Asymmetric Dissociative Tunneling Ionization of Tetrafluoromethane in ω - 2ω Intense Laser Fields.

Dissociative ionization of tetrafluoromethane (CF4) in linearly polarized ω-2ω ultrashort intense laser fields (1.4 × 1014 W/cm2, 800 and 400 nm) has been investigated by three-dimensional momentum ion imaging. The spatial distribution of C F 3 + produced by CF4 → C F 3 + + F + e- exhibited a clear asymmetry with respect to the laser polarization direction. The degree of the asymmetry varies by the relative phase of the ω and 2ω laser fields, showing that 1) the breaking of the four equivalent C-F bonds can be manipulated by the laser pulse shape and 2) the C-F bond directed along the larger amplitude side of the ω-2ω electric fields tends to be broken. Weak-field asymptotic theory (WFAT) shows that the tunneling ionization from the 4t 2 second highest-occupied molecular orbital (HOMO-1) surpasses that from the 1t 1 HOMO. This predicts the enhancement of the tunneling ionization with electric fields pointing from F to C, in the direction opposite to that observed for the asymmetric fragment ejection. Possible mechanisms involved in the asymmetric dissociative ionization, such as post-ionization interactions, are discussed.

Association Reaction of Gaseous C2H4 in Femtosecond Laser Filaments Studied by Time-of-Flight Mass Spectrometry.

Association reactions by femtosecond laser filamentation in gaseous C2H4 were studied by time-of-flight mass spectrometry of neutral reaction products. Direct sampling from the reaction cell to a mass spectrometer via a differential pumping stage allowed the identification of various hydrocarbon molecules C n H m with n = 3-7 and m = 4-7, which includes species not observed in the previous studies. It was found that products containing three and four carbon atoms dominate the mass spectrum with smaller yields for higher-mass species, suggesting that carbon chain growth proceeds through the reaction with C2H4 in the reaction cell. The product distribution showed a clear dependence on the laser pulse energy for filamentation.

Characterization of soft X-ray FEL pulse duration with two-color photoelectron spectroscopy.

The pulse duration of soft X-ray free-electron laser (FEL) pulses of SACLA BL1 (0.2-0.3 nC per bunch, 0.5-0.8 MeV) were characterized by photoelectron sideband measurements. The intensity of the He 1 s-1 photoelectron sidebands generated by a near-infrared femtosecond laser was measured as a function of the time delay between the two pulses using an arrival time monitor. From the width of the cross-correlation trace thus derived, the FEL pulse duration was evaluated to be 28 ± 5 fs full width at half-maximum in the photon energy range between 40 eV and 120 eV.

Selective bond breaking of CO2 in phase-locked two-color intense laser fields: laser field intensity dependence.

Selective bond breaking of CO2 in phase-locked ω-2ω two-color intense laser fields (λ = 800 nm and 400 nm, total field intensity I ∼ 1014 W cm-2) has been investigated by coincidence momentum imaging. The CO+ and O+ fragment ions produced by two-body Coulomb explosion, CO22+ → CO+ + O+, exhibit asymmetric distributions along the laser polarization direction, showing that one of the two equivalent C-O bonds is selectively broken by the laser fields. At a field intensity higher than 2 × 1014 W cm-2, the largest fragment asymmetry is observed when the relative phase ϕ between the ω and 2ω laser fields is ∼0 and π. On the other hand, an increase of the asymmetry and a shift of the phase providing the largest asymmetry are observed at lower field intensities. The selective bond breaking and its dependence on the laser field intensity are discussed in terms of a mechanism involving deformation of the potential energy surfaces and electron recollision in intense laser fields.

Imaging Electronic Excitation of NO by Ultrafast Laser Tunneling Ionization.

Tunneling-ionization imaging of photoexcitation of NO has been demonstrated by using few-cycle near-infrared intense laser pulses (8 fs, 800 nm, 1.1×10^{14} W/cm^{2}). The ion image of N^{+} fragment ions produced by dissociative ionization of NO in the ground state, NO (X^{2}Π,2π)→NO^{+}+e^{-}→N^{+}+O+e^{-}, exhibits a characteristic momentum distribution peaked at 45° with respect to the laser polarization direction. On the other hand, a broad distribution centered at ∼0° appears when the A^{2}Σ^{+} (3sσ) excited state is prepared as the initial state by deep-UV photoexcitation. The observed angular distributions are in good agreement with the corresponding theoretical tunneling ionization yields, showing that the fragment anisotropy reflects changes of the highest-occupied molecular orbital by photoexcitation.

Dermal and ocular irritation and skin sensitization studies of fullerene C60 nanoparticles.

CONTEXT: Widespread production and use of nanomaterials have caused the release of increasing amounts of nanomaterials into the environment. The introduction of novel materials into industry requires safety evaluations as well as an understanding of the impact of the nanomaterials on human health, because the unique properties and size of nanomaterials may also result in unique health risks. Skin and eyes have the highest risk of exposure to nanomaterials, because deposition to the superficial organs has the potential to be a major route of exposure during the manufacturing, use, and disposal of nanomaterials. However, information on the dermal and eye irritation and sensitization of fullerene C(60) nanoparticles is still lacking. OBJECTIVES: This study was performed to examine the potential irritating and sensitizing effects of fullerenes on the skin and eyes. METHODS: The dermal and eye irritation study was performed using rabbits according to the Organisation for Economic Co-operation and Development (OECD) Guidelines 404 and 405, respectively. The skin sensitization study was carried out in accordance to the OECD Guideline 406 using guinea pigs. The concentrations of the fullerenes in the test substances were the maximum allowable for administration. Fullerenes were applied at 50 mg in dermal irritation, 40 mg in skin sensitization, and 100 mg in eye irritation studies. RESULTS: No dermal responses, including erythema/eschar or edema, were found in rabbits treated with fullerenes. No rabbits exhibited corneal opacity, abnormality of the iris, or chemosis eye at any time point after the application of fullerenes. Fullerenes caused conjunctival redness and blood vessel hyperemia at 1 h, but not at 24 h. No erythema or edema was observed after the challenge with fullerenes in the fullerene-treated guinea pigs. CONCLUSION: Reversible minimal potential for acute irritation of the eyes was induced by fullerenes, but neither irritation nor sensitization was caused on the skin. Although the present study provided initial information on the acute irritation and acute sensitization of highly purified C(60) fullerenes, information on the toxicological effects of fullerenes and their derivatives is still limited. Further information is needed to clarify the potential for toxicity given the complex nature of fullerenes and their derivatives.

A magnetic-bottle multi-electron-ion coincidence spectrometer.

A novel multi-electron-ion coincidence spectrometer developed on the basis of a 1.5 m-long magnetic-bottle electron spectrometer is presented. Electrons are guided by an inhomogeneous magnetic field to a detector at the end of the flight tube, while a set of optics is used to extract counterpart ions to the same detector, by a pulsed inhomogeneous electric field. This setup allows ion detection with high mass resolution, without impairing the high collection efficiency for electrons. The performance of the coincidence spectrometer was tested with double ionization of carbon disulfide, CS(2) → CS(2)(2+) + e(-) + e(-), in ultrashort intense laser fields (2.8 × 10(13) W/cm(2), 280 fs, 1030 nm) to clarify the electron correlation below the rescattering threshold.

Evaluation of dermal and eye irritation and skin sensitization due to carbon nanotubes.

The present paper summarizes the results of our studies on dermal and eye irritation and skin sensitization due to carbon nanotubes (CNTs), whose potential applications and uses are wide and varied, including CNT-enhanced plastics, electromagnetic interference/radio-frequency (EMI/RFI) shielding, antistatic material, flexible fibers and advanced polymers, medical and health applications, and scanning probe microscopy. Skin and eyes have the highest risk of exposure to nanomaterials, because deposition of nanomaterials to the surficial organs has the potential to be a major route of exposure during the manufacturing, use, and disposal of nanomaterials. Two products composed of single-walled carbon nanotubes (SWCNTs) and two products composed of multi-walled carbon nanotubes (MWCNTs) were tested regarding acute dermal and acute eye irritation using rabbits, and skin sensitization using guinea pigs. The concentrations of the CNTs in the substances were the maximum allowable for administration. The two products of SWCNTs and one of the products of MWCNTs were not irritants to the skin or eyes. The other product of MWCNTs caused very slight erythema at 24h, but not at 72h, after patch removal in the dermal irritation experiments and conjunctival redness and blood vessel hyperemia at 1h, but not at 24h, in eye irritation experiments. These findings showed that one product of MWCNTs was a very weak acute irritant to the skin and eyes. No products of SWCNTs and MWCNTs exhibited skin-sensitization effects. Our knowledge of the toxicological effects of CNTs is still limited. Further information is needed to clarify the potential for irritation and sensitization given the complex nature of CNTs.

Time-resolved EUV photoelectron spectroscopy of dissociating I2 by laser harmonics at 80 nm.

Generation of single-order laser harmonics in extreme ultraviolet (EUV) and the application to the time-resolved photoelectron spectroscopy of I(2) are demonstrated. The EUV pulses at 80 nm were generated from Kr as the 5th order harmonics of intense 400 nm laser pulses and then separated from other harmonic orders by a thin indium foil. The pump-probe photoelectron spectroscopy of I(2) in the B (3)Π(0(u)(+)) and B" (1)Π(1(u)) states excited by visible laser pulses at 490 nm showed a rapid increase in the yield of atomic iodine (~400 fs), reflecting the dissociation dynamics evolving simultaneously in the two excited states.

Visualizing hydrogen atoms migrating in acetylene dication by time-resolved three-body and four-body Coulomb explosion imaging.

The visualization of ultrafast isomerization of deuterated acetylene dication (C(2)D(2)(2+)) is demonstrated by time-resolved Coulomb explosion imaging with sub-10 fs intense laser pulses (9 fs, 0.13 PW cm(-2), 800 nm). The Coulomb explosion imaging monitoring the three-body explosion process, C(2)D(2)(3+)→ D(+) + C(+) + CD(+), as a function of the delay between the pump and probe pulses revealed that the migration of a deuterium atom proceeds in a recurrent manner; One of the deuterium atoms first shifts from one carbon site to the other in a short timescale (∼90 fs), and then migrates back to the original carbon site by 280 fs, in competition with the molecular dissociation. Correlated motion of the two deuterium atoms associated with the hydrogen migration and structural deformation to non-planar geometry are identified by the time-resolved four-body Coulomb explosion imaging, C(2)D(2)(4+)→ D(+) + C(+) + C(+) + D(+).

Multiple explosion pathways of the deuterated benzene trication in 9-fs intense laser fields.

The fragmentation of deuterated benzene (C6D6) in ultrashort intense laser fields (9 fs, 1 x 10(15) W/cm2) is studied by the ion-coincidence momentum imaging technique. Five two-body and eight three-body Coulomb explosion pathways from the trication (C6D6(3+)), associated with the deprotonation and ring-opening reactions, are identified. It is found from the fragment momentum correlation that all the observed three-body explosion processes proceed sequentially via the two-body Coulomb explosion forming molecular dications, C(m)D(n)(2+), with (m,n) = (6,5), (5,5), (5,4), (4,4), (4,3), and (3,3), which further dissociate into pairs of monocations. The branching ratio of the fragmentation pathways estimated from the number of the observed coincidence events indicates that the fragmentation is nonstatistical.

Acetylene-vinylidene isomerization in ultrashort intense laser fields studied by triple ion-coincidence momentum imaging.

The isomerization of acetylene via hydrogen migration in intense laser fields (8 x 10(14) W/cm2) has been investigated by coincidence momentum imaging of the three-body Coulomb explosion process, C2H2 (3+)-->H+ + C+ + CH+. When ultrashort (9 fs) laser pulses are used, the angle between the momenta of C+ and H+ fragments exhibits a sharp distribution peaked at a small angle ( approximately 20 degrees ), showing that the hydrogen atom remains near the original carbon site in the acetylene configuration. On the other hand, a significantly broad distribution extending to larger momentum angles ( approximately 120 degrees ) is observed when the pulse duration is increased to 35 fs, indicating that the ultrafast isomerization to vinylidene is induced in the longer laser pulse.

Dalitz plot analysis of Coulomb exploding O3 in ultrashort intense laser fields.

The three-body Coulomb explosion of O3, O3(3+)-->O++O++O+, in ultrashort intense laser fields (2x10(15) W/cm2) is studied with two different pulse durations (9 and 40 fs) by the coincidence momentum imaging method. In addition to a decrease in the total kinetic energy release, a broadening in the Dalitz plot distribution [Philos. Mag. 44, 1068 (1953)] is observed when the pulse duration is increased from 9 to 40 fs. The analysis based on a simple Coulomb explosion model shows that the geometrical structure of O3 remains almost unchanged during the interaction with the few-cycle intense laser fields, while a significant structural deformation along all the three vibrational coordinates, including the antisymmetric stretching coordinate, is identified in the 40 fs intense laser fields. The observed nuclear dynamics are discussed in terms of the population transfer to the excited states of O3.

Visualizing recurrently migrating hydrogen in acetylene dication by intense ultrashort laser pulses.

We demonstrate the visualization of ultrafast hydrogen migration in deuterated acetylene dication (C2D2{2+}) by employing the pump-probe Coulomb explosion imaging with sub-10-fs intense laser pulses (9 fs, 0.13 PW/cm{2}, 800 nm). It is shown, from the temporal evolution of the momenta of the fragment ions produced by the three-body explosion, C2D2{3+}-->D{+} + C{+} + CD{+}, that the migration proceeds in a recurrent manner: the deuterium atom first shifts from one carbon site to the other in a short time scale (approximately 90 fs) and then migrates back to the original carbon site by 280 fs, in competition with the molecular dissociation.

Nanosecond rapid freezing of liquid benzene under shock compression studied by time-resolved coherent anti-Stokes Raman spectroscopy.

Nanosecond time-resolved coherent anti-Stokes Raman spectroscopy is used to investigate the shock-induced liquid-solid phase transition and crystallization of liquid benzene. Temporal evolution of the Raman shift of the ring-breathing and C-H stretching modes is investigated. A metastable supercompressed state and a liquid-solid phase transition are observed under shock compression. Time-resolved Raman spectra reveal that the liquid state is initially a metastable state and rapidly transforms to the solid state within 25 ns under shock compression at 4.2 GPa.

Electronic and nuclear responses of fixed-in-space H2S to ultrashort intense laser fields.

The Coulomb explosion dynamics of H2S, H2S3+-->H+ +S+ + H+, in ultrashort intense laser fields (12 fs, approximately 2 x 10(14) W/cm2) is studied by the coincidence momentum imaging of the three fragment ions. Different electronic and nuclear responses are identified depending on the direction of laser polarization epsilon in the molecular frame. The dependence can be interpreted in terms of the electronic and bonding characters of charge transfer states of H2S coupled to the electronic ground state.