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1.
Phys Rev Lett ; 123(12): 123201, 2019 Sep 20.
Artigo em Inglês | MEDLINE | ID: mdl-31633947

RESUMO

Femtosecond laser pulses have opened new frontiers for the study of ultrafast phase transitions and nonequilibrium states of matter. In this Letter, we report on structural dynamics in atomic clusters pumped with intense near-infrared (NIR) pulses into a nanoplasma state. Employing wide-angle scattering with intense femtosecond x-ray pulses from a free-electron laser source, we find that highly excited xenon nanoparticles retain their crystalline bulk structure and density in the inner core long after the driving NIR pulse. The observed emergence of structural disorder in the nanoplasma is consistent with a propagation from the surface to the inner core of the clusters.

2.
J Chem Phys ; 151(10): 104308, 2019 Sep 14.
Artigo em Inglês | MEDLINE | ID: mdl-31521092

RESUMO

We have investigated the ionization and fragmentation of a metallo-endohedral fullerene, Sc3N@C80, using ultrashort (10 fs) x-ray pulses. Following selective ionization of a Sc (1s) electron (hν = 4.55 keV), an Auger cascade leads predominantly to either a vibrationally cold multiply charged parent molecule or multifragmentation of the carbon cage following a phase transition. In contrast to previous studies, no intermediate regime of C2 evaporation from the carbon cage is observed. A time-delayed, hard x-ray pulse (hν = 5.0 keV) was used to attempt to probe the electron transfer dynamics between the encapsulated Sc species and the carbon cage. A small but significant change in the intensity of Sc-containing fragment ions and coincidence counts for a delay of 100 fs compared to 0 fs, as well as an increase in the yield of small carbon fragment ions, may be indicative of incomplete charge transfer from the carbon cage on the sub-100 fs time scale.

3.
Struct Dyn ; 6(5): 054101, 2019 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-31531387

RESUMO

We have performed a full-dimensional theoretical study of vibrationally resolved photoelectron emission from the valence shell of the water molecule by using an extension of the static-exchange density functional theory that accounts for ionization as well as for vibrational motion in the symmetric stretching, antisymmetric stretching, and bending modes. At variance with previous studies performed in centrosymmetric molecules, where vibrationally resolved spectra are mostly dominated by the symmetric stretching mode, in the present case, all three modes contribute to the calculated spectra, including intermode couplings. We have found that diffraction of the ejected electron by the various atomic centers is barely visible in the ratios between vibrationally resolved photoelectron spectra corresponding to different vibrational states of the remaining H2O+ cation (the so-called v-ratios), in contrast to the prominent oscillations observed in K-shell ionization of centrosymmetric molecules, including those that only contain hydrogen atoms around the central atoms, e.g., CH4. To validate the conclusions of our work, we have carried out synchrotron radiation experiments at the SOLEIL synchrotron and determined photoelectron spectra and v-ratios for H2O in a wide range of photon energies, from threshold up to 150 eV. The agreement with the theoretical predictions is good.

4.
Phys Chem Chem Phys ; 2019 Aug 22.
Artigo em Inglês | MEDLINE | ID: mdl-31436275

RESUMO

Momenta of ions from diiodomethane molecules after multiple ionization by soft-X-ray free-electron-laser pulses are measured. Correlations between the ion momenta are extracted by covariance methods formulated for the use in multiparticle momentum-resolved ion time-of-flight spectroscopy. Femtosecond dynamics of the dissociating multiply charged diiodomethane cations is discussed and interpreted by using simulations based on a classical Coulomb explosion model including charge evolution.

5.
Nanomaterials (Basel) ; 9(6)2019 May 31.
Artigo em Inglês | MEDLINE | ID: mdl-31159254

RESUMO

Memristors are two terminal electronic components whose conductance depends on the amount of charge that has flown across them over time. This dependence can be gradual, such as in synaptic memristors, or abrupt, as in resistive switching memristors. Either of these memory effects are very promising for the development of a whole new generation of electronic devices. For the successful implementation of practical memristors, however, the development of low cost industry compatible memristive materials is required. Here the memristive properties of differently processed porous silicon structures are presented, which are suitable for different applications. Electrical characterization and SPICE simulations show that laser-carbonized porous silicon shows a strong synaptic memristive behavior influenced by defect diffusion, while wet-oxidized porous silicon has strong resistance switching properties, with switching ratios over 8000. Results show that practical memristors of either type can be achieved with porous silicon whose memristive properties can be adjusted by the proper material processing. Thus, porous silicon may play an important role for the successful realization of practical memristorics with cost-effective materials and processes.

6.
Nat Commun ; 10(1): 2186, 2019 05 16.
Artigo em Inglês | MEDLINE | ID: mdl-31097703

RESUMO

The increasing availability of X-ray free-electron lasers (XFELs) has catalyzed the development of single-object structural determination and of structural dynamics tracking in real-time. Disentangling the molecular-level reactions triggered by the interaction with an XFEL pulse is a fundamental step towards developing such applications. Here we report real-time observations of XFEL-induced electronic decay via short-lived transient electronic states in the diiodomethane molecule, using a femtosecond near-infrared probe laser. We determine the lifetimes of the transient states populated during the XFEL-induced Auger cascades and find that multiply charged iodine ions are issued from short-lived (∼20 fs) transient states, whereas the singly charged ones originate from significantly longer-lived states (∼100 fs). We identify the mechanisms behind these different time scales: contrary to the short-lived transient states which relax by molecular Auger decay, the long-lived ones decay by an interatomic Coulombic decay between two iodine atoms, during the molecular fragmentation.

7.
J Phys Chem A ; 123(14): 3074-3079, 2019 Apr 11.
Artigo em Inglês | MEDLINE | ID: mdl-30807166

RESUMO

Nitroimidazole exhibits a remarkable regioselective fragmentation subsequent to valence ionization, which is characterized by ejection of NO. As NO is also considered to be an effective radiosensitizer, we investigated its production efficiency as a function of isomeric composition (the site of the NO2 nitro group). We observe strong dependence in the 8.6-15 eV binding energy range, and moreover, that the production of NO can be effectively suppressed by methylation of nitroimidazole. This behavior can be understood by modification of the valence electronic structure with respect to the dissociation threshold, which gives rise to varying effective density of dissociative states. We find the NO yield to follow the efficiency of the nitroimidazole dervivatives as radiosensitizers, found in preclinical studies.

8.
Phys Chem Chem Phys ; 20(14): 9591-9599, 2018 Apr 04.
Artigo em Inglês | MEDLINE | ID: mdl-29578226

RESUMO

The chemical bond scission of methylbenzoate (C6H5CO2CH3) following core excitation at the C and O K edges was examined from partial ion yield measurements across these edges using synchrotron radiation. Site-specific scission of the C-O bonds was observed at both edges. Theoretical X-ray absorption spectra (XAS) were obtained using density functional theory. Peak assignments in the observed spectra were found to be consistent with the theory. From core-excited state dynamics calculations, an elongation of the C-O bond was predicted and provides an explanation of the observed partial ion yield enhancement of CH3+ and C6H5CO+ at the core-excited resonances at both edges.

9.
J Chem Phys ; 147(19): 194302, 2017 Nov 21.
Artigo em Inglês | MEDLINE | ID: mdl-29166118

RESUMO

Photofragmentation of gas-phase acetamide and acetic acid clusters produced by a supersonic expansion source has been studied using time-of-flight mass spectrometry and the partial ion yield (PIY) technique combined with tunable vacuum-ultraviolet synchrotron radiation. Appearance energies of the clusters and their fragments were experimentally determined from the PIY measurements. The effect of clusterization conditions on the formation and fragmentation of acetic acid clusters was investigated. Ab initio quantum mechanical calculations were performed on both samples' dimers to find their neutral and ionized geometries as well as proton transfer energy barriers leading to the optimal geometries. In the case of the acetamide dimer, the reaction resulting in the production of ammoniated acetamide was probed, and the geometry of the obtained ion was calculated.

10.
J Mass Spectrom ; 52(11): 770-776, 2017 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-28763569

RESUMO

We present here the photofragmentation patterns of doubly ionized 4(5)-nitroimidazole and 1-methyl-5-nitroimidazole. The doubly ionized state was created by core ionizing the C 1s orbitals of the samples, rapidly followed by Auger decay. Due to the recent development of nitroimidazole-based radiosensitizing drugs, core ionization was selected as it represents the very same processes taking place under the irradiation with medical X-rays. In addition to the fragmentation patterns of the sample, we study the effects of methylation on the fragmentation patterns of nitroimidazoles. We found that methylation alters the fragmentation significantly, especially the charge distribution between the final fragments. The most characteristic feature of the methylation is that it effectively quenches the production of NO and NO+ , widely regarded as key radicals in the chemistry of radiosensitization by the nitroimidazoles.

11.
Phys Chem Chem Phys ; 19(30): 19707-19721, 2017 Aug 02.
Artigo em Inglês | MEDLINE | ID: mdl-28530728

RESUMO

Coulomb explosion of diiodomethane CH2I2 molecules irradiated by ultrashort and intense X-ray pulses from SACLA, the Japanese X-ray free electron laser facility, was investigated by multi-ion coincidence measurements and self-consistent charge density-functional-based tight-binding (SCC-DFTB) simulations. The diiodomethane molecule, containing two heavy-atom X-ray absorbing sites, exhibits a rather different charge generation and nuclear motion dynamics compared to iodomethane CH3I with only a single heavy atom, as studied earlier. We focus on charge creation and distribution in CH2I2 in comparison to CH3I. The release of kinetic energy into atomic ion fragments is also studied by comparing SCC-DFTB simulations with the experiment. Compared to earlier simulations, several key enhancements are made, such as the introduction of a bond axis recoil model, where vibrational energy generated during charge creation processes induces only bond stretching or shrinking. We also propose an analytical Coulomb energy partition model to extract the essential mechanism of Coulomb explosion of molecules from the computed and the experimentally measured kinetic energies of fragment atomic ions by partitioning each pair Coulomb interaction energy into two ions of the pair under the constraint of momentum conservation. Effective internuclear distances assigned to individual fragment ions at the critical moment of the Coulomb explosion are then estimated from the average kinetic energies of the ions. We demonstrate, with good agreement between the experiment and the SCC-DFTB simulation, how the more heavily charged iodine fragments and their interplay define the characteristic features of the Coulomb explosion of CH2I2. The present study also confirms earlier findings concerning the magnitude of bond elongation in the ultrashort X-ray pulse duration, showing that structural damage to all but C-H bonds does not develop to a noticeable degree in the pulse length of ∼10 fs.

12.
Langmuir ; 32(49): 13020-13029, 2016 12 13.
Artigo em Inglês | MEDLINE | ID: mdl-27951684

RESUMO

The effect of adsorption and confinement on ibuprofen was studied by immersion loading the molecules into porous silicon (PSi) microparticles. The PSi microparticles were modified into thermally oxidized PSi (TOPSi) and thermally hydrocarbonized PSi (THCPSi) to evaluate the effects of the loading solvent and the surface chemistry on the obtainable drug payloads. The payloads, location, and the molecular state of the adsorbed drug were evaluated using thermal analysis. The results showed that after the adsorption of ∼800 mg/cm3 (wdrug/vpores) of drug into the mesopores, depending on the solvent used in the immersion, the drug began to rapidly recrystallize on the external surface of the particles. Moderate concentrations, however, enabled payloads of 800-850 mg/cm3 without excessive surface crystallization, and thus, there was no need for rinsing the samples to remove the externally crystallized portion. The results showed that the confined ibuprofen forms nanocrystals inside of the mesopores after approximately 200 mg/cm3 payloads were obtained, accounting for half of the adsorbed drug amount. The presence of both crystalline and noncrystalline phases was further characterized using variable temperature solid-state nuclear magnetic resonance (NMR) measurements. The interactions between the drug molecules and the pore walls of TOPSi and THCPSi were observed using Fourier transform infrared and 1H NMR spectroscopies, and the hydrogen bonding between the silanol groups of TOPSi and the adsorbed ibuprofen was confirmed, but having only limited effect on the overall state of the confined drug. In vitro drug permeation studies in Caco-2 and Caco-2/HT29 cocultures showed that the adsorption onto hydrophilic or hydrophobic PSi microparticles had no significant effects on the ibuprofen permeation, whether the drug was partially nanocrystalline or completely in a liquidlike state.


Assuntos
Portadores de Fármacos/química , Ibuprofeno/química , Silício , Adsorção , Células CACO-2 , Cristalização , Humanos , Nanopartículas , Porosidade
13.
Phys Chem Chem Phys ; 18(37): 26026-26032, 2016 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-27711595

RESUMO

In this paper we report an experimental and computational study of liquid acetonitrile (H3C-C[triple bond, length as m-dash]N) by resonant inelastic X-ray scattering (RIXS) at the N K-edge. The experimental spectra exhibit clear signatures of the electronic structure of the valence states at the N site and incident-beam-polarization dependence is observed as well. Moreover, we find fine structure in the quasielastic line that is assigned to finite scattering duration and nuclear relaxation. We present a simple and light-to-evaluate model for the RIXS maps and analyze the experimental data using this model combined with ab initio molecular dynamics simulations. In addition to polarization-dependence and scattering-duration effects, we pinpoint the effects of different types of chemical bonding to the RIXS spectrum and conclude that the H2C-C[double bond, length as m-dash]NH isomer, suggested in the literature, does not exist in detectable quantities. We study solution effects on the scattering spectra with simulations in liquid and in vacuum. The presented model for RIXS proved to be light enough to allow phase-space-sampling and still accurate enough for identification of transition lines in physical chemistry research by RIXS.

14.
J Chem Phys ; 145(12): 124313, 2016 Sep 28.
Artigo em Inglês | MEDLINE | ID: mdl-27782678

RESUMO

Photofragmentation of small gas-phase acetamide clusters (CH3CONH2)n (n ≤ 10) produced by a supersonic expansion source has been studied using time-of-flight ion mass spectroscopy combined with tunable vacuum-ultraviolet (VUV) synchrotron radiation. Fragmentation channels of acetamide clusters under VUV photoionization resulting in protonated and ammoniated clusters formation were identified with the discussion about the preceding intramolecular rearrangements. Acetamide-2,2,2-d3 clusters were also studied in an experiment with a gas discharge lamp as a VUV light source; comparison with the main experiment gave insights into the mechanism of formation of protonated acetamide clusters, indicating that proton transfer from amino group plays a dominant role in that process. Geometry of the acetamide dimer was discussed and the most stable arrangement was concluded to be achieved when subunits of the dimer are connected via two N-H⋯O -C hydrogen bonds. Also, the influence of the photon energy on the stability of the clusters and their fragmentation channels has been examined.

15.
Nanoscale Res Lett ; 11(1): 413, 2016 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-27644239

RESUMO

Zinc oxide (ZnO) nanorods were manufactured using the aqueous chemical growth (ACG) method, and the effect of thermal acetylene treatment on their morphology, chemical composition, and optical properties was investigated. Changes in the elemental content of the treated rods were found to be different than in previous reports, possibly due to the different defect concentrations in the samples, highlighting the importance of synthesis method selection for the process. Acetylene treatment resulted in a significant improvement of the ultraviolet photoluminescence of the rods. The greatest increase in emission intensity was recorded on ZnO rods treated at the temperature of 825 °C. The findings imply that the changes brought on by the treatment are limited to the surface of the ZnO rods.

16.
Opt Express ; 24(11): 11768-81, 2016 May 30.
Artigo em Inglês | MEDLINE | ID: mdl-27410102

RESUMO

We built a two-mirror based X-ray split and delay (XRSD) device for soft X-rays at the Linac Coherent Light Source free electron laser facility. The instrument is based on an edge-polished mirror design covering an energy range of 250 eV-1800 eV and producing a delay between the two split pulses variable up to 400 femtoseconds with a sub-100 attosecond resolution. We present experimental and simulation results regarding molecular dissociation dynamics in CH3I and CO probed by the XRSD device. We observed ion kinetic energy and branching ratio dependence on the delay times which were reliably produced by the XRSD instrument.

18.
J Phys Chem Lett ; 6(15): 2944-9, 2015 Aug 06.
Artigo em Inglês | MEDLINE | ID: mdl-26267186

RESUMO

In recent years, free-electron lasers operating in the true X-ray regime have opened up access to the femtosecond-scale dynamics induced by deep inner-shell ionization. We have investigated charge creation and transfer dynamics in the context of molecular Coulomb explosion of a single molecule, exposed to sequential deep inner-shell ionization within an ultrashort (10 fs) X-ray pulse. The target molecule was CH3I, methane sensitized to X-rays by halogenization with a heavy element, iodine. Time-of-flight ion spectroscopy and coincident ion analysis was employed to investigate, via the properties of the atomic fragments, single-molecule charge states of up to +22. Experimental findings have been compared with a parametric model of simultaneous Coulomb explosion and charge transfer in the molecule. The study demonstrates that including realistic charge dynamics is imperative when molecular Coulomb explosion experiments using short-pulse facilities are performed.

19.
J Chem Phys ; 142(19): 194303, 2015 May 21.
Artigo em Inglês | MEDLINE | ID: mdl-26001457

RESUMO

Fragmentation of RNA nucleoside uridine, induced by carbon 1s core ionization, has been studied. The measurements by combined electron and ion spectroscopy have been performed in gas phase utilizing synchrotron radiation. As uridine is a combination of d-ribose and uracil, which have been studied earlier with the same method, this study also considers the effect of chemical environment and the relevant functional groups. Furthermore, since in core ionization the initial core hole is always highly localized, charge migration prior to fragmentation has been studied here. This study also demonstrates the destructive nature of core ionization as in most cases the C 1s ionization of uridine leads to concerted explosions producing only small fragments with masses ≤43 amu. In addition to fragmentation patterns, we found out that upon evaporation the sugar part of the uridine molecule attains hexagonal form.


Assuntos
Fotólise/efeitos da radiação , Uridina/química , Uridina/efeitos da radiação , Gases/química , Gases/efeitos da radiação , Conformação Molecular/efeitos da radiação , Análise Espectral , Raios X
20.
Dent Mater J ; 32(1): 165-72, 2013.
Artigo em Inglês | MEDLINE | ID: mdl-23370886

RESUMO

The effect of silica-coating by sol-gel process on the bond strength of resin composite to zirconia was evaluated and compared against the sandblasting method. Four groups of zirconia samples were silica-coated by sol-gel process under varied reagent ratios of ethanol, water, ammonia and tetraethyl orthosilicate and for different deposition times. One control group of zirconia samples were treated with sandblasting. Within each of these five groups, one subgroup of samples was kept in dry storage while another subgroup was aged by thermocycling for 6,000 times. Besides shear bond testing, the surface topography and surface elemental composition of silica-coated zirconia samples were also examined using scanning electron microscopy and X-ray photoelectron spectroscopy. Comparison of silica coating methods revealed significant differences in bond strength among the Dry groups (p<0.001) and Thermocycled groups (p<0.001). Comparison of sol-gel deposition times also revealed significant differences in bond strength among the Dry groups (p<0.01) and Thermocycled groups (p<0.001). Highest bond strengths were obtained after 141-h deposition: Dry (7.97±3.72 MPa); Thermocycled (2.33±0.79 MPa). It was concluded that silica-coating of zirconia by sol-gel process resulted in weaker resin bonding than by sandblasting.


Assuntos
Resinas Compostas/química , Colagem Dentária/métodos , Corrosão Dentária/métodos , Materiais Dentários/química , Silanos/química , Dióxido de Silício/química , Zircônio/química , Análise de Variância , Teste de Materiais , Microscopia Eletrônica de Varredura , Espectroscopia Fotoeletrônica , Resistência ao Cisalhamento , Propriedades de Superfície
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