obtained rate coefficients for the ET-mechanism of kETrec,0Ar 2.3 × 10−34T300−1.5and kETrec,0N2 3.5 × 10−34T300−1.5cm6molecule−2s−1and rate coefficients for the RC-mechanism of kRCrec,0Ar 1.7 × 10−34T300−3.2and kRCrec,0N2] 2.5 × 10−34T300−3.3cm6molecule−2s−1. The data bases for M He, CO2and SF6are less complete and only approximate separations of RC- and ET-mechanism were possible. The consequences of the present analysis for an analysis of isotope effects in ozone recombination are emphasized.

The combination reaction C6H5CH2 C6H5CH2M → C14H14M was studied over the pressure range 0.03–900 bar and the temperature range 250–400 K. Helium, argon, xenon, N2and CO2were employed as bath gases. Benzyl radicals were generated by H abstraction from toluene by Cl atoms formed via laser photolysis of Cl2at 308 nm. Benzyl radicals were monitored by time-resolved transient UV absorption at 253 nm. The observed combination rates were pressure independent over the range 0.04–0.45 bar in CO2and 0.03–5 bar in argon which identifies a limiting “high-pressure” value of rate constant within the energy-transfer mechanism, kET∞T 4.1 ± 0.3 × 10−11T300 K−0.23cm3molecule−1s−1. Although the reaction has clearly reached the “high-pressure limit” kET∞ at pressures below 1 bar, a further increase of the rate constants was observed when the pressure of the bath gas was raised above ∼5 bar. At much higher pressures finally the rate constants decrease when diffusion-controlled kinetics takes over. The degree of enhancement of the combination rates beyond the “high-pressure limit” was found to depend on the bath gas, increasing in the order He < N2 Ar < CO2. The enhancement was most prominent at low temperatures. Measurements of transient UV absorption spectra of benzyl radicals, over the pressure range 5–30 bar of CO2at 300 K, confirmed that an only small pressure-dependent solvatochromic shift of benzyl radicals cannot be responsible for the observation of enhanced rate constants. Instead, the results clearly point toward a significant effect of van der Waals clustering, i.e.of “radical-complex” formation, on the combination reaction kinetics in the gas–liquid transition range. An analysis in terms of statistical adiabatic channelclassical trajectory calculation SACMCT appears to provide a consistent description.

The formation of particles by the rapid expansion of supercritical solutions RESS of four organic substances n-undecane, naphthalene, trans-stilbene and benzoic acid dissolved in supercritical CO2has been investigated employing two complementary on-line in situdetection techniques. For the first time, a laser-based shadowgraphy LABS setup was applied, which allows the recording of the diameter, morphology, size distribution and concentration of particles down to sizes of roughly 8 μm. In addition, laser-based three-wavelength extinction measurements 3-WEM have been employed to determine the average particle diameter, the width of the particle size distribution and the concentration. The average particle sizes determined from 3-WEM for n-undecane, naphthalene, trans-stilbene and benzoic acid are 2660, 1260, 550 and 430 nm, respectively, and the standard deviation of the assumed logarithmic normal distributions 0.37, 0.42, 0.78 and 0.60. The particle size distributions at large diameters from LABS compare favourably with the tails of the 3-WEM distributions. An analysis of the morphology of trans-stilbene particles and n-undecane droplets reveals an, on average, slightly elliptical shape.

The formation of particles by the rapid expansion of supercritical solutions (RESS) of four organic substances (n-undecane, naphthalene, trans-stilbene and benzoic acid) dissolved in supercritical CO₂ has been investigated employing two complementary on-line in situ detection techniques. For the first time, a laser-based shadowgraphy (LABS) setup was applied, which allows the recording of the diameter, morphology, size distribution and concentration of particles down to sizes of roughly 8 μm. In addition, laser-based three-wavelength extinction measurements (3-WEM) have been employed to determine the average particle diameter, the width of the particle size distribution and the concentration. The average particle sizes determined from 3-WEM for n-undecane, naphthalene, trans-stilbene and benzoic acid are 2660, 1260, 550 and 430 nm, respectively, and the standard deviation of the (assumed) logarithmic normal distributions 0.37, 0.42, 0.78 and 0.60. The particle size distributions at large diameters from LABS compare favourably with the tails of the 3-WEM distributions. An analysis of the morphology of trans-stilbene particles and n-undecane droplets reveals an, on average, slightly elliptical shape.

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.

No Abstract

The combination reaction of two benzyl radicals has been studied in a wide range of pressures 0.01–1000 bar and temperatures 250–400 K in various bath gases Ar, N2and CO2. The measured second-order combination rate constants of benzyl radicals were independent of the pressure and the bath gas below 1 bar, as expected for a limiting “high pressure” rate constant of a termolecular combination process. However, the reaction becomes steadily faster when the pressure in Ar is further raised until it finally starts to decrease when densities corresponding to diffusion controlled kinetics are reached. Such a unique pressure dependence was more strongly accentuated in CO2and at lower temperatures. Our results seem to provide first clear indications for a contribution of the radical–solvent interaction in the combination reaction kinetics of such large radicals as benzyl in the gas to liquid transition range.

The combination reaction of two benzyl radicals has been studied in a wide range of pressures (0.01–1000 bar) and temperatures (250–400 K) in various bath gases (Ar, N₂ and CO₂). The measured second-order combination rate constants of benzyl radicals were independent of the pressure and the bath gas below 1 bar, as expected for a limiting “high pressure” rate constant of a termolecular combination process. However, the reaction becomes steadily faster when the pressure in Ar is further raised until it finally starts to decrease when densities corresponding to diffusion controlled kinetics are reached. Such a unique pressure dependence was more strongly accentuated in CO₂ and at lower temperatures. Our results seem to provide first clear indications for a contribution of the radical–solvent interaction in the combination reaction kinetics of such large radicals as benzyl in the gas to liquid transition range.