Hideki Ohmura

Coherent phase control of the product branching ratio in the photodissociation of dimethylsulfide

Coherent phase control of the photodissociation reaction of the dimethylsulfide has been achieved by means of quantum-mechanical interference between one- and three-photon transitions. Dimethylsulfide was irradiated by fundamental and frequency-tripled outputs of a visible laser (600.5-602.5 nm), simultaneously to yield CH3S+ and CH3SCH2+ fragment ions. The branching ratio of the two product channels could be modulated with variation of the phase difference between the light fields. This accounted for the difference between the molecular phases of the two product channels. The phase lag was observed to have a maximum value of 8 degrees at 601.5 nm. This is the first result of a selective bond breaking in a polyatomic molecule by the coherent phase control.

Control of photodissociation: vibrational mode selection and quantum interference

We present two methods for using lasers to control the photodissociation of small molecules and molecular complexes. The first method, which involves ternary aniline cluster cations consisting of aniline, water, and an aromatic molecule (benzene or pyrrole), utilizes the strong interaction between an NH stretching vibration and an adjacent hydrogen bond The infrared predissociation reactions of the cluster cations were investigated. Upon absorption of an infrared photon, the clusters dissociated and eliminated either water or the aromatic molecule. Measurements of the branching ratio for this reaction revealed that fission of the hydrogen bond was accelerated by excitation of the NH stretching vibration adjacent to the targeted hydrogen bond. The second method involves coherent control in the frequency domain. We investigated interference effects on the two-color dissociation of IBr using the fundamental (560nm) and its second harmonic (280nm). In the strong-excitation regime by the fundamental light (∼1.3 x 10 10 W cm -2 ), the yield of iodine atoms from IBr oscillated with the relative phase of the 560 and 280 nm light, whereas no change was observed in its angular distribution. This phase-dependent behavior can be explained by interference between the one-photon transition induced by the second-harmonic light and the dipole-forbidden two-photon transition induced by the strong fundamental light.

The interference effects induced by two-color excitation in the photodissociation of IBr

Abstract We have investigated the interference effects in the two-color dissociation of IBr using fundamental (560 nm) and its second-harmonic (280 nm) light. We have performed one-dimensional photofragment translational spectroscopy in the presence of two-color excitation. In the strong-excitation regime for the fundamental light (∼ 1.3×10 10 W cm −2 ), the yield of the spin–orbit excited iodine atoms dissociated from IBr molecules shows an oscillating behavior that is dependent on the relative phase between the fundamental and the second-harmonic light. This phase-dependent behavior can be explained by the interference between a one-photon transition induced by the second-harmonic light and a dipole-forbidden two-photon transition induced by the strong fundamental light.

Detection of trace substances adhered to a metal surface by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) was applied to inspect trace substances on aluminum alloy surfaces. Silicone oil, which is often used as a release agent, was employed as the adhered substance. Nanosecond laser pulses at UV wavelengths (KrF laser, λ = 248 nm) were employed for the LIBS measurements. Although the absorption of silicone oil at 248 nm was negligible, the Si emission of surface-adhered silicone oil was detected. The intensity ratio of the Si emission at 288 nm to the Al emission at 309 nm increased with increasing surface concentrations in the range 1–35 μg cm−2, and a linear dependence on the silicone oil surface concentration was observed at low surface concentrations (<5.0 μg cm−2). The limit of detection was evaluated to be 1.18 μg cm−2.

Molecular tunnelling ionization of allyl halides induced by phase-controlled two-colour laser fields

We have investigated the molecular tunnelling ionization (TI) of four allyl halide molecules (C3H5X; X = F, Cl, Br, I) induced by phase-controlled two-colour laser fields consisting of a fundamental light and a second-harmonic light with a pulse duration of 130 fs and an intensity of 1012?1013?W cm?2. The geometric structure of the highest occupied molecular orbital (HOMO) of the four allyl halides can be systematically changed by replacing the halogen atom. We observed phase-dependent orientation-selective molecular ionization in the four methyl halide molecules. The mechanism of molecular TI is discussed in connection with the geometric structure of the HOMO.

Quantum control of molecular tunneling ionization in the spatiotemporal domain

We report on a method that can control molecular photoionization in both space and time domains. The directionally asymmetric molecular tunneling ionization induced by intense (5.0 x 10{sup 13} W/cm{sup 2}) phase-controlled two-color laser pulses consisting of fundamental and second-harmonic light achieves the selective ionization of asymmetric molecules in the space domain, and manipulates the birth time and direction of photoelectron emission on an attosecond time scale. This method provides a powerful tool for tracking the quantum dynamics of photoelectrons by using phase-dependent oriented molecules as a phase reference in simultaneous ion-electron detection.

Detection of contaminants on pre-bond surface by LIBS

ABSTRACT To inspect surface contamination of pre-bond surface for adhesive bonding, laser-induced breakdown spectroscopy (LIBS) was examined. LIBS could detect silicone oil adhered at the surface of aluminium alloy plates as low as 11.8 mg/m2 in surface concentration. It was revealed that similar contamination caused weakened adhesive bonding. It proves that the LIBS can be applied for pre-bond surface inspection. Si signals can also be detected at failure parts of samples after tensile strength tests of adhesive bonding.

Directionally Asymmetric Tunneling Ionization and Control of Molecular Orientation by Phase-Controlled Laser Fields

Intense \((1{0}^{12} - 1{0}^{13}{\mathrm{W/cm}}^{2})\) phase-controlled laser fields consisting of a fundamental light and a second-harmonic light induce directionally asymmetric tunneling ionization and the resultant selective ionization of oriented molecules. It is demonstrated that selective ionization of oriented molecules induced by phase-controlled ω + 2ω laser fields reflects the geometric structure of the highest occupied molecular orbital. This method is robust, being free of both laser wavelength and pulse-duration constraints, and thus can be applied to a wide range of molecules.

Molecular orientation of CH3F induced by phase-controlled lights

We investigate molecular orientation of CH3F induced by phase-controlled two-color pulses consisting of a fundamental pulse (ω) and its second harmonic pulse (2ω) with an intensity of 1.0×1012 W/cm2 and a pulse-duration of 130 fs.