Sekika Yamamoto

Construction of a Soft X-Ray Emission Spectroscopy (SXES) Apparatus and Its Application for Study of Electronic and Atomic Structures of a Multilayer System

A soft X-ray emission spectroscopy (SXES) apparatus was constructed using a grating monochromator. The resolution was sufficient to show differences in valence electronic structures of Si compounds including pure Si crystal. A nondestructive analysis of a Ni/Si(111) specimen with heat treatment was carried out using either a clear difference in Si L2,3 SXES spectra of Ni silicide and Si single crystals or the fact that the soft X-ray production depth increases in a solid with increasing energy of a primary electron, Ep. The electronic and atomic structures of the surface and interface of specimens adopted were clarified with Ep varying between 1.5 and 10 keV.

Decay profiles of Mn2+ photoluminescence in CdMnTe

Decay profiles of Mn 2+ photoluminescence in Cd 1- x Mn x Te ( x =0.6) have been investigated under various excitation and observation energies. From the analysis of time resolved spectra, we find that the excited states of d-d transition relax to lower energy by the amount of 5 meV to 40 meV on the time scale of ∼10 µs and that the non-exponential decay profiles of Mn 2+ PL are understood as a result of the energy relaxation. At 2 K, the relaxation time under the lower energy excitation than the peak of the excitation spectrum (∼2.2 eV) is ∼40 µs and that of the high energy excitation is ∼15 µs. With increasing temperature, the relaxation time decreases. These behaviors explain well previous results of site selected and non-site selected photoluminescence obtained by CW measurements.

Broad Raman scattering and luminescence in β-carotene solution

Resonant secondary radiation spectra of dilute β-carotene solution (10−4–10−5 M) are measured under stationary excitation. The excitation energy is varied within 0–0 and 0–1 transition energies in the S2–S0 transition of β-carotene. When the excitation energy is varied from the peak of the 0–0 absorption band to the low-energy side at 60 K and 175 K, (a) the line shape of 0–0 emission band changes from symmetric to asymmetric, and (b) the intensity of luminescence rapidly decreases as compared with the intensity of Raman scattering by ν1, ν2, and ν3 intramolecular vibronic modes of β-carotene. When the 0–1 absorption band is excited, we successfully separate luminescence and broad Raman component (BR), which is resonant Raman scattering of low-frequency phonon modes in solution. The line shape of the density of vibronic states weighed by the coupling strength between electronic states in a dye molecule and vibrations of the surrounding atoms (WDOS) is determined by BR. Taking into account inhomogeneous br...

Pressure dependence of local structure in liquid carbon disulfide

High pressure x-ray diffraction measurements on liquid carbon disulfide up to 1.2 GPa are performed by using an energy dispersion method. The results are compared with a molecular dynamics calculation with usual Lennard-Jones potential. They give very good agreement for all pressures measured. It becomes clear that the liquid structure changes like hard core liquid up to the pressure just below crystallizing point. The relation between structural change and optical response at high pressure is discussed.

Energy Transfer between Mn Ions in Semimagnetic Semiconductors

Time-resolved measurement is performed for photoluminescence around 2.0 eV which originates from internal transition between 3d multiplet states in Mn based semimagnetic semiconductors. Observed emission spectra show red shift with delay time after excitation. Larger shift is observed in the sample with larger width of the emission spectrum, which implies that the shift is caused by the energy relaxation in the inhomogeneous distribution of the Mn energies. To confirm this, we perform simple Monte-Carlo simulation in a wide range of time scale. The results well explain the magnitude of the shift and its logarithmic dependence on delay time. The dependences on excitation energy, temperature and pressure are also investigated.

Pressure dependence of low-frequency light scattering in carbon disulfide

Abstract The pressure dependence of low-frequency vibrational density of states, ρ ( ω ), for liquid and crystalline carbon disulfide (CS 2 ) is determined by measurement of light-scattering spectra using diamond anvil cell. In the liquid, a broad band is observed and in crystalline CS 2 , sharp peaks are observed under high pressure. Molecular dynamics calculations predicted that these sharp spectra come from the librational mode. We indicate experimentally that the origins of spectra obtained in liquid and crystalline CS 2 are the same.

OPTICAL KERR EFFECT IN CARBON DISULFIDE UNDER HIGH PRESSURE

The optical heterodyne spectroscopy of optical Kerr effect (OKE) in liquid carbon disulfide (CS2) was performed under various high pressures using a diamond anvil cell (DAC). The relaxation time of orientation was determined up to 1.20 GPa at 295 K using OKE signals. It increased from 1.6 ps at atmospheric pressure to 10.7ps at 1.20 GPa. Low-frequency light scattering (LFLS) spectra were measured in order to compare the intermolecular dynamical processes in liquid CS2 with those of crystalline CS2. A damped oscillation in OKE is found as a precursor of librational motion observed in crystalline state. This change shows that the intermolecular dynamical processes in the liquid vary gradually to that in crystal with increasing pressures.

Highly enhanced visible luminescence in Zn1?xMgxO nanocrystals

Intensified trap luminescence in Zn1−xMgxO nanocrystals compared with nondoped ZnO nanocrystals is investigated using static and time-resolved luminescence measurements at room temperature. As the Mg concentration x increases from 0 to 0.14, the visible luminescence increases by 10 times and its quantum efficiency reaches 22% while the spectrum blueshifts and broadens. The time-resolved measurements reveal that the spectrum redshifts and narrows with time after the excitation. These temporal changes are ascribed to a large distribution of the trap energy induced by Mg doping, not to the energy distribution in the conduction band caused by crystal size fluctuation.

Exciton Luminescence Dynamics in ZnO Crystal Observed under One- and Two-Photon Excitation

Free-exciton luminescence dynamics at the surface and deep inside of a ZnO single crystal are investigated by one- and two-photon excitation mothods. The free-exciton lifetime is carefully evaluated at each position by considering the excitonic diffusion effect and the trapping process inside the sample. The obtained temperature dependence of the lifetime indicates that the photon recycling effect dominates the exciton lifetime inside the sample while the K-selection rule determines the lifetime at the surface.

Pressure Dependence of the Birefringence in AgGaS2

We measured the pressure dependence of polarized transmission spectra on AgGaS2 at 2 K. These spectra show an oscillatory behavior around the zero-birefringence energy E0 where the ordinal and extraordinal refractive indices coincide. Using this oscillation we determined E0 under hydrostatic pressures. From the difference of pressure dependences of E0 and the energy gap for the upper valence band, it is expected that the pressure dependence of the energy gap for the next valence bands is smaller than that for the upper one. Thus the tetragonal crystal field splitting decreases with increasing pressure. We discuss it in terms of p–d hybridization in the valence bands.