Hiroshi Akagi

Theoretical Study on the Structure and the Frequency of Isomers of the Naphthalene Dimer

The structures of the naphthalene monomer and dimer were investigated with performing vibrational analysis. The MP2 optimization showed that the naphthalene monomer has a nonplanar geometry in the 6-31G, 6-31G*, 6-31+G*, and 6-311G basis sets, while it has a planar geometry in the 6-31G*(0.25) and Dunnings correlation consistent basis sets. The MP2/cc-pVDZ calculation showed the presence of the four stable isomers, which were part of the isomers in the previous MP2/6-31G* calculation (Walsh, T. R. Chem. Phys. Lett. 2002, 363, 45). The presence of extra structures in the MP2/6-31G* calculation is attributed to a poor description of the potential energy surface, which is evident from the nonplanar structure of the monomer in the MP2/6-31G* calculation. The relative stability among the isomers in the MP2/cc-pVDZ calculation without counterpoise correction was maintained in both the single-point calculation at the MP2/aug-cc-pVDZ//MP2/cc-pVDZ level and the counterpoise-corrected optimization at the MP2/cc-pVDZ level. The relative stability among the isomers suggested an enhancement of the π-π interaction in the structure with lower symmetry, which could be explained using a molecular-orbital model. The vibrational analysis in MP2/cc-pVDZ without the counterpoise correction suggested that the isomers of the naphthalene dimer were distinguishable by the observation of the infrared spectrum in the low-frequency region (150-600 cm(-)(1)).

Femtosecond laser ablation dynamics of fused silica extracted from oscillation of time-resolved reflectivity

Femtosecond laser ablation dynamics of fused silica is examined via time-resolved reflectivity measurements. After optical breakdown was caused by irradiation of a pump pulse with fluence F{sub pump} = 3.3–14.9 J/cm{sup 2}, the reflectivity oscillated with a period of 63 ± 2 ps for a wavelength λ = 795 nm. The period was reduced by half for λ = 398 nm. We ascribe the oscillation to the interference between the probe pulses reflected from the front and rear surfaces of the photo-excited molten fused silica layer. The time-resolved reflectivity agrees closely with a model comprising a photo-excited layer which expands due to the formation of voids, and then separates into two parts, one of which is left on the sample surface and the other separated as a molten thin layer from the surface by the spallation mechanism. Such oscillations were not observed in the reflectivity of soda-lime glass. Whether the reflectivity oscillates or not probably depends on the layer viscosity while in a molten state. Since viscosity of the molten fused silica is several orders of magnitude higher than that of the soda-lime glass at the same temperature, fused silica forms a molten thin layer that reflects the probe pulse, whereas the soda-lime glass is fragmented into clusters.

Frequency-resolved optical gating for characterization of VUV pulses using ultrafast plasma mirror switching.

We propose and experimentally demonstrate a method for characterizing vacuum ultraviolet (VUV) pulses based on time-resolved reflection spectroscopy of fused silica pumped by an intense laser pulse. Plasma mirror reflection is used as an ultrafast optical switch, which enables us to measure frequency-resolved optical gating (FROG) traces. The VUV temporal waveform can be retrieved from the measured FROG trace using principal component generalized projections algorithm with modification. The temporal profile of the plasma mirror reflectivity is also extracted simultaneously.

Separation of ionization and subsequent electronic excitation for formation of electronically excited ethanol cation in intense laser fields

Ionization and subsequent electronic excitation occurring within the same laser pulse (400 nm, 96 fs, 1.3 ~ 18 TW cm?2) are separately investigated by measuring in coincidence an electron and a product ion produced from C2H5OH. It is revealed that the nascent population in the electronically excited C2H5OH+ prepared by the ionization decreases as the laser intensity increases, while the subsequent electronic excitation is enhanced through the resonant electronic transitions. Ionization and electronic excitation mechanisms are described based on the electronic-state distributions of C2H5OH+.

Practical Method for Producing a Large Amount of Isotopically Enriched Silicon by Infrared Multi-Photon Dissociation of Hexafluorodisilane

This paper presents a practical method for producing highly enriched silicon isotopes utilizing laser irradiation. One- or two-frequency CO2 laser irradiation has been employed to separate the desired isotope of silicon by means of infrared multi-photon dissociation (IRMPD) of hexafluorodisilane (Si2F6) molecules using a flow reaction system. The production of Si2F6 with a 28Si fraction of 99.1% at a rate of 0.67 g (28Si)/h was successfully accomplished with a yield of about 63% by two-frequency laser irradiation. Enriched SiF4 gas with 30Si exceeding 31% was also continuously obtained at a production rate of 0.12 g/h by one-frequency laser irradiation.

Non-thermal effects on femtosecond laser ablation of polymers extracted from the oscillation of time-resolved reflectivity

The dynamics of femtosecond laser ablation of transparent polymers were examined using time-resolved reflectivity. When these polymers were irradiated by a pump pulse with fluence above the ablation threshold of 0.8–2.0 J/cm2, we observed the oscillation of the reflectivity caused by the interference between the reflected probe pulses from the sample surface and the thin layer due to the non-thermal photomechanical effects of spallation. As the fluence of the pump pulse increased, the separation velocity of the thin layer increased from 6 km/s to the asymptotic value of 11 km/s. It is suggested that the velocities are determined by shock-wave velocities of the photo-excited layer.

Isotopically Selective Infrared Multiphoton Dissociation of 2,3-Dihydropyran

Oxygen isotopic selectivity on infrared multiphoton dissociation of 2,3-dihydropyran has been studied by the examination of the effects of excitation frequency, laser fluence, and gas pressure on the dissociation probability of 2,3-dihydropyran and isotopic composition of products. Oxygen-18 was enriched in a dissociation product: 2-propenal. The enrichment factor of 18O and the dissociation probability were measured at a laser frequency between 1033.5 and 1057.3 cm-1, the laser fluence of 2.2-2.3 J/cm2, and the 2,3-dihydropyran pressure of 0.27 kPa. The dissociation probability decreases as the laser frequency being detuned from the absorption peak of 2,3-dihydropyran around 1081 cm-1. On the other hand, the enrichment factor increases with detuning the frequency. The enrichment factor of 18O increases with increasing the 2,3-dihydropyran pressure at the laser fluence of 2.7 J/cm2 or less and the laser frequency of 1033.5 cm-1, whereas the yield of 2-propenal decreases with increasing the pressure. A very high enrichment factor of 751 was obtained by the irradiation of 0.53 kPa of 2,3-dihydropyran at 2.1 J/cm2. Collisional effect of vibrationally excited molecules with ambient molecules on isotopic selectivity is discussed on the basis of a rate equation model including a collisional vibrational de-excitation process.

Time-resolved VUV reflection spectroscopy for spatio-temporal characterization of ultrafast plasma formation

Recently, high-harmonics of femtosecond laser pulses are actively utilized in time-resolved photoelectron spectroscopy and transient absorption spectroscopy for revealing ultrafast electronic dynamics of atoms and molecules. Short-wavelength pulses generated as high-harmonics are also sensitive probes into laser plasma with high density of excited electrons on the surface of solid materials, because the plasma resonance frequency is proportional to the square root of the density of excited electrons. We demonstrated that time-resolved vacuum ultraviolet (VUV) reflection spectroscopy is applicable to extraction of time-dependent reflectivity change in ultrafast plasma formation as well as to characterization of VUV pulses [1]. In this study, we turn our attention to the spatial mode of plasma mirror reflection in addition to the temporal characteristics.

Frequency-resolved optical gating with plasma mirror for VUV pulse measurement

We demonstrate a new type of frequency-resolved optical gating based on time-resolved reflection measurement with a plasma mirror. A VUV waveform is retrieved with a modified principal component generalized projections algorithm.