Yoshihiko Hatano

Interaction of VUV photons with molecules: Spectroscopy and dynamics of molecular superexcited states

Abstract A survey is given of recent progress in experimental studies of spectroscopy and dynamics of molecules in the superexcited states which are produced in the interaction of vacuum ultraviolet photons with molecules. Molecules studied here ranged from simple diatomic and triatomic molecules to polyatomic molecules such as hydrocarbons, alcohols, ethers, and some Si-containing compounds in the gas phase. Most of the observed superexcited states are assigned to high Rydberg states which are vibrationally (or/and rotationally), doubly, or inner-core excited, and converge to each of ion states. Non-Rydberg superexcited states are also observed. Some remarks are also presented of superexcited states as reaction intermediates or collision complexes in electron-ion recombination, electron attachment, and Penning ionization processes.

Spectroscopy and dynamics of molecular superexcited states. Aspects of primary processes of radiation chemistry

Abstract A survey is given of recent progress in experimental studies of spectroscopy and dynamics of molecules in the superexcited states. The Platzman’s idea of the superexcited states deduced from the theoretical analysis of the interaction of ionizing radiation with matter has been recently substantiated experimentally. Most of the observed superexcited states are assigned to high Rydberg states which are vibrationally (or/and rotationally), doubly, or inner-core excited, and converge to each of ion states. Non-Rydberg superexcited states are also observed. The dissociation dynamics of these states as well as the products are highly different from those of the states excited below ionization thresholds. The neutral dissociation is unexpectedly important in comparison with ionization. Some remarks are presented of superexcited states in the condensed phase from the viewpoint to understand the primary processes in radiation chemistry.

Acidity effects on the fluorescence properties and adsorptive behavior of rhodamine 6G molecules at the air-water interface studied with confocal fluorescence microscopy.

The effects of acidity on fluorescence originated from rhodamine 6G (R6G) molecules adsorbed at the air-water interface of extremely low-concentration aqueous solutions have been studied with confocal fluorescence microscopy. Similarities and differences in the observed acidity effects between R6G molecules at the interface and those in the bulk solution have been discussed. With increasing the subphase-pH from 1 to 6, height and frequency of photon bursts as well as intensity of the interface-originated fluorescence change in two steps, while bulk fluorescence changes in one step and a little change in the number of adsorbed R6G molecules is verified with surface tension measurements. The results suggest that there is an interface-specific equilibrium among the chemical forms of R6G molecules. Chemical forms contributing to the interface-originated fluorescence above pH 5 are discussed.

Pulse-height distribution of output signals in positive ion detection by a microchannel plate

Abstract Positive ions produced by a 120xa0eV electron impact on H 2 , He, CH 4 , N 2 , CO 2 , and Kr were detected with a microchannel plate (MCP), and the pulse-height distributions (PHDs) of the output signals were observed. As a result, the half width at half maximum (HWHM) of the region above the peak of the PHD was almost independent of the kind of ion. This finding demonstrates that the HWHM is due only to the amplification of the secondary electrons that emerged as a result of the collision of the ions with the surface of the MCP. The dependence of the peak position and the HWHM against the potential difference applied between the front and back of the MCP was also examined. It was found that the plots of the HWHM as a function of the potential difference applied to the MCP could be fitted by a linear line very well. A formula that can reproduce the observed PHDs is presented as a function of the potential applied to the MCP and the mass number. A relation of the mean peak height in the PHD to the potential applied to the MCP and the mass number is also presented.

Forbidden doubly excited states of molecular nitrogen dissociating into two neutral atoms in electron collisions

The ordinary and coincident electron-energy-loss spectra of N2 have been measured in an expected energy range for the formation of doubly excited states to investigate their formation and decay processes in electron-N2 collisions. The present results are compared with those obtained by photoexcitation experiments. Existence of the optically forbidden doubly excited states and their dissociative character have been revealed clearly for the first time.

Second harmonic generation from insoluble films of a rhodamine dye at the air-water interface: Effect of sodium dodecylsulfate

The second harmonic generation (SHG) from the insoluble monolayers of bis-(N-ethyl, N-octadecyl)rhodamine perchlorate (RhC18) formed on the surface of sodium dodecylsulfate (SDS) solutions of different concentrations has been studied. An enhancement of the second harmonic response was observed in the mixed films of RhC18/SDS compared to the pure-dye layer. To clarify the origin of the phenomenon, the films were characterized by surface pressure-area isotherm and reflection-absorption spectroscopy studies. The analysis of surface pressure-area isotherms of RhC18/SDS layers showed that incorporated SDS molecules essentially influence the rheological properties of the dye monolayer. The film parameters, such as the molecular surface area, maximum surface pressure, and solid-condensed phase composition, are the functions of SDS bulk concentration. A joint analysis of the SHG results and the reflection-absorption spectra revealed that the structural ordering within films was responsible for the enhancement of the nonlinear optical response, whereas the contributions from the spectral shifts and increased absorption upon aggregate formation are of less importance.

Quantitative Analysis of CF4 Produced in the SiO2 Etching Process Using c-C4F8, C3F8, and C2F6 Plasmas by In Situ Mass Spectrometry

The use of CF32+ as a specific product ion to selectively quantify CF4 produced in the SiO2 etching process using plasmas of perfluorocompounds (PFCs), such as c-C4F8, C3F8, and C2F6, has been proposed and investigated in the present experiments by measuring mass spectra inside and outside the plasmas. It is known that the CF32+ ion does not appear in the mass spectra of any stable PFCs, except for CF4. It is confirmed in the present experiments that the quantity of CF32+ originating from the CF3 radical in the mass spectra measured in situ is negligible. Other unstable chemical species in the plasmas are too small in quantity to explain the intensity of CF32+ appearing in the mass spectra measured in situ, even if they could produce stable CF32+ by ionization. It is therefore concluded that CF32+ can be used as a fingerprint of CF4 in mass spectrometry. Application of this new method for the quantitative analysis of CF4 produced in the SiO2 etching process using PFC plasmas results in CF4 production advancing significantly not only in the etching region of SiO2 but also in the downstream region of the plasmas.

Fragment ion-photon coincidence investigation of trifluoromethane by controlled electron impact

Fragment ion-photon coincidence (FIPCO) spectra by 120xa0eV electron impact on trifluoromethane (CHF3) have been observed, in which optical emission in the 200-600xa0nm region has been detected. The CF3+, CHF2+, CF2+ and CF+ ions were observed in the FIPCO spectra. The kinetic energy distributions of these ions were estimated on the basis of the Monte Carlo simulation of their observed band shapes. Considering the total kinetic energy release in the dissociation processes producing CF3+ and CHF2+, we have concluded that the production of CF3+ and CHF2+ follows the D-B and D-C emissions of the parent ion, respectively. The CF2+ and CF+ ions were correlated with the Balmer emission from the excited hydrogen atom. These dissociation processes producing CF2+ and CF+ take place after the excitation to singly charged superexcited statesxa0ofxa0CHF3.

Phase transitions in Langmuir monolayers of a rhodamine dye as studied by a second harmonic generation technique

Abstract Phase transitions in the Langmuir monolayer of a xanthene dye alkylated with two long hydrophobic chains have been studied by a second harmonic generation (SHG) technique at various temperatures. Owing to the sensitivity of the SHG technique to the molecular ordering at an interface, transitions between phases of different orientational organizations of dye chromophores have been observed in the shape of SHG signals. The method of the derivative of an SHG signal has been proposed to determine the points of phase transitions in the Langmuir monolayer. Obtained results are in good agreement with those derived from ( π – A ) isotherms. Several additional orientational phase transitions have been found upon compression of a liquid phase of the layer. Detailed comparison of the SHG signals with the ( π – A ) isotherms has allowed us to conclude that the phase transitions at a low surface pressure may not involve the rearrangement of dye chromophores. Formation of a denser phase likely occurs by means of the orientational rearrangement of flexible alkyl chain network of the layer. However, a transition to the solid phase is accompanied by profound changes in the chromophore arrangement. The amplitude of SHG signals generated by the solid phase increases with a decrease in the temperature, suggesting the formation of more ordered layers at lower temperatures.