H. Baumgärtel

Homogeneous nucleation rates of supercooled water measured in single levitated microdroplets

Homogeneous nucleation rates are determined for micrometer sized water droplets levitated inside an electrodynamic Paul-trap. The size of a single droplet is continuously measured by analyzing the angle-resolved light scattering pattern of the droplets with classical Mie theory. The freezing process is detected by a pronounced increase in the depolarization of the scattered light. By statistical analysis of the freezing process of some thousand individual droplets, we obtained the homogeneous nucleation rate of water between 236 and 237 K. The values are in agreement with former expansion cloud chamber measurements but could be determined with considerably higher precision. The measurements are discussed in the light of classical nucleation theory in order to obtain the size and the formation energy of the critical nucleus.

Electronic spectroscopy of fluorobenzene Van der Waals molecules by resonant two-photon ionization

Abstract 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 monomers vibronic origin of the S 1 (ππ*) ← S 0 transition (λ 00 = 2644 A). The heterogeneous clusters FB·Ar n ) ( n 00 induced by vdW interaction (FB·Ar: −23 cm −1 : FB·Ar 2 : −46 cm −1 : FB·Ar 3 : +4.6 cm −1 ). Additional satellite bands appeared due to intermolecular photofragmentation. A second band found for FB·Ar at 20 cm −1 was assigned to a vdW vibration (ν vdW = 43 cm −1 ). Similar results were obtained for FB·Kl n ??? ( n 2 and FB 3 were more complex. The dimer spectrum showed two broad spectral features, one blue- the other red-shifted relative to λ 00 . Each one is probably due to a different isomer. The blue-shifted contained progressions, which were tentatively assigned to a vdW vibration with 20 cm −1 in the ground and 15 cm −1 in the excited state. The trimer spectrum showed a broad blue-shifted absorption maximum with prominent bands at −2.6, −20, −29 and −50 cm −1 . From the observed spectra the feasibility of cluster-specific spectroscopy is discussed.

Ar 2p spectroscopy of free argon clusters

Total electron and total and partial ion yield spectra of Ar clusters (with average size up to 600±200) in the region of Ar 2p excitation have been measured using synchrotron radiation and time‐of‐flight mass spectrometry. As the average cluster size increases, the x‐ray absorption spectrum changes systematically from that of atomic Ar to that of solid Ar. The shape of the Ar 2p3/2→4s region is found to be a sensitive monitor of the cluster sizes present in a molecular beam of Ar clusters. Extended x‐ray absorption fine structure (EXAFS) is detected in the spectra of the larger clusters. There is a strong correlation between the intensity of the components of the Ar 2p3/2→4s signal associated with clusters and the intensity of the Fourier filtered first shell Ar 2p EXAFS signal. A low amplitude, high frequency fine structure is observed in the Ar 2p continuum of the heaviest clusters which corresponds closely to that observed in solid Ar. This signal develops with cluster size more slowly than the Ar 2p E...

Polycyclic aromatic hydrocarbons: negative ion formation following low energy (0–15 eV) electron impact

Abstract Long-lived (autodetachment lifetimes > 10 −6 s) parent negative ions of a number of polycyclic aromatic hydrocarbons (M) have been observed when the electron affinity (EA) of M exceeds a few tenths of an electronvolt. For the larger hydrocarbons with EA (M) ⩾ 0.5 eV the long-lived parent negative ion was found to form via as many as three negative ion states above 0.0 eV. The production of long-lived parent negative ions at electron energies well above 0.0 eV and with much larger cross sections than at 0.0 eV is attributed to electron-excited Feshbach resonances (i.e., two-electron negative ion states). Results are also reported on the formation of (MH) − from M; (MH) − is the only dissociative attachment negative ion observed.

In-situ infrared spectroscopic studies of thymine adsorption on a Au(111) electrode

Abstract We have investigated the adsorption of thymine on Au(111) with in-situ infrared spectroscopy. Using IR spectroscopy we have been able to probe the structure and bonding of the ‘chemisorbed’ thymine phase. IR bands in the 1800–1550 cm −1 region, which have a major contribution from stretching vibrations of the carbonyl groups of thymine, exhibit large spectral shifts between thymine in aqueous solution and the chemisorbed phase. By contrast, ‘physisorbed’ thymine shows no IR bands in the spectral region studied, which is consistent with a flat-lying adsorbate. In-situ IR spectroscopy provides compelling evidence for the orientational change of adsorbed thymine occurring concomitantly with the most pronounced anodic current peak in the voltammogram. We have compared the in-situ IR spectra for chemisorbed thymine with IR spectra of metal co-ordination complexes of thymine and uracil. These comparisons provide evidence that chemisorbed thymine bonds to the Au(111) surface through both carbonyl functionalities and a deprotonated N3.

Charge separation in core excited argon clusters

Charge separation in core excited argon clusters is reported. Neutral argon clusters have been prepared in a supersonic molecular beam. Photoionization with monochromatized synchrotron radiation in the L3/L2 regime (240–260 eV) initiates various single and double ionization processes. The photoion–photoion‐coincidence (PIPICO) technique is applied to measure dissociative double ionization processes in core excited argon clusters. Three series of charge separation channels are observed: (i) Ar+/Ar+n, (ii) Ar+2/Ar+n, and (iii) Ar+3/Ar+n. Kinetic energy releases from charge separation reactions as well as the relative intensities of the PIPICO signals are discussed in relation to fragmentation mechanisms, resonant Auger spectra, and properties of cluster dications, such as cluster dication fragmentation energies and charge separation distances.

The photofragmentation of naphthalene and azulene monocations in the energy range 7–22 eV

Abstract Photoion mass spectrometry was used to study the fragmentation of naphthalene and azulene monocations over the excitation energy range 7–22 eV. Fifteen fragmentation processes in naphthalene and twelve in azulene have been examined in detail. The photoionization mass spectra at 20.58 eV are quasi-identical for the two isomers. This, and the constant value of the difference between the fragment appearance energies (AE) for naphthalene and azulene, equal to the difference in the heats of formations of the neutral parents, suggest that identical products are formed. The unimolecular dissociations fall mainly into (i) a “low energy” group, (AE 18 eV). The reactions in (i) have in common the bicyclic precursor C 10 H + 8 ion 18 which decays via rupture of one ring. The group (ii) reactions involve rupture of both rings to give an open chain precursor, the 1,6-bis-ethinyl-hexatriene radical cation 20 . Thermodynamic and mechanistic arguments are given to propose specific reaction pathways and product structures. Two general schemes rationalize the low-energy and high-energy ionic decompositions.

Photoionization of aromatic van der waals complexes in a supersonic jet

Abstract The cluster photoionization mass spectra and ionization potentials (IP) of benzene (BZ), toluene (TOL), and fluorobenzene (FB) van der Waals complexes have been measured using the molecular beam photoionization technique with synchrotron radiation: IP(BZ)2 = 8.84, IP(TOL)2 = 8.61, IP(FB)2 = 8.91, IP(BZ·FB) = 8.93, IP(TOL·FB) = 8.68, IP(BZ·TOL) = 8.61 eV. The ionization potentials of the complexes are discussed in relation to those of the monomer species. For all cluster species reported in this study standard heats of formation of the neutral and cationic species have been evaluated.

K‐shell spectroscopy of Ar clusters

Total ion yield spectra of a supersonic beam of Ar clusters have been recorded in the Ar K‐shell (1s) spectral region using synchrotron radiation. The spectrum of the cluster component of beams containing argon clusters of mean size N≊400 atoms is similar to that of the x‐ray absorption spectrum of solid Ar. Analysis of the Ar 1s extended x‐ray absorption fine structure (EXAFS) indicates an average first shell Ar–Ar distance in the cluster similar to that of the solid, but there is an increased Debye–Waller (disorder/thermal motion) term as well as changes in the higher coordination shell signals.Time‐of‐flight mass spectra and photoion–photoion coincidence (PIPICO) spectra of argon cluster beams are reported and the charge separation mechanisms of multiply charged argon clusters are discussed. The results are compared to those from recent studies of the Ar 2p spectra of Ar clusters.

Classification of growth behaviour for copper on various substrates with in-situ scanning probe microscopy

In-situ scanning probe microscopy has been used to directly examine the deposition of copper on gold, polypyrrole and gold substrates which have been precovered with a multilayer copper film. The growth of the copper deposits, from electrolytes with and without additives, have been classified for these different substrates in accordance with Bauer [1] as either: (A) the Stranski-Krastanov mechanism involving the deposition of one or more layers followed by the formation of three-dimensional islands; (B) Frank-van der Merwe layer-by-layer growth of either individual monolayers (B1); or multilayer films (B2); or (C) the Volmer-Weber mechanism involving the formation of bulk islands directly on-top of the substrate. We show that the growth behaviour can be rationalised in terms of simple thermodynamic models which take into account, the strength of the deposit-substrate interaction and the crystallographic misfit between the deposit and the underlying substrate surface.