We present direct evidence for the formation of the covalent bonded styrene (isoprene)(2) oligomer and the isoprene dimer ions following resonance ionization of the gas phase styrene-isoprene binary clusters. The application of resonance ionization to study polymerization reactions in clusters provides new information on the structure and mechanism of formation of the early stages of polymerization and holds considerable promise for the discovery of new initiation mechanisms and for the development of novel materials with unique properties.

The styrene-water binary clusters SW(n), with n = 1-5 have been studied by the (one-color) resonant two-photon ionization technique using the resonance of styrene. The structures and energetics of the neutral clusters are investigated using a search technique that employs Monte Carlo procedure. The strong tendency for water molecules to form cyclic hydrogen-bonded structures is clearly observed in the SW(n) structures starting from n =3. The results indicate that the spectral shifts correlate with the interaction energies between styrene and the water subcluster (W(n)) within the SW(n) clusters. Evidence is presented that points to (1) the formation of a covalent bonded styrene radical cation dimer following the 193 nm MPI of styrene neutral clusters, (2) proton transfer from the styrene dimer cation to the water or methanol subcluster, resulting in the formation of protonated water or methanol clusters and a styrene dimer radical, and (3) extensive solvation of the styrene dimer radical within the protonated solvent molecules. The proton-transfer reactions may explain the strong inhibition effects exerted by small concentrations of water or methanol on the cationic polymerization of styrene. These results provide a molecular level view of the inhibition mechanism exerted by protic solvents on the cationic polymerization of styrene.

Well-resolved spectra of styrene-methanol binary clusters SMn, with n = 1-9, have been obtained by the (one-color) resonant two-photon ionization technique using the 0(0)(0) resonance of styrene. The spectra reveal a rapid increase in complexity with the number of methanol molecules in the cluster, associated with van der Waals modes and isomeric forms. Two distinct isomers are identified for each of the clusters studied with the exception of SM4 and SM8. The progressive addition of methanol molecules to the SM complex leads to the formation of stable cyclic and branched cyclic methanol subclusters within the SM, clusters. The spectral shift of the cluster origin reflects the nature of the intermolecular interactions within the binary cluster. Blue shifts are observed for the SM, SM2, and SM3 Clusters and are consistent with hydrogen bonding interactions between the OH groups and the styrene pi-system. A remarkable switch in the spectral shift from blue to red is observed at the SM4 cluster and is consistent with the ring structure of the methanol tetramer. Evidence is provided for intracluster dissociative proton-transfer reactions within the S2Mn+(n > 2) clusters that generate protonated methanol clusters. These reactions may explain the strong inhibition effects exerted by small concentrations of methanol on the cationic polymerization of styrene.

Resonant two-photon ionization (R2PI) spectra of styrene–water (SWn) and styrene–methanol (SMn) binary clusters with n=1,2 are reported. The results indicate that the SWn clusters exhibit different structures as compared to the benzene (water)n clusters. Ab initio calculations of the lowest energy structure of the SW complex confirm that water interacts mostly with the ethylene group. Two distinct isomers are identified for the SM2 cluster. The favorable interactions of water and methanol with the olefin group of styrene may explain the strong inhibition effects observed by trace concentrations of water or methanol on the cationic polymerization of styrene.

Electron impact and multiphoton ionization techniques are used for a comparative study of the acetic acid–water and acetic acid–benzene clusters generated by supersonic beam expansion. In acetic acid–water clusters, hydrogen-bonding interaction is the driving force in determining the structures of the clusters. The protonated and the methyl cation containing clusters are characterized by 6-membered cyclic and 8-membered bicyclic structures. The similar magic number patterns observed for the protonated and methyl cation containing clusters suggest that the cyclic structures are stabilized by the charge interaction. A remarkable periodicity in the ion intensity of benzene (acetic acid)n clusters is observed. The clusters containing an even number of acetic acid molecules exhibit enhanced ion intensities. This effect is attributed to the formation of multiple cyclic dimers as a result of clustering from a dimer-rich vapor phase. Protonated acetic acid clusters are generated following the three-photon absorption by the binary clusters, which leads to dissociative charge transfer followed by proton transfer within the ionized acetic acid clusters. Evidence is presented for the special stability of the B2A8 ion, which is proposed to consist of a benzene dimer cation entrapped between two acetic acid tetramers.

The dependence of quantum size effects on bonding structure in oxidized silicon nanoclusters is established by correlating photoluminescence data with photon-yield electronic structure measurements at the advanced light source. The nanoclusters were synthesized using a laser ablation technique that utilizes a convective He environment to control the size of the particles. After removal from the growth chamber, our ex situ photoluminescence (PL) results indicate that, as the nanoclusters oxidize, the main PL peak moves from 1.83 to 1.94 eV in energy. The central focus of the present work is to establish the origin of the main PL peak, and to determine why its energy shifts as the nanoclusters are allowed to oxidize slowly in air. The changes in the morphology and bonding structure of the clusters was established using soft-X-ray fluorescence spectroscopy (SXF) and photon-yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, which probe the element-specific density of occupied (SXF) and unoccupied (NEXAFS) electronic structure. Our conclusion is that the as-synthesized nanoclusters consist of a pure, crystalline Si core within a nearly pure SiO2 shell, with little or no sub-oxides present. As the nanoclusters oxidize, the radius of the crystalline core decreases in size, which gives rise to the change in the position of the PL signal.

Web-like aggregates of Si nanocrystals produced by laser vaporization–controlled condensation technique are allowed to oxidize slowly in air and the photoluminescence (PL) is measured. A significant shift in the PL red band from 1.83 to 1.94 eV is observed. The bonding structure is established by correlating the PL data with the photon-yield electronic structure measurements using soft-X-ray fluorescence (SXF) and photon-yield near-edge X-ray absorption fine structure (NEXAFS) techniques. The results indicate that as the nanoparticles oxidize, the radius of the crystalline core decreases, which gives rise to a larger bandgap and consequently to the observed blue shift in the PL band.

The decay dynamics and the quenching of the photoluminescence (PL) from Si nanocrystals are investigated. Electron accepters whose reduction potentials lie below the conduction band (CB) edge of the Si nanocrystals quench the red emission from the Si nanocrystals. The quenching rate constants obtained from Stern-Volmer analyses for 3,5-dinitrobenzonitrile, 4-nitrophthalonitrile, 1,4-dinitrobenzene, 4-nitrobenzonitril 2,3-dinitrotoluene, 3,4-dinitrotoluene, 2,4-dinitrotoluene, and 2,6-dinitrotoluene are in the range of 10(6)-10(7) M(-1)s(-1) The quenching mechanism occurs via an electron transfer from the CB band of the Si nanocrystals to the vacant orbitals of the quenchers. The PL decay profiles of the Si nanocrystals, in the presence and absence of the quencher, are well described by the stretched exponential decay law. The band gap of the Si nanocrystals estimated from the present study is larger than the PL peak energy. The results are consistent with a quantum-confinement model, where recombination of electrons and holes occurs in a surface state. The ability of nitrotoluenes to quench the PL from Si nanocrystals could be used to develop a sensor based on Si nanostructures for the detection of explosives.

Web-like aggregates of coalesced Si nanocrystals produced by a laser vaporization-controlled condensation technique show luminescence properties that are similar to those of porous Si. The results are consistent with a quantum confinement mechanism as the source of the red photoluminescence (PL) in this system. The oxidized Si nanoparticles do not exhibit the red PL that is characteristic of the surface-oxidized Si nanocrystals. The nanoparticles are allowed to oxidize slowly, and the PL is measured as a function of the exposure time in air. A significant blue shift in the red PL peak is observed as a result of the slow oxidation process. The dependence of quantum size effects on the bonding structure is established by correlating the PL data with the photon-yield electronic structure measurements made at the Advanced Light Source. The results indicate that as the nanoparticles oxidize, the radius of the crystalline core decreases in size, which gives rise to a larger bandgap and consequently to the observed blue-shift in the PL band. The correlation between the FL, SXF, and NEXAFS results provides further support for the quantum confinement mechanism as the origin of the visible PL in Si nanocrystals.

Weblike aggregates of coalesced Si nanocrystals are prepared by the Laser Vaporization-Controlled Condensation (LVCC) method. Upon excitation with visible or UV light, the Si nanocrystals show red photoluminescence whose multiexponential time decays are characterized by lifetimes that range from 20–80 μs, depending on the emission wavelength. This red emission can be quenched with electron acceptors like 1,4-dinitrobenzene and WO3 nanoparticles. The quenching rate constants obtained from a Stern- Volmer analysis are 7.65 × 106 (Ms)−1 and 14.1 × 106 (Ms)−1 for 1,4-dinitrobenzene and WO3 nanoparticles, respectively. The quenching mechanism appears to proceed via an electron transfer from the CB band of the Si nanocrystals to the quenchers.

Resonant two-photon ionization (R2PI) spectra of benzene-acetic acid binary clusters BA,, with n = 1-6 are reported. The results indicate that cluster formation from acetic acid vapor proceeds mainly via the association of preformed hydrogen-bonded dimers. The spectral shift of the BA complex is consistent with pi-hydro,gen bonding interaction with the benzene ring. The spectral shifts of the BA(2) and BA(3) dusters are consistent with a predominantly dispersion interaction which is enhanced upon electronic excitation of the benzene molecule. Spectroscopic evidence for the formation of (Ne).(C6H6).(CH3COOH), clusters, with n 4 and 6, is presented. The direction and magnitude of the spectral shift indicate that the C6H6 Ne and the C6H6... (CH3COOH), structures are retained in the multicomponent clusters, suggesting no perturbations due to the weak C6H6... Ne interaction.

Weblike aggregates of Si nanocrystals and tungsten oxide nanoparticles are produced by a laser vaporization controlled condensation technique. XRD, FTIR, and Raman results confirm that the Si nanoparticles have a diamond-like crystalline structure and WO3 nanoparticles exhibit a monoclinic crystalline structure. Due to the quantum size effect, the band gaps of Si nanocrystals and WO3 nanoparticles shift to higher energies by 0.68 and 0.55 eV, respectively, from the bulk values. The red emission of Si nanocrystals is quenched by adding successive amounts of the WO3 nanoparticles to Si nanoparticles suspended in a methanol solution. The quenching follows Stern−Volmer kinetics and the quenching rate constants are 14.1 × 106 and 3.2 × 106 (Ms)-1 for WO3 and W2O5, respectively. The quenching mechanism is explained by an electron transfer from the CB of the Si to the CB of the WO3 nanoparticles.