Femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) is used as a probe for monitoring the vibrational dynamics of room temperature ionic liquids (ILs). The experiments are performed on a series of 1,3-dialkylimidazolium ILs containing the bis(trifluoromethylsulfonyl)imide [NTf2] anion. The effect of methylation of the cationic C2 position on the dephasing time is studied analyzing [NTf2]-ILs of 1-ethyl-3-methylimidazolium [EMIM], 1-ethyl-2,3-dimethylimidazolium [EMMIM], 1-butyl-3-methylimidazolium [BMIM], and 1-butyl-2,3-dimethylimidazolium [BMMIM]. Raman coherences are excited around ∼1400 cm(-1), and the vibrational dephasing of the modes in the fingerprint region is monitored as a function of time. The results indicate that vibrational energy transfer occurs governed by the interionic interactions. This is suggested by mode beating involving vibrations beyond the excitation spectrum as well as systematic differences in the temporal dephasing behavior. In contrast, the length of the cationic alkyl side chain has a negligible impact on the vibrational dynamics.

Polycyclic aromatic hydrocarbons (PAHs) play an important role in interstellar chemistry and are subject to high energy photons that can induce excitation, ionization, and fragmentation. Previous studies have demonstrated electronic relaxation of parent PAH monocations over 10–100 femtoseconds as a result of beyond-Born-Oppenheimer coupling between the electronic and nuclear dynamics. Here, we investigate three PAH molecules: fluorene, phenanthrene, and pyrene, using ultrafast XUV and IR laser pulses. Simultaneous measurements of the ion yields, ion momenta, and electron momenta as a function of laser pulse delay allow a detailed insight into the various molecular processes. We report relaxation times for the electronically excited PAH*, PAH+* and PAH2+* states, and show the time-dependent conversion between fragmentation pathways. Additionally, using recoil-frame covariance analysis between ion images, we demonstrate that the dissociation of the PAH2+ ions favors reaction pathways involving two-body breakup and/or loss of neutral fragments totaling an even number of carbon atoms.

The ultrafast vibrational dynamics of the room-temperature ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate, [EMIm][EtSO 4 ] are studied using time-resolved femtosecond coherent anti-Stokes Raman scattering spectroscopy. Raman coherences are excited around 1040 and 1400 cm À1 , the vibrational dephasing of the modes in the fingerprint region is monitored as a function of time.The results indicate that beating pairs of Raman modes typically correspond to either the cation or the anion. The assignment of mixed mode pairs is not unambiguous and, moreover, the beating patterns of such possible mixed pairs are very weak. Interestingly, no evidence for interionic vibrational energy transfer is found despite the fact that the interactions in the ion pair are characterized by strong hydrogen bonds.

We report on a direct time-domain measurement of the temporal properties of a seeded free-electron laser pulse in the extreme ultraviolet spectral range. Utilizing the oscillating electromagnetic field of terahertz radiation, a single-shot THz streak-camera was applied for measuring the duration as well as spectral phase of the generated intense XUV pulses. The experiment was conducted at FLASH, the free electron laser user facility at DESY in Hamburg, Germany. In contrast to indirect methods, this approach directly resolves and visualizes the frequency chirp of a seeded free-electron laser (FEL) pulse. The reported diagnostic capability is a prerequisite to tailor amplitude, phase and frequency distributions of FEL beams on demand. In particular, it opens up a new window of opportunities for advanced coherent spectroscopic studies making use of the high degree of temporal coherence expected from a seeded FEL pulse.

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Purpose -The purpose of this study is to investigate the influences of the supports on tensile strength (TS) of stereolithography (SL) parts. Design/methodology/approach -The shape of specimens (tensile specimens) is according to the American Society Testing and Materials (ASTM) D638 standard. Some parts have support trace on one side and some have support trace on both sides. To achieve this target, some parts are fabricated on other parts; therefore, support of the upper part is fabricated on upper face of lower part. Findings -Influences of supports traces on mechanical properties aren't "zero". Supports affect TS of SL parts by affecting surface roughness of parts. After experimentation and analyzing experimental results, it is concluded that the TS of the parts which have support on both sides is slightly lower than the parts which have support just on one side. Originality/value -This is the first attempt toward investigation of supports effects on the mechanical properties in SL parts.

Purpose Stereolithography (SLA) is a broadly used technology in the field of rapid prototyping. One of the disadvantages of SLA is poor mechanical properties of its products. To approach the mechanical properties of original part, the mechanical properties of SLA part, such as tensile strength, should be optimized. In this process, there are many parameters that affect the tensile strength of parts. However, the “layer thickness”, “fabrication orientation” and “post curing time” are the most significant ones. Hence, the purpose of this study is to investigate the influence of these parameters on tensile strength of SLA parts. Design/methodology/approach According to the obtained results from experiments based on the “full factorial” method, an empirical equation was developed for the tensile strength in terms of the effective parameters by using regression analysis. Considering this empirical equation, the process parameters were optimized to maximize the tensile strength by using genetic algorithm. Finally, the tensile tests of the specimens were simulated via the general-purpose finite element package of ABAQUS. Findings The outputs of the numerical simulations were in good agreement with experimental results. Both experimental and numerical results show that the increase of layer thickness and the decrease in post curing time increase the tensile strength. Furthermore, the tensile strength of parts produced in vertical orientation is higher than that of parts produced in horizontal orientation. Originality/value This is a complete study about the tensile strength of the SLA parts from experimental and analytical viewpoints.

We investigated the dissociation of dications and trications of three polycyclic aromatic hydrocarbons (PAHs), fluorene, phenanthrene, and pyrene. PAHs are a family of molecules ubiquitous in space and involved in...