Numerous equally-spaced emission lines can be generated by stimulated Raman scattering and subsequent four-wave Raman mixing. The spectral domain extends from the near-infrared to the deep-ultraviolet, and then an ultimately-short optical pulse can be generated by phase locking in the process of four-wave Raman mixing. In fact, an intense 17-fs optical pulse has been generated using this technique. A comb of 1.6-fs optical pulses, i.e., the shortest optical pulses, has also been generated by other researchers. On the other hand, it is possible to generate highly-repetitive pulses using a continuous wave (CW) laser as a pump source and molecular hydrogen as a Raman medium in a high-finesse cavity. Coherent superposition of the two-color beam comprising of the fundamental beam and the Stokes beam provides a sinusoidal wave modulated at 17 THz. It is also possible to generate more than three emission lines simultaneously, which should generate an impulsive wave. It is noted that such Raman emission can be generated in a hollow-core fiber, making the device extremely simple. Such a device may have a potential for use in basic science and technology, e.g., the generation of a three-primary-color laser for display.
KEYWORDS: Ionization, Spectroscopy, Picosecond phenomena, Ions, Molecules, Chemical analysis, Pulsed laser operation, Dye lasers, Mass spectrometry, Chemical species
Dioxins, which are already known to exist in the environment, are strongly carcinogenic, and, therefore, it is requested to develop a new analytical instrument for their trace analysis. There are many isomers in dioxins and their toxicities are very different, and, as a result, a selective analytical means is essential for analysis. At the same time, the concentrations of dioxins are generally very low, and it is needed to develop a sensitive analytical means as well. Gas chromatography combined with mass spectrometry is currently used for analysis of dioxins. It is, however, difficult to apply this method to on-line real-time monitoring of dioxins emitting from an incinerator. On the other hand, supersonic jet spectrometry combined with multiphoton ionization/mass spectrometry (SSJ/MPI/MS) is suitable for this purpose, because of its high selectivity given by optically-selective ionization, allowing on-line real-time monitoring of dioxins and their precursors. In this study, we developed SSJ/MPI/MS and used in the detection of chemical species formed in the chemical reaction of dioxin precursors in the presence and absence of a catalyst. It was found that chlorination and dimerization reactions occur efficiently in the presence of FeCl3. Thus, this approach based on SSJ/MPI/MS provides a useful analytical means for continuous monitoring of dioxins and their precursors. We also developed several analytical techniques for the improvement of sensitivity, which include designs of a new type of picosecond dye laser and of ion focusing optics, and a digital counting technique.
Dioxins and their precursors such as halogenated benzenes and phenols are suspected to be carcinogenic, and therefore they are required to be measured at ultratrace levels. Supersonic jet spectrometry combined with multiphoton ionization mass spectrometry is developed in this study for solving this problem. In order to improve the ionization yield, either 150-fs or 500-fs laser pulse is used as an excitation/ionization source, and the results are compared with those obtained using a 15-ns laser pulse. The ionization yield is substantially improved by decreasing the laser pulsewidth, which is explained by the short lifetime of the excited state for halogenated benzenes, which is arising from spin-orbit coupling and fast relaxation to the triplet state. In order to improve the sensitivity, i.e., to increase the number of ions induced by laser irradiation, a collinear ionization scheme is studied, in which the axis of jet expansion is designed to be identical to the direction of the ion drifting in the time-of-flight tube. The enhancement, by a factor of 50, in the ionization signal is achieved, which coincides with an expected value of 36 from a geometry. Supersonic jet spectrometry is also used for the detection of halogenated aromatic hydrocarbons formed by thermal decomposition of polyvinyl chloride and chlorinated polyvinyl chloride. Interestingly, chlorinated aromatic hydrocarbons are not detected from polyvinyl chloride but detected only from chlorinated polyvinyl chloride. Isomer specific determination of dichlorobenzene is also demonstrated in this study.
Different ways to produce high-power short laser pulses have been investigated using low-Q short cavity dye lasers and a nanosecond laser pumping. A powerful 500-fs widely tunable ultrashort pulsed laser device is developed using a single standard nanosecond Nd:YAG laser as a pumping source. Two pulse-shortening techniques are combined within this compact and cheap dye laser device. The first technique, called the spectro-temporal selection, is extended to generation of < 100 ps pulses at wavelengths adjustable in the 320-700 nm spectral range. The second technique makes use of an extra- cavity pulse treatment of low-Q dye laser microcavity output pulses by nonlinear resonant processes in dye media to generate single 500-fs pulses. High-power continuously tunable subpicosecond pulses in the 430-850 nm spectral range are produced by supercontinuum generation, spectral selection and amplification in dye amplifiers. Using the pumping pulses from the nanosecond Nd:YAG laser, such a system has achieved 5-GW peal power levels between 50 nm and 670 nm.
Indirect fluorescence detection of electrically neutral compounds separated by micellar electrokinetic chromatography is performed using a semiconductor laser as an exciting light source. Oxazine 750 is used as a visualizing agent of which absorption maximum is near 680 nm. A surfactant, tetradecyltrimethylammonium chloride, is used to form micelles and to prevent adsorption of oxazine 750 with a positive charge on the capillary wall negatively charged. This surfactant coats on the capillary wall so that oxazine 750 is repulsed electrically on the capillary wall. In this technique, some aromatic compounds with relatively polar functional groups, such as aniline and nitrobenzene, could be separated and detected, while nonpolar compounds such as benzene and toluene can not be detected. The range of the detection limit is from 4.2 X 10-4 to 1.6 X 10-3 M (S/N equals 3) for the aromatic compounds. The detection mechanism is based on enhancement of the fluorescence intensity in the micellar solution and on exclusion of the fluorophore attached at the hydrophilic moiety of the micelle by a hydrophilic sample.
In order to simplify a laser fluorometric detector for capillary electrophoresis a semiconductor laser emitting at 660 nm is used as an exciting source. A labeling reagent fluorescent in the deep-red region is newly synthesized and used for determination of amino acids. The separation resolution is several hundred thousand and the detection limit is subattomole levels. Protein is also labeled by the dye developed and is measured at subfemtomole levels. Contrarily, nonfluorescent samples are detected by indirect fluorometry. Amino acids, flavin adenine dinucleotide (FAD), and deoxyadenosine monophosphate (dAMP) are determined, the detection limit being subpicomole levels.
There are a lot of optical sensors for the selective determination of ion species. Some organic compounds also have been determined by optical sensors using the enzymatic and immunological reactions. On the other hand, calixarenes are well known as novel host molecules, and specific guest ions or molecules can be incorporated inside the cavity of calixarenes. This specific recognition function of calixarene has been applied to the development of electrochemical and optical ion sensors. However, an optical sensing of organic molecules using this host-guest system is a new approach at present. In this study, a sensing membrane containing a fluorescent probe and a calixarene derivative is prepared, and it is attached on a distal end of an optical fiber. An organic compound, which specially interacts with the calixarene derivative, is optically determined. The response mechanism of the sensor is discussed.
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