Polycyclic aromatic hydrocarbon-isomer fragmentation pathways: case study for pyrene and fluoranthene molecules and clusters

We report on measurements of the ionization and fragmentation of polycyclic aromatic hydrocarbon (PAH) targets in Xe(20+)?+ C(16)H(10) and Xe(20+)?+ [C(16)H(10)](k) collisions and compare results for the two C(16)H(10) isomers: pyrene and fluoranthene. For both types of targets, i.e., for single PAH molecules isolated in vacuum or for isomerically pure clusters of one of the molecules, the resulting fragment spectra are surprisingly similar. However, we do observe weak but significant isomer effects. Although these are manifested in very different ways for the monomer and cluster targets, they both have at their roots small differences (<2.5 eV) between the total binding energies of neutral, and singly and multiply charged pyrene and fluoranthene monomers. The results will be discussed in view of the density functional theory calculations of ionization and dissociation energies for fluoranthene and pyrene. A simple classical over-the-barrier model is used to estimate cross sections for single- and multiple-electron transfer between PAHs and ions. Calculated single and multiple ionization energies, and the corresponding model PAH ionization cross sections, are given.     

Importance of properties of the lowest and higher singlet excited states on the resonant two-photon ionization of stilbene and substituted stilbenes using two-color two-lasers

Radical cations of trans-stilbene and substituted trans-stilbenes (stilbenes and the radical cations denote Sand S(*+), respectively) were generated from the resonant two-photon ionization (TPI) in acetonitrile with irradiation of one-laser (266- or 355-nm laser) and with simultaneous irradiation of two-color two-lasers (266- and 532-nm or 355- and 532-nm lasers) with the pulse width of 5 ns each. The formation yields of S(*+), the TPI efficiency, depended on the properties of S in the lowest and higher singlet excited state (S(S(1)) and S(S(n))), generated from one-photon excitation with 266- or 355-nm laser and from two-photon excitation with simultaneous irradiation of 266- and 532-nm or 355- and 532-nm lasers, respectively. The TPI efficiency using two-color two-lasers increased compared with that using one-laser. It is confirmed that the TPI proceeds through two-step two-photon excitation with the S(0) --> S(1) --> S(n)() transition. In addition to the electronic character of S(S(0)) which depends on the substituent of S, oxidation potential, and molar absorption coefficient of the S(0) --> S(1) absorption as well-known important factors for the TPI efficiency, it is shown that properties of S(S(1)) and S(S(n)) such as lifetimes, electronic characters of S(S(1)) and S(S(n)), molar absorption coefficient of the S(1) --> S(n) absorption, and ionization rate from S(S(n)) are also important.     

Laser multiphoton ionization of aromatic molecules in nonpolar liquids

The laser multiphoton ionization (MPI) of fluoranthene in tetramethylsilane (TMS) and of azulene in n-tridecane, n-pentane, 2,2,4,4-tetramethylpentane, TMS and tetramethyltin is reported. Three distinct types of MPI mechanisms have been identified: two-photon ionization, stepwise three-photon ionization and mixed two- and three-photon ionization. The stepwise three-photon process consists of two-photon excitation, relaxation to a lower lying excited state with a lifetime comparable to the laser pulse duration (for azulene this state is the S₂ while for fluoranthene both the S₁ and S₂ states) and subsequent ionization with the absorption of a third photon. The ionization threshold of azulene in each liquid has been determined and found to vary linearly with the V₀ of the liquid.     

Electronic transmission of substituent effects through the pyrrole ring Ionization constants of some 4- and 5-substituted-2-pyrrolcarboxylic acids

The ionization constants of several 4- and 5-substituted-2-pyrrolcarboxylic acids were determined in water at 25·0°. A good fit in the Hammett equation was found using σ_m, and σ_P constants. The value (1·65) has been compared with those for the ionization of benzene-, furane-, thiophene- and selenophene-carboxylic acids.     

Fabrication and chemical crosslinking of electrospun <Emphasis Type="Italic">trans</Emphasis>-polyisoprene nanofiber nonwoven

In this work, the optimal electrospinning conditions of trans-polyisoprene (TPI) solutions were evaluated nevertheless its lower glass transition temperature than the room temperature. Subsequently, chemical crosslinking of TPI nonwovens was firstly investigated by vulcanizing at high temperatures in the case of the persistence of nanofiber structure. For this purpose, curing agents of TPI were embedded in TPI nanofibers by co-electrospinning, and then a protect layer was coated on TPI nanofibers by filtering gelatin solution going through TPI nonwoven before the vulcanization at 140?160 °C. The results showed that the vulcanization of TPI fibrous nonwoven at high temperatures did not destroy the fiber morphology. Interestingly, TPI fibrous nonwovens after vulcanization showed excellent mechanical properties (~17 MPa of tensile strength) that could be comparable to or even higher than that of some bulk rubber materials.     

几种芳香烃的激光电离飞行时间质谱研究

利用一步紫外激光脱附和电离的飞行时间质谱法,测定了几种芳香烃分子:苯并[e]芘、荧蒽、肉桂酸和2,5-二羟基苯甲酸的质谱。实验结果发现,苯并[e]芘发生有效的“软”电离,属于双光子电离过程。而荧蒽需吸收三个光子才能电离。对2,5-二羟基苯甲酸和肉桂酸,在紫外脉冲激发作用下除了自身电离外,还发生了分子离子反应。     

Study of the N2 bΠu state via 1 + 1 multiphoton ionization

Medium-resolution spectra of the N₂ b¹Π_u-X¹Σ_g⁺ band system were recorded by 1 + 1 multiphoton ionization. In the spectra we found different linewidths for transitions to different vibrational levels in the b ¹Π_u state: Δν₀ = 0.50 ± 0.05 cm⁻¹, Δν₁ = 0.28 ± 0.02 cm⁻¹, Δν₂ = 0.65 ± 0.06 cm⁻¹, Δν₃ = 3.2 ± 0.5 cm⁻¹, Δν₄ = 0.60 ± 0.07 cm⁻¹, and Δν₅ = 0.28 ± 0.02 cm⁻¹. From these linewidths, predissociation lifetimes τ_ν were obtained: τ₀ = 16 ± 3 ps, τ₁ > 150 ps, τ₂ = 10 ± 2 ps, τ₃ = 1.6 ± 0.3 ps, τ₄ = 9 ± 2 ps, and τ₅ > 150 ps. Band origins and rotational constants for the b ¹Π_uν = 0 and 1 levels were determined for the ¹⁴N₂ and ¹⁴N¹⁵N molecules.     

Solubility, dissociation and complexation with Nd(III) and Th(IV) of oxine, thenoyltrifluoroacetone and 1,10-phenanthroline in 5.0 m NaCl

Equilibria in the system of Nd(III) and Th(IV) with 8-hydroxyquinoline (oxine), thenoyltrifluoroacetone (HTTA) and 1,10-phenanthroline (phen) in 5.0 m NaCl solution have been investigated by spectroscopy and potentiometry. The solubility and deprotonation constants of the three organics were measured to be: pK_s = 3.09 ± 0.01, pK_a1 = 5.82 ±0.02, pK_a2= 10.00 ±0.01 for oxine; pK_s = 2.49 ± 0.01, pK_a1 = 6.47 ±0.03 for HTTA; pK_s = 2.86 ± 0.02, pK_a2 = 5.82 ± 0.05 for phen. The stabilities of the corresponding metal complexes are in the order M(oxine) > M(TTA) > M(phen), where M = Nd(III), Th(IV). For all three organic ligands, the Th(IV) complexation is stronger than that of Nd(III).     

Persistent oxidation dications from twisted fluoranthenes, benzo[k]fluoranthene and dimethyldibenzo[j.l]fluoranthene: charge delocalization mode, tropicity, and formation of novel 8,8'-bifluoranthenyls. An NMR and theoretical study.

First examples of persistent oxidation dications from fluoranthene-PAHs namely 1,3,4,6,7,10-hexamethyl- 2 and 3,4-dichloro-1,6,7,10-tetramethylfluoranthene 3, benzo[k]fluoranthene 6, and 3,6-dimethyldibenzo[j,l]fluoranthene 9 are reported. Charge delocalization mode and tropicity in the resulting nonalternant dications are examined. Quenching of the superacid solutions of the dications resulted in the formation of novel 8,8'-bifluoranthenyls 2a-4a. AM1 was used as an initial guide for dication generation (DeltaDeltaH(f) degrees and ionization potentials) and for probing the structures of the crowded fluoranthene substrates and their bifluoranthenyls. In selected cases, the dications and their neutral precursors were calculated at the B3LYP/6-31G(d,p) level. Charge delocalization mode (difference in NPA charges) and DFT/GIAO-derived NMR chemical shifts were obtained for comparison with experiment; aromaticity was assessed via nucleus independent chemical shift (NICS) calculations.     

L-subshell Coster-Kronig yields measured with synchrotron radiation

The synchrotron photoionization method was applied to measure L-subshell Coster-Kronig yields. This method is based on the capability of tuning the energy of the synchrotron photons producing a selective subshell ionization. Two foil samples of Yb and Ta were irradiated and their characteristic spectra were recorded. Data were analyzed using a new formalism (based on a matrix representation) for expressing X-ray fluorescence intensities involving Coster-Kronig transitions. The results obtained in this work are f₁₂ = 0.249 ± 0.021, f₁₃ = 0.408 ± 0.055 and f₂₃ = 0.186 ± 0.040 for Yb, and f₁₂ = 0.168 ± 0.039, f₁₃ = 0.322 ± 0.072 and f₂₃ = 0161 ± 0.053 for Ta. These data are very reliable and represent a valuable information for spectroscopists, considering the lack of data for L-shell parameters.     

Effects of benzyl ether type dendrons as hole-harvesting antennas, and shielding for the neutralization of stilbene core radical cations with chloride ion during two-photon ionization of stilbene dendrimers having stilbene core and benzyl ether type dendrons

The two-photon ionization (TPI) process (308 and 266 nm) of stilbene dendrimers having a stilbene core and benzyl ether type dendrons has been investigated in an acetonitrile and 1,2-dichloroethane mixture (3:1) in order to elucidate the dendrimer effects. The quantum yield of the formation of stilbene core radical cation during the 308-nm TPI was independent of the dendron generation of the dendrimers, whereas a generation dependence of the quantum yield of the radical cation was observed during the 266-nm TPI, where both the stilbene core and benzyl ether type dendron were ionized, suggesting that the subsequent hole transfer occurs from the dendron to the stilbene core, and that the dendron acts as a hole-harvesting antenna. The neutralization rate of the stilbene core radical cation with the chloride ion, generated from the dissociative electron capture by 1,2-dichloroethane, decreased with the increase in the dendrimer generation, suggesting that the dendron is an effective shield of the stilbene core radical cation against the chloride ion.     

Measurement of an autoionizing triplet state in D2 by photoexcitation of a neutralized ion beam

Neutralized ion beam techniques have been extended to observe the laser ionization spectrum of the c ³Π_u state of D₂. The autoionizing triplet→nd Rydberg series has been observed. From the break in the continuous ionization signal the ionization energy of the c ³Π_u state is found to be 3.675±0.020 eV.     

Multiphoton ionization detected by fourier-transform mass spectrometry

Ions produced by multiphoton ionization (MPI) of naphthalene, fluoranthene and triphenylene have been detected by Fourier-transform mass spectrometry (FT MS). Paret ions are produced very efficiently at 250 and 222 nm with pulse energies as low as 1 mJ. With FT MS a complete, high-resolution mass spectrum is obtained for each laser pulse.     

Mechanical relaxations in trans-1,4-polyisoprene crystals

Dynamic mechanical relaxation spectra were obtained for solution-grown crystals of trans-1,4-polyisoprene (TPI) in the α and β form. For single crystal mats three relaxations were observed. The highest temperature relaxation peak was characterized as due to the crystalline regions, whereas the intermediate peak was assigned to the primary amorphous relaxation which originates from the fold regions. The nature of the amorphous regions was elucidated by examining the effect of epoxidation on the lamellar fold surface. For an epoxidized single crystal mat, the intermediate relaxation maximum shifted to a higher temperature which corresponds to the glass transition of the almost completely epoxidized TPI. These results are discussed in terms of the fold structure of the TPI single crystals.     

A theoretical investigation of the enol content of acetic acid and the acetate ion in aqueous solution

Ab initio molecular orbital calculations and statistical Monte Carlo simulations employing a combined quantum and molecular mechanical potential were used to determine the enol contents of acetic acid and the acetate ion in aqueous solution. A pK_E of 19.3 ± 0.3 was predicted for the keto-enol equilibrium of acetic acid, and 21.8 ± 0.8 for the acetate ion in water. The results are found to be in good accord with Guthrie's calculations based on disproportionation reactions and kinetic data. Combining with the experimental pK_a value of acetic acid, we obtained pK_a^k = 26.6 for ionization of acetic acid as a carbon acid in water, and pK_a^E = 7.3 for ionization of the enol of acetic acid.     

Determination of the first ionization energy of furan (C4H4O) from an extrapolation of two nd Rydberg series observed in the mass-resolved (2 + 1) resonance enhanced multiphoton ionization spectrum

The first ionization energy of furan (C₄H₄O) has been determined from a short extrapolation of two nd (n=6–22) Rydberg series observed in the mass-resolved (2 + 1) resonance enhanced multiphoton ionization spectrum as IE=71673 ± 3 cm⁻¹. This value confirms the higher of the two values in the literature.     

Conformational identification and dipole moment of 4-fluoro-1-butene by microwave spectroscopy

The microwave spectrum of 4-fluoro-1-butene has been recorded in the frequency region 26.5–40.0 GHz. A-type rotational transitions in the ground vibrational state have been assigned. The effective rotational constants and centrifugal distortion constants were determined to be A = 19 196 ± 323, B = 2132.48 ± 0.01, C = 2112.52 ± 0.01 MHz, D_J = 0.70 ± 0.03 and D_JK = −26.16 ± 0.05 kHz. Analysis of the measured Stark effects gave a dipole moment of 1.80 D with components of |u_a| = 1.62 ± 0.01, |u_b| = 0.68 ± 0.05 and |u_c| = 0.39 ± 0.14 D. From the experimental rotational constants and dipole moments, it is suggested that the assigned spectra have resulted from the skew—trans conformer.     

Energy of the quasi-free electron in supercritical argon near the critical point

Field ionization of high-n CH₃I Rydberg states doped into argon is presented as a function of argon number density along the critical isotherm. These data exhibit a decrease in the argon induced shift of the dopant ionization energy near the critical point. We show that this decrease is due to the interaction between argon and the quasi-free electron arising from field ionization of the dopant. The energy of the quasi-free electron in argon near the critical point is calculated in a local Wigner–Seitz model containing no adjustable parameters to within ±0.2% of experiment.     

A density functional study of the structures and ionization energies of some scandium compounds with hydrogen and oxygen

The geometries, frequencies and energies of various Sc species with hydrogen and oxygen which might exist in a flame doped with Sc were studied with density functional methods. Two functionals, B3LYP and BP86, equipped with two basis sets, TZVP and 6-311++G(3df,3pd), were employed. In predicting geometries and frequencies, the BP86 functional performs consistently better than does the B3LYP functional, but the B3LYP functional performs better for energy calculations. For most species, the effects of using different basis-set effects are minor. The calculated ionization energies of 6.47 eV for Sc and 6.6 eV for ScO are in good agreement with the experimental values of 6.561 for Sc and 6.6±0.3 eV for ScO. For other species, suggested values of the calculated ionization energies are 6.3 eV for ScH, 6.1 for ScOH, 9.3 for HScO, 6.5 for HScOH, 9.5 for OScOH, 6.2 for Sc(OH)₂, 9.6 for HSc(OH)₂ and 10.1 eV for Sc(OH)₃, all with an approximate error of 0.2–0.3 eV. These high values indicate that the formation of Sc-containing ions in flames by thermal (collisional) ionization is unlikely.     

Depth-profiling of organic layers on microparticles with SNMS

Summary Previous work on the quantification and localization of organically bound elements with plasma-based SNMS for the characterization of microparticles has been continued. Relative detection factors for 10 elements have been determined. Except for lead and bromine a principle proportionality to atomic ionization probabilities is shown. A moderate matrix dependence of less than 40% variation was found even when inorganic and organic materials are included. For depth calibration, erosion rates of organic materials were determined from the time interval necessary to sputter through planar single-layers and Langmuir-Blodgett multi-layer systems with known thickness, as well as from gravimetric powder measurements. Depth propagation rates were 0.7 nm·sec^–1 for polymers and 2.3 nm·sec^–1 for aromatic compounds, when 400 eV argon ion bombardment with 0.7 mA·cm^–2 was used. A depth resolution of 30 nm has been obtained. Model particles of 5 m size have been coated with fluoranthene. Inspection with SNMS revealed an incomplete coating covering only 20% of the microparticle surface with an average thickness of the partial coating of 300 nm. Subsequent characterization using laser-induced fluorimetry confirmed the amount of fluoranthene coating measured by SNMS.