Akira Kuwako

Supersensitive detection of sodium in water with use of dual-pulse laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy has been used to detect sodium (Na) in water. Laser-induced breakdown was formed by dual-pulse and crossed beam Nd:YAG lasers on a water film. To improve the detection sensitivity, the fluorescence intensity dependence on timing between laser pulses, the delay time of fluorescence detection timing, the gate width of fluorescence detection period, and the laser energy were investigated. Under the optimized conditions, the detection limit of Na in water was achieved in the range of 0.1 parts per billion. The developed system is applicable for quick and supersensitive detection of Na atoms in water.

Analysis of trace metal elements in water using laser-induced fluorescence of laser-breakdown plasma

Laser induced breakdown and laser-excited atomic fluorescence spectroscopy were used to analyze trace metal atoms in water. Laser breakdown plasma was generated by a Q-switched Nd:YAG laser and an optical parametric oscillator (OPO) laser was used to excite target atoms in water. The wavelength of second laser was tuned to the absorption line of the target atom, and its time delay from the first one was optimized in order to maximize the fluorescence signal under low background light. Furthermore, the optimum transition lines were selected in terms of oscillator strength, branching ratio, and level population of target atom. In the case of Fe doped solution, the Fe concentration of less than a hundred ppb was detected. With this method, a lower detection limit of Fe was achieved than that in the case of the method using only laser-induced breakdown spectroscopy. The developed system is applicable for quick and supersensitive detection of trace metal atoms in water.

Measurement of Dipole-Moment in Atomic Transitions under Strong External Magnetic Field

Obtaining an accurate value of the electric dipole moment μ is essential in the fields of laser application technologies. A direct way of measuring the electric dipole moment μ is to observe the Rabi-oscillation which manifests itself in the coherent photo-excitation behavior of atoms. In the case of the elements which have large angular momenta, identifying the Rabi-oscillation in their excitation behavior becomes rather difficult. We proposed an accurate and straightforward method of determining the electric-dipole moment μ between multifold degenerate levels. The point is to remove the degeneracy by applying an external magnetic field with the aid of the Zeeman effect and, then, to realize a degeneration free coherent excitation. As a result, we can observe the Rabi-oscillations explicitly in the excitation vs. laser-fluence curves. The present method provides a reliable basis of experimental determination of μ. As an example, we applied the present method to a transition to 0–17,362 cm−1 level in uran...

Polarized laser spectroscopy of two-step electric-dipole excitation of atoms

A simple and accurate method of measuring the total angular momentum J of atomic energy levels is described. The method is essentially based on the J and photon-polarization dependence of the laser-induced two-step excitation in a three-level system. Four different combinations of polarizations in two lasers, which are tuned to each of the two Bohr frequencies, are utilized. They are co-circulating, counter-circulating polarizations and mutually parallel and perpendicular linear polarizations. It has been confirmed that the J-value of a level in the three-level system can be determined uniquely from relative strengths of the fluorescence originating in the top level. The determination of J is carried out by recognition of patterns in the excitation behaviour, which is peculiar to each J-value. The merit of the present method lies in the high reliability inherent in the pattern recognition procedure and also in the relative ease of the experiment.

Precise determination of electric dipole moment in atomic transitions

Most conventional methods of determining the dipole moment μ are unsatisfactory when applied to transitions between degenerate levels; this is because the overlapping of transitions among magnetic sub‐levels modifies the photo‐excitation behavior in a complex way. To overcome this difficulty, two novel methods of determination are presented. One makes use of differently polarized laser light, and yields a set of excitation curves specific to each polarization. By treating μ as a parameter and fitting the curves through calculations, one can obtain the value of μ. In the other method, a magnetic field is applied to the atom under study in order to raise its degeneracy. As a result, simple Rabi oscillations manifest themselves and one can derive μ directly from their frequencies. These two methods of determining μ are tested on absorption lines in uranium, demonstrating that they work quite well and improve the accuracy of measured dipole moments.

Modelling and Simulation of Electron Impingement Pressure.

Pressure due to impingement of high-energy electrons onto the metal surface is considered to have the influence of deforming the molten metal surface, especially in cases where an electron beam is employed as an energy driver to evaporate molten metal. Here, a modelling of such electron impingiment pressure was proposed in terms of relativistic collision theory. By Monte Carlo simulation, electron impingement pressure, which was calculated as momentum transfer of high-energy electrons, was compared with vapor pressure, another factor that mainly influences deformation of the molten metal surface. According to the calculated results, although electron impingement pressure is smaller than vapor pressure in general cases, surface concavity induced by electron impingement pressure in addition to vapor pressure might develop and penetrate into the molten metal when the electron beam is focused and current density is increased to some degree.