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A newly established in situ technique using Raman spectroscopy was employed for the detailed kinetic investigation of mechanochemical reaction pathways. This approach was applied for the systematic investigation of the direct influence of colliding balls on the reaction rate constants of a mechanochemical cocrystallization reaction. As a model reaction, the mechanochemical cocrystallization of felodipine and the coformer imidazole was investigated. Keeping the total ball mass constant by varying the number of milling balls, our study reveals that the impact of each single collision has a more significant influence on the reaction kinetics than expected.

We present a facile alternative to other well known strategies for synthesizing flexible thermoelectric materials. Instead of printing thin active layers on flexible substrates or doping conductive polymers, we produce thermoelectric pastes, using a mixture of graphite, copper(I) oxide and polychlorotrifluoroethene. The Seebeck coefficient of the investigated pastes varies between 10 and 600 μV K(-1), while the electrical conductivity spans over an even wider range of 10(-4) to 10(2) S m(-1). Here, the influence of phenomena such as percolation on the electrical transport is revealed. The resulting power factor reaches 5.69 × 10(-4) ± 0.70 × 10(-4) μW m(-1) K(-2) for the graphite-polymer paste, with an unexpected minimum at a graphite molar fraction of approximately 0.4. The values are comparable to those of the powder mixtures, which are slightly higher, but less precisely tunable. Such compounds are further evaluated for practical applications. The graphite-polymer paste is used to exemplify, how a flexible thermoelectric sensor can be easily manufactured, step by step. Our results represent a proof of principle, that thermoelectric pastes are viable alternatives to current solutions. A further expansion of the scope for the composites can be achieved by using high performance thermoelectric materials and conductive polymers.

Pulsed laser ablation in liquids (PLAL) is a versatile route to stable colloids without the need for stabilizing agents. The use of suspensions instead of bulk targets further simplifies the experimental set-up and even improves the productivity. However, the utilization of this approach is hindered by limited knowledge about the underlying mechanisms of the nanoparticle formation. We present the synthesis of copper(i) iodide nanoparticles via ns-pulsed laser irradiation of CuI powder suspended in water or ethyl acetate. A thorough study of the nanoparticle size by transmission electron microscopy reveals a log-normal distribution with a mean diameter of 31 nm (±11 nm) in water and 18 nm (±7 nm) in ethyl acetate. The duration of the laser irradiation appears to have only a minor influence on the size distribution. Selected area diffraction and electron energy-loss spectroscopy verify the chemical composition of the generated CuI nanoparticles. While comparable precursors like CuO and Cu3N follow a reductive ablation mechanism, a fragmentation mechanism is found for CuI. With a productivity of 1.7 μg J(-1) this pulsed laser fragmentation in liquids (PLFL) proves to be an efficient route to colloidal CuI nanoparticles.

We present a study on the catalytic reduction of 4-nitrophenol (Nip) to 4-aminophenol (Amp) by sodium borohydride (BH4–) in the presence of metal nanoparticles in aqueous solution. This reaction which proceeds via the intermediate 4-hydroxylaminophenol has been used abundantly as a model reaction to check the catalytic activity of metallic nanoparticles. Here we present a full kinetic scheme that includes the intermediate 4-hydroxylaminophenol. All steps of the reaction are assumed to proceed solely on the surface of metal nanoparticles (Langmuir–Hinshelwood model). The discussion of the resulting kinetic equations shows that there is a stationary state in which the concentration of the intermediate 4-hydroxylaminophenol stays approximately constant. The resulting kinetic expression had been used previously to evaluate the kinetic constants for this reaction. In this stationary state there are isosbestic points in the UV/vis-spectra which are in full agreement with most published data. We compare the full...

Experimental results for the hypersonic impact of singly charged, mass selected, water clusters at a hard surface demonstrate cooling of the heated cluster by sequential loss of a water molecule. The trends in the experiment are most simply summarized as an evaporation. This behavior is in contrast to the shattering phenomena observed for rare gas clusters. The same information theoretic approach that predicted the shattering transition is fully consistent with the results for the water clusters. It shows that the relatively high binding energy per water monomer is the key reason for the anomalous fragmentation. The tetra coordinated oxygen atom in a cluster does however imply that the fragmentation is not a pure evaporation but has residual elements of a shattering transition. For water clusters the transition is broadened to span the finite range in energy between the onset of boiling and the energy when the solvation of the hydronium ion is over and it is bare.

Mechanochemical reactions often result in 100% yields of single products, making purifying procedures obsolete. Mechanochemistry is also a sustainable and eco-friendly method. The ever increasing interest in this method is contrasted by a lack in mechanistic understanding of the mechanochemical reactivity and selectivity. Recent in situ investigations provided direct insight into formation pathways. However, the currently available theories do not predict temperature T as an influential factor. Here, we report the first determination of an apparent activation energy for a mechanochemical reaction. In a temperature-dependent in situ study the cocrystallisation of ibuprofen and nicotinamide was investigated as a model system. These experiments provide a pivotal step towards a comprehensive understanding of milling reaction mechanisms.

A new cocrystal of pyrazinamide with oxalic acid was prepared mechanochemically and characterized by PXRD, Raman spectroscopy, solid-state NMR spectroscopy, DTA-TG, and SEM. Based on powder X-ray diffraction data the structure was solved. The formation pathway of the reaction was studied in situ using combined synchrotron PXRD and Raman spectroscopy. Using oxalic acid dihydrate the initially neat grinding turned into a rapid self-accelerated liquid-assisted grinding process by the release of crystallization water. Under these conditions, the cocrystal was formed directly within two minutes.

A one-step electrochemical grafting process using Grignard reagents has been performed to achieve a complete monolayer methyl-terminated Si(111) surfaces. Anodic treatment (0.5 mA/cm2 for 300 s) ha...

We report on the formation of silver subsurface ion-exchanged metal oxide (silver SIMO) glasses and their surface-enhanced Raman scattering (SERS) activity. The samples were prepared by a combined thermal and chemical three-step methodology and characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), environmental electron scanning microscopy (ESEM), and UV/Vis spectroscopy. This unique method provides SERS substrates with protection against contamination and strong, reliable and reproducible SERS enhancement. The Raman enhancement factors of the long-term stable SIMO glasses were estimated to approximately 10(7).

The dissolution of molecules of biological interest in supercritical carbon dioxide is investigated using pulsed molecular beam mass spectrometry. Due to the mild processing temperatures of most supercritical fluids, their adiabatic expansion into vacuum permits to transfer even thermally very sensitive substances into the gas phase, which is particularly attractive for pharmaceutical and biomedical applications. In addition, supercritical CO2constitutes a chemically inert solvent that is compatible with hydrocarbon-free ultrahigh vacuum conditions. Here, we report on the dissolution and pulsed supersonic jet expansion of caffeine (C8H10 N4O2), the provitamin menadione (C11 H8O2), and the amino acid derivative l-phenylalanine tert-butyl ester hydrochloride (C6H5CH2CH(NH2)COOC(CH3)3·HCl), into vacuum. An on-axis residual gas analyzer is used to monitor the relative amounts of solute and solvent in the molecular beam as a function of solvent density. The excellent selectivity and sensitivity provided by mass spectrometry permits to probe even trace amounts of solutes. The strong density variation of CO2 close to the critical point results in a pronounced pressure dependence of the relative ion currents of solute and solvent molecules, reflecting a substantial change in solubility. © 2008 Elsevier B.V. All rights reserved.

The reaction of Ru(10−10) with oxygen has been investigated under high-pressure conditions (≤1 bar) with thermal desorption spectroscopy (TDS), low electron energy diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and measurements of the work function changes ΔΦ, derived from ultraviolet photoelectron spectroscopy (UPS). For oxygen exposures between 0.1 and 1000 L, only chemisorption is observed independent of the sample preparation temperature TP (425 K 525 K. At exposures larger than 1000 L and temperatures below 525 K, the formation of an intermediate oxygen state as a precursor to the actual oxide is observed with XPS and TDS. It is accompanied by a perturbation of the long-range order of the Ru surface structure. Significant changes of the work function, ΔΦ, indicate that a Ru(10−10) surface exposed to oxygen at TP < 525 K resembles properties of a defect-rich Ru(0001) surface. The minimum ...

A large number of scientific contributions is dedicated to syntheses, characterization and applications of metal nanoparticles. In contrast, only few studies on their formation mechanisms have been reported. In general, concepts to describe particle growth processes are rare. Commonly used models are not able to explain the influences of reaction parameters on the growth and the final particle size. In this contribution it is shown how the growth of colloidal metal nanoparticles can be illustrated using an approach based on colloidal stability. In the first part, investigations of various syntheses of colloidal nanoparticles (including Rh, Pd, Pt, Cu, Ag and Au) show that growth due to aggregation and coalescence is the governing principle of nanoparticle formation if the monomer supply is faster than the actual growth. In the second part of this contribution, the influences of various parameters on the growth of Au nanoparticles are studied and it is demonstrated how the colloidal stability approach can illustrate the impact of synthesis parameters on the final particle size.

The spontaneous formation of complex interfacial patterns from thermally deposited Sb4 clusters on HOPG is controlled by the deposition conditions (i.e., coverage and deposition rate) at constant t...

Mixed bismuth–molybdenum oxide clusters containing one or two molybdenum atoms have been produced in a molecular beam. Their compositions can be formulated in terms of (Bi2O3)mBiOMoO3+, ( Bi 2 O 3 ) m BiO ( MoO 3 ) 2 + (m⩾2) , (Bi2O3)mOnMoO3+, and ( Bi 2 O 3 ) m O n ( MoO 3 ) 2 + (m⩾3, n=0,1) . Reactivities of these molybdenum-doped and pure bismuth oxide cluster cations towards ethene have been investigated and rate constants have been calculated, e.g., for Bi6O9+ and Bi6O9MoO3+. Remarkably, the reactivity of the highly active bismuth oxide cluster Bi6O9+ is only slightly varied by addition of one {MoO3} unit, providing evidence for the independence of the different metal oxide centres in the cluster.

Gas-phase vanadium oxide clusters VxOy+ in the size range 4 ⩽ x ⩽ 14 have been produced via laser vaporisation in a continuous flow of He/O2 carrier gas. The cluster distributions were subsequently characterised by their scattering behaviour at low-density gas targets (10−4 − 2 × 10−3 mbar). The experimental scattering cross sections were used to study differences in the interactions with rare gases, hydrocarbons (alkanes: methane, propane, n-butane and alkenes: ethene, propene, but-1-ene) and also small inorganic molecules (N2, O2, CO, NO, SO2). The compositions of the dominant oxide clusters reveal the predominance of vanadium atoms in the formal oxidation states +4 and +5. The interaction patterns of these oxide clusters with rare gases show no indications for fragmentation at collision energies corresponding to a cluster beam velocity of 900 ms−1. Clusters with a composition of (V2O5)n+ have increased cross sections for the interaction with alkanes and alkenes. Indeed, for the alkenes stable adducts have been detected under the applied scattering conditions. The increased cross sections have been interpreted in terms of a strong reactivity towards hydrocarbons, which is in agreement with previous results on smaller vanadium oxide clusters (R. C. Bell, K. A. Zemski, K. P. Kerns, H. T. Deng and A. W. Castleman, Jr., J. Phys. Chem. A, 1998, 102, 1733).

Fragment ion distributions of antimony clusters Sbn+ (n = 3–12) and bismuth clusters Bin+ (n = 3–8) after collision with different solid target surfaces at hyperthermal energies are reported. The measured fragmentation patterns for antimony and bismuth clusters of the same size are similar. Main fragmentation channels identified in our study at impact energies up to 150 eV correspond to the sequential unimolecular dissociation of the colliding clusters with a preferred loss of stable neutral tetrameric and dimeric cluster units. The efficiency of the energy transfer from kinetic energy into internal cluster energy during surface collision was found to be up to 10%. The fragmentation patterns are independent of the employed target surface. In contrast, the integral scattered ion yield strongly depends on the surface type. High scattered ion yields are observed for a silicon oxide surface with high work function, whereas graphite or metallic gold target surfaces result in considerably lower scattered ion yields. For a given surface the integral ion yield also shows strong variations as function of the size of the colliding clusters.

Pulsed laser ablation in liquids (PLAL) has developed to a convenient and efficient method for the synthesis of colloidal solutions. So far, in most cases, the laser pulse is focused on bulk targets like metal plates. An interesting alternative is the use of suspended μm-sized precursors. This leads to higher production rates and simpler setups. A thorough understanding of the mechanism is essential in order to gain control over the characteristics of the synthesized nanoparticles. Therefore, we investigated the formation of copper colloids by PLAL of CuO, Cu3N, Cu(N3)2, and Cu2C2 powders in organic liquids. Thus, we can compare copper precursors based on elements of the 4th, 5th, and 6th main group. The chemical composition of the resulting nanoparticles is revealed by electron energy loss spectroscopy (EELS). The presented investigations point to a reductive ablation process followed by laser-driven aggregation and coalescence steps instead of a simple fragmentation mechanism.

The size dependent evolution of the electronic structure of isolated gas-phase rhenium clusters Re n − ( n =2–40) is measured by photoelectron (PE) spectroscopy at 4.99 and 6.42 eV photon energies. For very small clusters the photodetachment threshold energies show a strong size dependence varying between 1.6 eV (Re 2 ) and 2.6 eV (Re 13 ). As for larger clusters, electron affinities (EA) increase smoothly toward bulk work function (WF). Bulk-like electronic features can already be observed around very small cluster sizes.

The formation of rhenium oxide cluster anions is studied in a pulsed molecular beam experiment as a function of source conditions and oxygen content (0.1‐10%) in the carrier gas. RenOm clusters are formed directly in plasma reactions in the laser vaporization cluster source. With both increasing oxygen content and stagnation pressure, larger oxide clusters are formed (m^n) and the distribution is dominated by certain mass peaks of high intensity indicating a high stability. At lower pressure and less oxygen content, a large variety of oxides is observed. Most stable clusters contain Re atoms with a formal oxidation state of 17 and oxygen in the 22 oxidation state. Oxygen rich clusters can be attributed to a series (ReO 4)l(Re2O7)m(O)n with l50 ‐1, m50 ‐ 4, and n50 ‐1. The results are explained by the nature of the Re‐O bond which is important for the alkene metathesis reaction mechanism. (Int J Mass Spectrom 209 (2001) 1‐ 4) © 2001 Elsevier Science B.V.

The activation of molecular oxygen by bismuth oxide cluster cations has been observed in the presence of unsaturated hydrocarbon molecules. Reactivities of gas-phase bismuth oxide cluster cations BixOy+ (x = 3 or 4, y ≤ 7) with hydrocarbons have been studied by measuring scattering cross sections and analyzing products formed under multiple collision conditions. The integral cross sections for the scattering of the cations BixOy+ (x = 3 or 4, y ≤ 7) from argon, alkanes (methane, propane, butane), and alkenes (ethene, propene, 1-butene, 1,3-butadiene) have been measured. In particular, the cation Bi4O6+ has a significantly increased cross section for the scattering from the alkenes as compared to the other oxides Bi4Oy+. The larger total cross section is explained by assuming an increased cross section for reactive collisions, which could be caused by a specific reactivity of that cation. Products for the reactions of small bismuth oxide cluster cations BixOy+ (x = 3 or 4) with ethene and propene have been...

We present a mass-spectrometric investigation of the reactions of manganese cluster anions Mnx (in the size range x 5 2‐12) with molecular oxygen in the gas phase. The manganese and manganese oxide cluster anions are studied in a pulsed supersonic molecular beam experiment and generated in a conventional laser vaporization cluster source with an additional flow tube reactor. Relative reactivities are determined at near room temperature as a function of oxygen partial pressure, cluster size, and composition. We observed for the first time the controlled formation of at least four stable manganese oxide series: MnxO 2 ,M n xO2 ,M n xO3 , and MnxO4 ( x 5 2‐12). The reactivities decrease rapidly with cluster size reaching a minimum around Mn4 and Mn5 . They increase smoothly with further increasing cluster size and show no significant pressure dependence for clusters of x . 5. The adsorption of molecular oxygen on the cluster surface most probably takes place in a dissociative way resulting in the formation of distinct Mn‐O bonds. The Mn5 cluster exhibits a special behavior. It has the highest intensity in mass spectra and shows the lowest reactivity towards oxygen. Another interesting feature is the evolution of an intense Mn2O 2 peak with increasing oxygen pressure. (Int J Mass Spectrom 185/186/187 (1999) 673‐ 683) © 1999

The formation of positively charged antimony oxide clusters has been investigated as a function of oxygen partial pressure using time of flight mass spectrometry. With increasing oxygen partial pressure magic number patterns are observed, which can be attributed to the clusters of (Sb2O3)n+ and (Sb2O3)n(SbO)+ with 2⩽n

The formation of positively charged antimony oxide clusters has been investigated as a function of oxygen partial pressure. At low oxygen concentration oxide clusters with a large variety of stoichiometries are observed. With increasing oxygen pressure some mass peaks become prominent. The special stability of these antimony oxide clusters can be explained by simple valence bond arguments, all the antimony atoms being in a formal +III and all the oxygen atoms being in a formal −II oxidation state. A further increase in the oxygen partial pressure shifts the distribution to mixed valence oxide clusters with one or more of the antimony atoms in the formal +V oxidation state. According to this model it is possible to predict discrete structures for these clusters. They have no direct Sb–Sb bonds, instead all the Sb-atoms are bridged by O-atoms. Further oxygen can then be added in a terminal position to form the mixed valence oxides. This model is also supported by bulk and gas-phase data of antimony oxides.

The reaction of bismuth clusters, produced in a gas aggregation source, with pure oxygen resulted in enhanced stable poly oxides following the series BiO + (Bi 2 O 3 ) n up to n =8. Reactions of these magic clusters with propylene in a scattering cell, under single collision conditions, lead to a significant reduction of the oxide peaks. Unexpectedly this finding is similar to the known catalytic reactions of propylene and solid Bi 2 O 3 . In the case of antimony oxide clusters no special reactivity can be found.

Quinaldine (2-methyl quinoline) is a liquid at room temperature, which can be supercooled to reach finally the glassy state. By heating the glass above the glass transition temperature T(g) = 180 K the sample performs two subsequent transitions into, likewise, dielectrically active phases. Thus, the reorientational relaxations of these phases as well as the kinetics of the phase transitions can be tracked in a highly resolved way by dielectric spectroscopy. X-ray diffraction analysis clearly shows two structurally different crystalline phases in addition to the supercooled liquid. Calorimetric measurements support the notion of first order phase transitions, occurring irreversibly in the supercooled regime, and suggest that the intermediate crystalline phase is metastable, too. Analyzing the quite distinct dielectric relaxation strengths, we discuss the possible nature of the two crystalline phases. Additionally, a very similar behavior to quinaldine is observed for 3-methyl quinoline, indicating a broad field of polymorphism among the quinoline derivatives.

A unique direct electrodeposition technique involving very high current densities, high voltages and high electrolyte concentrations is applied for highly selective mass synthesis of stable, isolable, surfactant-free, single-crystalline Bi hexagons on a Cu wire at room temperature. A formation mechanism of the hexagons is proposed. The morphology, phase purity, and crystallinity of the material are well characterized by FESEM, AFM, TEM, SAED, EDX, XRD, and Raman spectroscopy. The thermal stability of the material under intense electron beam and intense laser light irradiation is studied. The chemical stability of elemental Bi in nitric acid shows different dissolution rates for different morphologies. This effect enables a second way for the selective fabrication of Bi hexagons. Bi hexagons can be oxidized exclusively to α-Bi(2)O(3) hexagons. The Bi hexagons are found to be promising for thermoelectric applications. They are also catalytically active, inducing the reduction of 4-nitrophenol to 4-aminophenol. This electrodeposition methodology has also been demonstrated to be applicable for synthesis of bismuth-based bimetallic hybrid composites for advanced applications.

CTAB-stabilized gold nanoparticles were synthesized by applying the seeding-growth approach in order to gain information about the size dependence of the catalytic reduction of p-nitrophenol to p-aminophenol with sodium borohydride. Five different colloidal solutions of stabilized gold nanoparticles have been characterized by TEM, AFM, UV-Vis, SAXS, and DLS for their particle size distributions. Gold nanoparticles (mean sizes: 3.5, 10, 13, 28, 56 nm diameter) were tested for their catalytic efficiency. Kinetic data were acquired by UV-Vis spectroscopy at different temperatures between 25 and 45 °C. By studying the p-nitrophenol to p-aminophenol reaction kinetics we determined the nanoparticle size which is needed to gain the fastest conversion under ambient conditions in the liquid phase. Unexpectedly, CTAB-stabilized gold nanoparticles with a diameter of 13 nm are most efficient.

Scattering o f small potassium clusters by water molecules has been studied in a molecular beam experiment for the first time. In the size range up to eight atom s, trends for cross sections have been obtained by investigating low energy collisions. Significant differences in relative cross sections can be assigned to inelastic collisions. As in case o f scattering o f sodium clusters by argon (Goerke, A. et al., J. Chem. Phys. 98, 9653 (1993)), rotational excitation of the clusters seems to be the dominating interaction.

We introduce a straightforward experimental approach for determining the mean flow velocity of a supersonic jet with very high precision. While time measurements easily can achieve accuracies of Δt/t ≤ 10(-4), typically the absolute flight distances are much less well-defined. This causes significantly increased errors in calculations of the mean flow velocity and mean kinetic energy. The basic concept to improve on this situation is changing the flight distance in vacuo by precisely defined increments employing a linear translation stage. We demonstrate the performance of this method with a flight path that can be varied by approximately 15% with a tolerance of setting of 50 μm. In doing so, an unprecedented accurate value for the mean flow velocity of Δv/ < 3 × 10(-4) has been obtained without prior knowledge of the total distance. This very high precision in source pressure, temperature, and particle speed facilitates an improved energetic analysis of condensation processes in supersonic jet expansions. The technique is also of broad interest to other fields employing the strong adiabatic cooling of supersonic beams, in particular, molecular spectroscopy. In the presented case study, a thorough analysis of arrival time spectra of neutral helium implies cluster formation even at elevated temperatures.

Supersonic molecular beams constitute a very powerful technique in modern chemical physics. They offer several unique features such as a directed, collision-free flow of particles, very high luminosity, and an unsurpassed strong adiabatic cooling during the jet expansion. While it is generally recognized that their maximum flow velocity depends on the molecular weight and the temperature of the working fluid in the stagnation reservoir, not a lot is known on the effects of elevated particle densities. Frequently, the characteristics of supersonic beams are treated in diverse approximations of an ideal gas expansion. In these simplified model descriptions, the real gas character of fluid systems is ignored, although particle associations are responsible for fundamental processes such as the formation of clusters, both in the reservoir at increased densities and during the jet expansion. In this contribution, the various assumptions of ideal gas treatments of supersonic beams and their shortcomings are reviewed. It is shown in detail that a straightforward thermodynamic approach considering the initial and final enthalpy is capable of characterizing the terminal mean beam velocity, even at the liquid-vapor phase boundary and the critical point. Fluid properties are obtained using the most accurate equations of state available at present. This procedure provides the opportunity to naturally include the dramatic effects of nonideal gas behavior for a large variety of fluid systems. Besides the prediction of the terminal flow velocity, thermodynamic models of isentropic jet expansions permit an estimate of the upper limit of the beam temperature and the amount of condensation in the beam. These descriptions can even be extended to include spinodal decomposition processes, thus providing a generally applicable tool for investigating the two-phase region of high supersaturations not easily accessible otherwise.

We report the determination of ionization potentials for molecular beam isolated clusters of cadmium by photoionization mass spectrometry. The mass spectra reveal multimodal size distributions indicating particular stabilities for clusters with 20, 40, 70 and 92 electrons, which are shell closings in the electron shell model. A rapid convergence of the ionization potentials towards the values expected for the nonclassical metallic sphere model is observed. Ionization potentials for clusters with up to 30 atoms are reported.

The vacuum-UV-photoelectron spectra of Se2, Se5, Se6, Se7, and Se8 have been recorded at a photon energy of h ν= 10.0 eV. The isolated molecules are examined in a supersonic molecular beam employing a new photoelectron-photoion coincidence technique. The structure of the photoelectron spectra of selenium molecules with even and odd numbers of atoms differs in a characteristic manner. While the spectra of Se6 and Se8 show one single broad band, three separated bands with different intensities are observed for Se5 and two for Se7. The spectra are compared to molecular orbital energy calculations based on theoretically supposed geometries. The comparison indicates that Se6 and Se8 have Dnd-symmetrical ring structures, whereas Se5 and Se7 are C1h-symmetrical rings

Pulsed, supersonic beams of pure carbon monoxide and carbon dioxide at stagnation conditions above their critical point have been investigated by time-of-flight measurements as a function of pressure and temperature. Although both molecules form clusters readily in adiabatic expansions, surprisingly large speed ratios (above 100) indicative of very low translational temperatures (below 0.1 K) have been achieved. In particular, the supersonic expansion of CO(2) at stagnation temperatures slightly above the phase transition to the supercritical state results in unprecedented cold beams. This efficient cooling is attributed to the large values of the heat capacity ratio of supercritical fluids in close vicinity of their critical point.

The gas-phase infrared spectroscopy of V4O10+ produced by laser vaporization has been studied in the spectral region from 7 to 16 µm. Mass-selected V4O10+ cations were stored in a helium filled radio frequency hexadecapole ion trap and excited using tunable infrared radiation from a free electron laser. The photodissociation spectrum was recorded by monitoring the V4O8+ yield (O2 loss) as a function of the excitation wavelength. Two absorption bands at 842 and 1032 cm−1 are observed, which are assigned to resonant excitation of the antisymmetric V–O–V stretching and VO stretching vibrations, respectively. Comparison to recent theoretical and experimental studies indicate that the absorbing species consists of a V4O8+ ionic core weakly bound to an oxygen molecule.

Laser-induced resonant two-photon ionization of toluene and fluorobenzene cooled in a seeded supersonic expansion of Ar, combined with time-of-flight mass spectrometry. reveal the isotopic shifts of the vibronic origin of the ‘B7 - ‘A, transition: 13C-toluene: +2.5 cm-’ ; r3C2-toluene: +5.4 cm-l ; r3C-fluorobenzene: +1.9 cm-‘. Large separation factors are observed for isotopes containing one and two 13C atoms

We report time-resolved fluorescence decay lifetimes from photoselected states of jet-cooled 4-ethyl-trans-stilbene (4ETS) and 4-n-propyl-trans-stilbene (4PTS) in the energy range Ev = 0–2800 cm−1 above the S1 origin. Alkyl substitution enhances the non-radiative decay rates. These data can be fitted by the statistical RRKM theory in conjunction with the reduction of the photoisomerization threshold from Et = 1300±50 cm−1 for trans-stilbene to Et = 1100±100cm−1 for 4ETS, and Et = 1000±100 cm−1 for 4PTS.

Van der Waals (vdW) clusters of fluorobenzene (FB), synthesized in a seeded supersonic rare-gas expansion were studied by laser-induced, resonant two-photon ionization (R2PI) combined with TOF-mass spectrometry. The molecules were excited near the FB monomer's vibronic origin of the S1(ππ*) ← S0 transition (λ00 = 2644 A). The heterogeneous clusters FB·Arn) (n

The electronic absorption spectrum of the van der Waals dimer Hg2 has been measured in the UV–spectral range by a new supersonic jet direct absorption technique. The data led to an improved analysis of the X 10+g–G 10+u transition, yielding the parameters ω’e =79 cm−1, ωex’e =0.22 cm−1, and Re =3.00 A for the Morse potential of the G 10+u state.