Optical properties of a ZnSe/MgS superlattice (SL) were studied by reflection and photoluminescence (PL) spectroscopies. Excitonic structures were clearly observed in the reflection spectra up to room temperature, and the exciton absorption peak energy and linewidth were well coincident with the PL peak energy and linewidth. The origin of the lowest luminescence peak was indicated to be C1-HH1 exciton up to room temperature. The lineshape broadening due to longitudinal-optical-phonon scattering of excitons is much decreased in this SL compared to bulk ZnSe.
Excitonic properties of newly developed zinc-blende ZnSe/MgS superlattices ͑SL's͒ were measured by reflection spectroscopy. The modification of the excitonic peaks by the multiple reflection in the SL films was treated theoretically and a fitting method to estimate the exciton absorption peak positions and exciton linewidths was developed. Although zinc-blende MgS did not exist before, excellent optical properties were observed in the ZnSe/MgS SL's. Excitonic structures were clearly observed from 13 K up to the measured room temperature. In spite of strong ionicity of MgS barrier layers in the ZnSe/MgS SL's, the reduction of exciton-LO-phonon coupling was clearly observed in narrower wells. This indicates that the quantum confinement effect on excitons is large due to the large band offsets and it overcomes the enhancement of exciton-LO-phonon scattering due to high ionicity in barrier layers. ͓S0163-1829͑97͒03307-9͔
We have produced large samples of ultracold (<20 mK) ArH molecular ions, by sympathetic cooling and crystallization via laser-cooled Be + ions in a linear radio-frequency trap. As technique, we used chemical reactions with sympathetically cooled noble gas atomic ions or N + 2 and O + 2 molecular ions. These ultracold molecules are interesting targets for high-precision measurements in fundamental physics and may open new routes for the study of state-selective chemical reactions relevant to interstellar chemistry. S1242 B Roth et al was demonstrated with an accuracy at the few per cent level, by measuring the Zeeman effect in low-lying rotational transitions of ArH + and ArD + , produced in a discharge source [4]. The use of ultracold ArH + ions may allow us to improve precision of such measurements. Precision measurements of one or several ro-vibrational transition frequencies over time could serve to test the constancy of the nuclear-to-electron mass ratio. A technique to perform highprecision spectroscopy on non-fluorescing ions has recently been demonstrated [5]. Another interesting perspective is to use certain ultracold heteronuclear diatomics, in 1,2,3 or 2 states which are among the most frequent electronic ground states in molecules, as model systems for the implementation of schemes for internal state manipulation [3,2]. For this purpose, molecules with a relatively simple hyperfine structure of the ro-vibrational transitions are more favourable, in order to limit the number of laser sources required for internal cooling schemes.Being among the most abundant molecules in interstellar clouds, the chemistry of hydrogen molecular ions is relevant to astronomy. At present, the interstellar gas-phase chemistry of H + 3 and its deuterated isotopomers is not completely understood [6,7]. Measurements of state-specific reactions of H + 3 via high-resolution infrared spectroscopy can provide valuable input for theories of ion-molecule gas-phase chemistry and precise calculations of molecular transition frequencies. Such measurements could so far only be performed on warm samples [8]. Ultracold ensembles of triatomic hydrogen molecular ions, possibly cooled to their ro-vibrational ground state using cryogenic techniques, could lead to improved studies. As an example, translationally ultracold molecular ions in lowlying ro-vibrational levels (populated at room temperature) could be excited to higher rovibrational levels (not populated at room temperature), using standard IR laser sources. Chemical reactions between state-prepared ultracold molecular ions and (state-prepared) ultracold neutral molecules, which are endothermic when the ions are in low-lying vibrational levels, but exothermic for the excited vibrational levels, could then be studied.A powerful method for cooling molecular ions to translational temperatures in the millikelvin regime is sympathetic cooling. The translational energy of molecular ions can be reduced by interaction with directly cooled (laser-cooled) atomic ions. Under strong cooling, i.e....
Heterointerface properties of ZnSe/MgS superlattices (SLs) grown by -0.52pt metalorganic vapor phase epitaxy (MOVPE) are characterized using X-ray diffraction measurements and in situ monitoring of optical multiple reflection in the SL films. These studies showed the formation of MgSe-related extra interface layers in the SLs. It was found that a purge with a Zn precursor effectively suppresses the extra interface layer formation. By a purge with a Zn precursor, linewidths of the photoluminescence (PL) spectra became much narrower than those of SLs grown without purging. Monolayer exciton PL peaks were clearly observed in a SL with narrow wells. This demonstrates that a purge with a Zn precursor effectively improves the abruptness and the atomic flatness of the ZnSe/MgS heterointerfaces.
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