Katsumi Tanimura

Transmission-electron diffraction by MeV electron pulses

We have developed a compact ultrafast electron diffractometer, consisting of a laser-driven rf photocathode that generates 3.0 MeV probe electron pulses, three-stage lens optics, and a custom-built detector for relativistic electrons. High-quality single-shot transmission electron diffraction has been detected from 180-nm-thick Si single crystals, with an excellent special resolution for diffracted beams; the spot width of 0.02u2002A−1 is obtained. The pulse width is estimated to be 100 fs in duration. Characteristics of the electron beam and other diffractometer features are discussed.

Two slowly decaying luminescence bands in alkali iodides

Effects of optical excitation of the self-trapped excitons on the intensities of the two slowly-decaying luminescence bands in RbI and KI have been measured. The excitation-induced fractional decreases in the intensity of the two luminescence bands in RbI are found to be almost the same, irrespective of the excitation photon energy. The degree of polarization of the decrements of the two luminescene bands in RbI induced by the hole-excitation was found to be identical. The decay times of the two slowly-decaying luminescence bands in Na-doped KI and K-doped RbI are also measured and it is found that impurity-associated two slowly-decaying luminescence bands are induced in each of these doped crystals. We conclude that both of the two slowly-decaying luminescence bands in RbI are intrinsic. It is suggested that the exciton in RbI is relaxed either into the three-center-type self-trapped exciton as well as into the two-center-type self-trapped exciton.

Excitation-Induced Atomic Motion of Self-Trapped Excitons in RbCl: Reorientation and Defect Formation

Effects of electron excitation of self-trapped excitons on the π-luminescence and the optical absorption due to electron transition from their lowest excited states in RbCl have been investigated. Similarly to KCl and KBr, it is found that the yield of the F centers creation is the highest when the electron is excited to the 2 p π u orbital lying in the (100) plane. The experimental results are analyzed using kinetic equations involving the non-radiative transitions between the excited states of the self-trapped exciton and the transitions to the F - H pair and to the ground state. The general trend of the non-radiative de-excitation process in RbCl is found to be similar to that in KCl, except that the reorientation is induced when the electron is excited to the 2 p π u orbital lying in the (110) plane. It is suggested that the reorientation is induced by the configuration interaction or the Auger transition in which a hole excited state is involved.

Time-resolved two-photon photoelectron spectroscopy of the Si(001)-(2 × 1) surface

Abstract The electron dynamics of the Si(0xa00xa01)-(2xa0×xa01) surface has been studied by means of two-photon photoelectron spectroscopy. The photoelectron peak from the unoccupied surface state ( D down ) of down-Si atoms of the dimers is clearly resolved spectroscopically and temporally. Time-resolved study shows that the D down state is populated within 10 ps of excitation, and de-populated primarily with a time constant of 210 ps at 300 K. The rate of populating D down states is excitation-intensity dependent, and has been ascribed to electron–electron scattering processes.

Bond rupture of threefold-coordinated Si atoms at intrinsic sites on the Si(1 1 1)-(2 × 1) induced by 1.16-eV photon excitation

Abstract A scanning tunneling microscopy study reveals the electronic bond rupture of Si atoms threefold coordinated at intrinsic sites on the Si(1xa01xa01)-(2xa0×xa01) surface under 1064-nm excitation resulting in the lowest valence excitation of Si. The bond rupture generates monovacancies at surface Si sites with a rate dependent superlinearly on excitation intensity, and preferential bond rupture near the vacancies leads to the formation of vacancy clusters progressively. The mechanism of the electronic process is discussed in terms of the two-hole localization on this surface.

Optical Studies of Self-Trapped Excitons in CsCl and CsBr

Optical properties of self-trapped excitons (STEs) in CsCl and CsBr have been studied. An intrinsic luminescence band due to STEs is found, for the first time, to occur at 2.92 eV in CsCl. Transient optical-absorption spectra originating in the initial state of the STE luminescence are measured; the lowest absorption peak appears at 1.74 eV in CsCl and at 1.19 eV in CsBr. Temperature dependences of the intensity and decay time of the emission bands are also measured and analyzed to get parameters characterizing interlevel transition and quenching processes at the initial state of the luminescence. The relaxed configuration of excitons in cesium halides is discussed on the basis of the obtained results, in connection with the current model on STEs proposed for alkali halides.

Femtosecond time-resolved spectroscopy of photoinduced ionic-to-neutral phase transition in tetrathiafulvalen-p-chloranil crystals

Recent experimental results are reviewed of the photoinduced phase transformation between ionic and neutral phases in tetrathiafulvalen-p-chloranil crystals. Spectroscopic studies have revealed different features of Frenkel-type and charge transfer (CT)-type excited states in inducing photo-induced structural phase transition; the transition can be induced only above threshold-excitation intensities in the case of CT excitation. The thresholds imply that non-linear processes of CT excited states are crucial in the transition. On the other hand, time-resolved studies have demonstrated a dynamical aspect of the transition; optical reflectance signals that probe the evolution of the transition show dumped-oscillatory changes before establishing N-phase domains at around 100 ps after excitation. Mechanism of the transition has been discussed based on these results.

Time-resolved spectroscopic study of excitonic luminescence centers in RbI crystals

Luminescence and optical absorption associated with relaxed excitons in pure RbI have been studied by means of time-resolved spectroscopy. It is found that the emission band near 3.0 eV consists of an intrinsic component and an extrinsic component arising from Br - impurities. The intrinsic component, characterized by the same decay time as that of the π luminescence, exhibits an emission band peaking at 3.07 eV. The results indicate the presence of three intrinsic emission bands in RbI, the 3.07 eV, π and σ bands. The optical absorption originating from the initial states for the 3.07-eV and π luminescence is compared with the corresponding absorption in other alkali halides by means of the Mollwo-Ivey relation. It is shown that the absorption bands from the initial state of the π luminescence in alkali halides, except NaBr and NaI, can be categorized into two groups: one includes that for the 3.07-eV luminescence and the other that for the π luminescence.

Restoration of Fluorescence from the Lowest Singlet State in the Self-Trapped Exciton by Perturbation with Monovalent Cation Impurities in Alkali Halides

The electronic structure and intersystem crossing at excited states of the relaxed exciton, ( V k e ) A , perturbed by monovalent cation impurity in KCl and KBr have been studied by pulsed laser irradiation of samples populated with the lowest triplet ( V k e ) A . It is found that the σ-polarized luminescence, at 3.03 eV and 2.93 eV in KCI:Na and KBr:Na, respectively, that decays in less than 10 ns is produced upon the photoexcitation of the lowest triplet ( V k e ) A . The fast luminescence is ascribed to the fluorescence (referred to as 1 -luminescence) from the lowest singlet ( V k e ) A . The degree of polarization of the optical absorption due to the lowest triplet ( V k e ) A is determined and it is shown that the electron orbitals of ( V k e ) A are in the order of b 2 u , b 3 u , b 1 u and b * 3 u from lower energy. The S 1 luminescence is found to be produced most efficiently by the electron excitation from the a g orbital to the b * 3 u orbital.

Excitation-induced structural instability of semiconductor surfaces

The present work reviews laser-induced electronic processes of structural instability on covalent semiconductor surfaces. In particular, we concentrate on the mechanism of the instability that takes place at the intrinsic sites of reconstructed surface structures. In order to elucidate the primary processes involved, we focus our attention on experimental results obtained by scanning tunnelling spectroscopy studies, to reveal surface structural changes at the atomic level, and by post-ionization spectroscopy studies, to probe desorption processes with high sensitivity. First, the results obtained for reconstructed Si surfaces and {110} surfaces of III?V compound semiconductors are systematically surveyed. The instability is characterized by local bond rupture at intrinsic surface sites that show important common features. Also, we find significantly different aspects of the instability, which depend on the basic properties of surfaces. Based on the characteristics revealed by the experimental results, we then propose a mechanistic model based on the generalized two-hole localization mechanism, and demonstrate, through the quantitative analysis of typical experimental results, that the model describes all the important features quantitatively and consistently.