Ken-ichi Saitow

Study of inhomogeneity of supercritical water by small-angle x-ray scattering

Small-angle x-ray scattering (SAXS) experiments using synchrotron radiation were carried out for supercritical water along isotherms at the temperatures T=660.0, 661.5, 663.0, 677.0, and 687.5 K, from a gaslike density region to a liquidlike one, including an intermediate density region. The high-temperature and high-pressure sample holder for SAXS measurements suitable for supercritical water was redesigned for more precise measurements. The curves illustrating the density dependence of density fluctuations and correlation lengths show a slight shift of the maximum from critical isochore. The deviations become larger with increasing temperature. The results for the density fluctuations and correlation lengths for supercritical water are compared with those for supercritical CO2 and CF3H at T/Tc=1.02 and 1.06. The comparison allows us to draw the conclusion that the behavior in the long-range inhomogeneity of water in the supercritical state is in discord with the ordinary behaviors for other molecular su...

Raman spectral changes of neat CO2 across the ridge of density fluctuation in supercritical region

Density dependence of Raman spectra for carbon dioxide was studied along the isotherm in the temperature region 0.96⩽T/Tc⩽1.06 and the density region 0<ρ/ρc<2. Spectrum in the small-wavenumber region below 100 cm−1 and that in the region of the 2ν2 intramolecular band, i.e. one of the Fermi dyad, were carefully measured and analyzed. The results are interpreted by assuming the presence of the gas- and liquid-like regions in the supercritical region separated by the ridge of density fluctuation.

PHOTO-INDUCED REACTIONS OF CH2I2 IN SOLUTION STUDIED BY THE ULTRAFAST TRANSIENT ABSORPTION SPECTROSCOPY

Abstract We have studied the photo-induced reactions of CH 2 I 2 upon excitation at 268 nm in CH 3 CN by femtosecond transient absorption spectroscopy. The time profile of transients observed at 400 nm consists of three components: fast rise (200 fs), fast decay (500 fs), and slow rise (7–13 ps). The first two components are independent of the solute concentration and are assigned to the photofragment CH 2 I radical. The slow rise becomes faster by increasing concentration and is assigned to the formation of a charge-transfer complex between the photofragment I and CH 2 I 2 in solute aggregate. This is followed by full electron transfer from CH 2 I 2 to I forming CH 2 I 2 + .

White-blue electroluminescence from a Si quantum dot hybrid light-emitting diode

A silicon (Si) quantum dot (QD)-based hybrid inorganic/organic light-emitting diode (LED) was fabricated via solution processing. This device exhibited white-blue electroluminescence at a low applied voltage of 6 V, with 78% of the effective emission obtained from the Si QDs. This hybrid LED produced current and optical power densities 280 and 350 times greater than those previously reported for such device. The superior performance of this hybrid device was obtained by both the prepared Si QDs and the optimized layer structure and thereby improving carrier migration through the hybrid LED and carrier recombination in the homogeneous Si QD layer.

Terahertz absorption spectra of supercritical CHF3 to investigate local structure through rotational and hindered rotational motions

Abstract Far infrared absorption spectra of neat supercritical fluoroform (CHF3) are measured with terahertz (THz) radiation. The spectra covering from 10 to 100 cm −1 are obtained at reduced temperature T/Tc=1.02 on densities varied by a factor of 200. As density increases, dominant component of spectra changes from rotational to hindered-rotational motions. However, the change is nonlinear to the density. Such specificity arises from difference between bulk and local densities, and the most enhanced local density is observed near the thermodynamic state of maximum density fluctuation. In the pure fluid system, the relationship between density fluctuation and local density enhancement is experimentally presented.

Local density enhancement in neat supercritical fluid due to attractive intermolecular interactions

Abstract Short-range structure in the neat supercritical fluid consisting of polar molecules is investigated by measuring Raman spectrum of fluoroform (CHF3) at reduced temperature Tr=T/Tc=1.02. The spectra are analyzed by the Schweizer–Chandler model, and the spectral shift is decomposed into attractive and repulsive components as a function of density varied by a factor of 50. The attractive component shows larger shift than the one anticipated from the uniform molecular distribution. A local density enhancement due to the attractive intermolecular interactions is definitely observed in the neat supercritical fluid and evaluated against bulk density.

Formation of benzene dimer cations in neat liquid benzene studied by femtosecond transient absorption spectroscopy

Abstract Photoionization of neat liquid benzene has been studied by femtosecond pump-probe experiments in the near infrared region. A transient species is formed within 1 ps following two-photon excitation of liquid benzene at 268 nm, which shows a broad absorption band with a peak at 920 nm and extended intensities at wavelengths longer than 1000 nm. This band is attributed to a charge resonance band of a localized benzene dimer cation. The transient species show essentially no evidence of the recombination with electrons in a time shorter than 50 ps.

Attractive and repulsive interactions among methanol molecules in supercritical state investigated by Raman spectroscopy and perturbed hard-sphere theory

The short-range structure of supercritical methanol (CH(3)OH) is investigated by measuring the spontaneous Raman spectra of the C-O stretching mode. The spectra are obtained at a reduced temperature, T(r)=T/T(c)=1.02 (522.9 K), which permits the neat fluid to be studied isothermally as a function of density. As the density increases, the spectral peaks shift toward the lower energy side and the spectra broaden. In the supercritical region, the amount of shifting shows nonlinear density dependence and the width becomes anomalously large. We use the perturbed hard-sphere model to analyze these density dependencies along the vibrational coordinate. The amount of shifting is decomposed into attractive and repulsive components, and the changes in attractive and repulsive energies are evaluated as functions of density and packing fraction, both of which are continuously varied by a factor of 120. Here we show that the shift amount consists principally of the attractive component at all densities, since the attractive energy is about eight times the repulsive energy. The density dependence of the widths is analyzed by calculating homogeneous and inhomogeneous widths as a function of density. The results show that, although vibrational dephasing and density inhomogeneity contribute similarly to the width at low and middle densities, at high density the main contributor turns out to be the vibrational dephasing. We estimate the local density enhancements of supercritical CH(3)OH as function of bulk density by two methods. The results of these analyses show common features, and both the estimated local density enhancements of CH(3)OH are considerably larger than the local density enhancements of simple fluids, i.e., those having nonhydrogen bonding. It is revealed that the local density of supercritical CH(3)OH is 40%-60% greater than the local densities of the simple fluids. We also estimate the local density fluctuation using the obtained values of attractive shift, inhomogeneous width, and local density. The density fluctuation in the vicinity of a vibrating molecule is compared to the fluctuation of bulk density, which is obtained from the thermodynamic calculation.

Ultrafast dynamics of photoexcited trans-1,3,5-hexatriene in solution by femtosecond transient absorption spectroscopy

Abstract We have investigated the ultrafast dynamics of trans-1,3,5-hexatriene in cyclohexane and acetonitrile solutions following photoexcitation to the 1 1 B u state. The transient absorption in the visible region consists of an instantaneous rise (

Correlation time of density fluctuation for supercritical ethylene studied by dynamic light scattering

Time dependence of density fluctuation for neat supercritical ethylene (C2H4) is investigated by dynamic light scattering at various densities in isothermal conditions of reduced temperatures Tr=T/Tc=1.02, 1.04, and 1.06. Time correlations of the density fluctuation for all thermodynamic states decay as single exponential functions with the time constant of submicrosecond. Critical slowing down of diffusive motions is observed in the time domain, and the correlation time of the density fluctuation becomes maximum at the extension of the gas–liquid coexistence curve on the P–T phase diagram. It is revealed that the time dependence of the density fluctuation just corresponds to the magnitude of static density fluctuation obtained by small-angle x-ray scattering measurements. By measuring correlation times as a function of scattering angle, the critical slowing down is thermodynamically discussed. It is elucidated experimentally that the critical slowing down of diffusive motions considerably correlates to t...