Oleg Kostko

Symmetry and Electronic Structure of Noble-Metal Nanoparticles and the Role of Relativity

We present high resolution UV-photoelectron spectra of cold mass selected Cun-, Agn-, and Aun- with n=53-58. The observed electron density of states is not the expected simple electron shell structure, but is strongly influenced by electron-lattice interactions. Only Cu55- and Ag55- exhibit highly degenerate states. This is a direct consequence of their icosahedral symmetry, as is confirmed by density functional theory calculations. Neighboring sizes exhibit perturbed electronic structures, as they are formed by removal or addition of atoms to the icosahedron and therefore have lower symmetries. Gold clusters in the same size range show completely different spectra with almost no degeneracy, which indicates that they have structures of much lower symmetry. This behavior is related to strong relativistic bonding effects in gold, as demonstrated by ab initio calculations for Au55-.

Thermal decomposition of CH3CHO studied by matrix infrared spectroscopy and photoionization mass spectroscopy.

A heated SiC microtubular reactor has been used to decompose acetaldehyde and its isotopomers (CH(3)CDO, CD(3)CHO, and CD(3)CDO). The pyrolysis experiments are carried out by passing a dilute mixture of acetaldehyde (roughly 0.1%-1%) entrained in a stream of a buffer gas (either He or Ar) through a heated SiC reactor that is 2-3 cm long and 1 mm in diameter. Typical pressures in the reactor are 50-200 Torr with the SiC tube wall temperature in the range 1200-1900 K. Characteristic residence times in the reactor are 50-200 μs after which the gas mixture emerges as a skimmed molecular beam at a pressure of approximately 10 μTorr. The reactor has been modified so that both pulsed and continuous modes can be studied, and results from both flow regimes are presented. Using various detection methods (Fourier transform infrared spectroscopy and both fixed wavelength and tunable synchrotron radiation photoionization mass spectrometry), a number of products formed at early pyrolysis times (roughly 100-200 μs) are identified: H, H(2), CH(3), CO, CH(2)=CHOH, HC≡CH, H(2)O, and CH(2)=C=O; trace quantities of other species are also observed in some of the experiments. Pyrolysis of rare isotopomers of acetaldehyde produces characteristic isotopic signatures in the reaction products, which offers insight into reaction mechanisms that occur in the reactor. In particular, while the principal unimolecular processes appear to be radical decomposition CH(3)CHO (+M) → CH(3) + H + CO and isomerization of acetaldehyde to vinyl alcohol, it appears that the CH(2)CO and HCCH are formed (perhaps exclusively) by bimolecular reactions, especially those involving hydrogen atom attacks.

First-principles determination of the structure of NaN and NaN− clusters with up to 80 atoms

We have performed an extensive computational search for the global minimum (GM) structures of both neutral and anionic sodium clusters with up to 80 atoms. The theoretical framework combines basin hopping unbiased optimizations based on a Gupta empirical potential (EP) and subsequent reoptimization of many candidate structures at the density functional theory level. An important technical point is that the candidates are selected based on cluster shape descriptors rather than the relative stabilities of the EP model. An explicit comparison of the electronic density of states of cluster anions to experimental photoemission spectra suggests that the correct GM structures have been identified for all but two sizes (N = 47 and 70). This comparison validates the accuracy of the proposed methodology. Furthermore, our GM structures either match or improve over the results of previous works for all sizes. Sodium clusters are seen to accommodate strain very efficiently because: (a) many structures are based on polyicosahedral packing; (b) others are based on Kasper polyhedra and show polytetrahedral order; (c) finally, some (N + 1)-atom structures are obtained by incorporating one adatom into the outermost atomic shell of a compact N-atom cluster, at the cost of increasing the bond strain. GM structures of neutrals and anions differ for most sizes. Cluster stabilities are analyzed and shown to be dominated by electron shell closing effects for the smaller clusters and by geometrical packing effects for the larger clusters. The critical size separating both regimes is around 55 atoms. Some implications for the melting behavior of sodium clusters are discussed.

Vacuum-Ultraviolet Photoionization Measurement and ab Initio Calculation of the Ionization Energy of Gas-Phase SiO2

In this work we report on the detection and vacuum-ultraviolet (VUV) photoionization of gas-phase SiO(2) generated in situ via laser ablation of silicon in a CO(2) molecular beam. The resulting species are investigated by single-photon ionization with tunable VUV synchrotron radiation and mass analyzed using reflectron mass spectrometry. Photoionization efficiency (PIE) curves are recorded for SiO and SiO(2), and ionization energy estimates are revealed from such measurements. A state-to-state ionization energy of 12.60 +/- 0.05 eV is recorded by fitting two prominent peaks in the PIE curve for the following process: (1)Sigma O-Si-O --> (2)Pi(g) [O-Si-O](+). Electronic structure calculations aid in the interpretation of the photoionization process and allow for identification of the symmetric stretch of (2)Pi(g) [O-Si-O](+), which is observed in the PIE spectrum to be 0.11 eV (890 cm(-1)) above the ground state of the cation and agrees with the 892 cm(-1) symmetric stretch frequency calculated at the CCSD(T)/aug-cc-pVTZ level.

Transition from a Bloch-Wilson to a free-electron density of states in Znn− clusters

We present photoelectron spectroscopy studies on Zn(n) (-) in the size range of n=3-117. We show that zinc clusters exhibit a distinct transition in their electronic structure as a function of size. At small sizes (up to n=18) the clusters follow the Bloch-Wilson picture of the development of a metal from closed-shell atoms, exhibiting a gradual decrease of the gap between the fully occupied s band and the empty p band. For large sizes (n approximately or > 32) the band overlap allows the valence electrons to fully delocalize. This leads to an almost perfect free-electron density of states, as is demonstrated by discussing the spectra in the light of standard free-electron models and by comparison to the results obtained on sodium clusters.

Probing Methanol Cluster Growth by Vacuum Ultraviolet Ionization

The ability to probe the formation and growth of clusters is key to answering fundamental questions in solvation and nucleation phenomena. Here, we present a mass spectrometric study of methanol cluster dynamics to investigate these two major processes. The clusters are produced in a molecular beam and ionized by vacuum ultraviolet (VUV) radiation at intermediate distances between the nozzle and the skimmer sampling different regimes of the supersonic expansion. The resulting cluster distribution is studied by time-of-flight mass spectrometry. Experimental conditions are optimized to produce intermediate size protonated methanol and methanol-water clusters and mass spectra and photoionization onsets and obtained. These results demonstrate that intensity distributions vary significantly at various nozzle to ionization distances. Ion-molecule reactions closer to the nozzle tend to dominate leading to the formation of protonated species. The protonated trimer is found to be the most abundant ion at shorter distances because of a closed solvation shell, a larger photoionization cross section compared to the dimer, and an enhanced neutral tetramer precursor. On the other hand, the protonated dimer becomes the most abundant ion at farther distances because of low neutral density and an enhanced charged protonated monomer-neutral methanol interaction. Thomsons liquid drop model is used to qualitatively explain the observed distributions.

Isotope effects and spectroscopic assignments in the non-dissociative photoionization spectrum of N2

Photoionization efficiency spectra of (14)N2, (15)N(14)N, and (15)N2 from 15.5 to 18.9 eV were measured using synchrotron radiation at the Advanced Light Source at Lawrence Berkeley National Laboratory with a resolution of 6 meV, and significant changes in peak energies and intensities upon isotopic substitution were observed. Previously, we reported the isotope shifts and their applications to Titans atmosphere. Here, we report more extensive experimental details and tabulate the isotope shifts of many transitions in the N2 spectrum, including those for (15)N(14)N, which have not been previously reported. The isotope shifts are used to address several long-standing ambiguities in spectral peak assignments just above the ionization threshold of N2. The feature at 15.677 eV (the so-called second cathedral peak) is of particular interest in this respect. The measured isotope shifts for this peak relative to (14)N2 are 0.015 ± 0.001 eV for (15)N2 and 0.008 ± 0.001 eV for (15)N(14)N, which match most closely with the isotope shifts predicted for transitions to the (A (2)Πu v = 2)4sσ(g) (1)Π(u) state using Herzberg equations for the isotopic differences in harmonic oscillator energy levels plus the first anharmonic correction of 0.0143 eV for (15)N2 and 0.0071 eV for (15)N(14)N. More generally, the isotope shifts measured for both (15)N2 and (15)N(14)N relative to (14)N2 provide new benchmarks for theoretical calculations of interferences between direct and indirect autoionization states which can interact to produce intricate resonant structures in molecular photoionization spectra in regions near ionization thresholds.

Photoelectron spectroscopy of the structure and dynamics of free size selected sodium clusters

Photoelectron spectroscopy on free cold size-selected sodium clusters has led to a detailed knowledge of their electronic and geometric structures in a very broad size range. Even more information about the electronic structure of the clusters has been obtained now by angle resolved photoelectron spectroscopy, which in principle allows one to gain information about the character of the electronic wavefunctions. The results demonstrate that sodium clusters have an electronic structure close to that of an finite size free electron gas, which makes them perfect model systems for the study of many particle dynamics. One such measurement is the time-resolved study of the cooling of the hot electron gas in the cluster, which allowed to determine the size dependence of the electron phonon coupling strength.

Probing Ionic Complexes of Ethylene and Acetylene with Vacuum-Ultraviolet Radiation

Mixed complexes of acetylene-ethylene are studied using vacuum-ultraviolet (VUV) photoionization mass spectrometry and theoretical calculations. These complexes are produced and ionized at different distances from the exit of a continuous nozzle followed by reflectron time-of-flight mass spectrometry detection. Acetylene, with a higher ionization energy (11.4 eV) than ethylene (10.6 eV), allows for tuning the VUV energy and initializing reactions either from a C2H2(+) or a C2H4(+) cation. Pure acetylene and ethylene expansions are separately carried out to compare, contrast, and hence identify products from the mixed expansion: these are C3H3(+) (m/z = 39), C4H5(+) (m/z = 53), and C5H5(+) (m/z = 65). Intensity distributions of C2H2, C2H4, their dimers and reactions products are plotted as a function of ionization distance. These distributions suggest that association mechanisms play a crucial role in product formation closer to the nozzle. Photoionization efficiency (PIE) curves of the mixed complexes demonstrate rising edges closer to both ethylene and acetylene ionization energies. We use density functional theory (ωB97X-V/aug-cc-pVTZ) to study the structures of the neutral and ionized dimers, calculate their adiabatic and vertical ionization energies, as well as the energetics of different isomers on the potential energy surface (PES). Upon ionization, vibrationally excited clusters can use the extra energy to access different isomers on the PES. At farther ionization distances from the nozzle, where the number densities are lower, unimolecular decay is expected to be the dominant mechanism. We discuss the possible decay pathways from the different isomers on the PES and examine the ones that are energetically accessible.