Vachagan V. Harutunyan

Study of optical absorption of corundum in the vacuum ultraviolet region of synchrotron radiation

Abstract The optical system with a modified normal-incidence monochromator, installed in the synchrotron radiation channel, has been developed. Some changes in the optical system make it possible to use the monochromator as a double-beam spectrophotometer maintaining the polarization of synchrotron radiation at the exit. Experimental control and the recording of irradiation and spectra are automated by a computer. The high degree of polarization of synchrotron radiation, in contrast with other sources, makes it possible to detect experimentally the optical absorption bands at 4.8, 5.4, 6.1, 6.3 and 7 eV photon energies in corundum single crystals grown by the method of horizontally directed crystallization and irradiated with high-energy particles. It is proved that at the orientation of the electrical vector E of synchrotron radiation parallel to the optical axis ( C 3 ) of the crystal, the intensity of the 6.3 eV absorption band is larger than that at the orthogonal orientation. In the case of E ⊥ C 3 the optical absorption bands at 5.4 and 7 eV are more outlined. Optical bleaching in the F-band results in the reduction of F-center concentration and in the increase of the intensity of the 4.86, 5.4, 6.3 and 7 eV absorption bands. The results show that the 7 eV band is also due to electron transition in the F+-center.

Vacuum ultraviolet luminescence excitation spectra of α-Al2O3 single crystals

Abstract The luminesence excitation spectra of corundum single crystals irradiated with 50 MeV electrons were studied. In the region of fundamental absorption there appeared excitation bands (blue and ultraviolet) which correlated well with the optical absorption bands of the F + -centre in the absorption (reflection) spectra. The excitonic peaks observed in the excitation spectra of red luminescence were explained by the charge energy transfer by Cr 2+ and Cr 4+ ions at excitonic excitation. At hv > 9.5 eV sharp drops appeared which were interpreted as nonirradiative near-surface recombinations, the probability of which sharply increased at high absorption coefficients. The enhancement of the luminescence efficiency in the high-energy region was connected with the decay effect of electron excitations as well as with the influence of volumetric excitations leading to the enhancement of recombination glow.

VUV luminescence of as-grown and electron irradiated corundum single crystals

The optical properties of as-grown and electron-irradiated α-Al203 (corundum) single crystals were investigated at low temperatures, using vacuum-W (VUV) time-resolved luminescence spectroscopy under the synchrotron radiation excitation. It has been shown that in the electron-irradiated crystals the excitation of self-shrunk exciton emission extends to the region of interband transitions. The decay time of the 7.6-eV emission of the self-shrunk excitons in as-grown crystals was found to be about 7 and 23.8 ns respectively for the excitations in the excitonic band and at energies at which several electron-hole pairs are generated. In the irradiated crystals, these lifetimes were by about 2 ns longer than in the as-grown ones. For both crystal states, the nature and dynamics of the electronic excitations are discussed.

The study of corundum single crystal reflection spectra in the corundum fundamental absorption region

Abstract With the use of the highly polarized synchrotron radiation the reflection spectra of a nominally pure corundum single crystal were measured. The electron irradiation is shown to lead to the formation of an F + -centre (6.3 and 7 eV) and to the shift of the maximum of the excitonic peak as well as to the display of anisotropy of high-energy bands in the reflection spectra depending on the orientation of the electric vector E to the optical axis C 3 . The observed effects in the reflection spectra in the fundamental absorption region are explained by the change in the density of 2p electron oxygen states, which is due to the increased concentration of defects in the near-surface layers owing to radiation effects.

Comparative analyses of physical properties of natural zeolites from Armenia and USA

Dielectric permittivity, specific conductivity, luminescence and infrared absorption properties of natural zeolites from Armenia and the US were studied. Optical spectra and dielectric permittivity including specific conductivity are compared for samples of both origins. It was shown that measured properties determine clinoptilolite content in the sample. In the sample of US origin there were more impurities than in Armenian ones. In the samples of Armenian origin clinoptilolite properties are more prominent.

Radiation processes on the surface of irradiated corundum monocrystals

Abstract The reflectivity of corundum (α-Al2O3) was measured by a spectral-kinetical set up, using the synchrotron radiation (SR). It is shown, that as a result of irradiation, in the surface layers the stoichiometry is disturbed due to the elastic interaction with crystal lattice. The radiation-stimulated oxygen desorption occurs and restored Al fills vacancy pores and dislocation loops on the surface of irradiated corundum, thereby increasing the reflectivity in the energy range of E > 25 eV.

Luminescence Excitation and Influence of Radiation on the Stimulated Processes in Corundum Crystals

The excitation spectra of luminescence in irradiated and non-irradiated corundum crystals are investigated by means of highly polarised synchrotron radiation. In the fundamental absorption region double-exciton peaks are observed at 90 K in the region 8.5 to 9.2 eV for crystals irradiated by neutrons. At 9.5 eV sharp drops appeared, which were interpreted as non-irradiative near-surface recombination, the probability of which sharply increased at high absorption coefficients. The enhancement of the luminescence efficiency in the decay effect of electron excitations as well as the influence of bulk excitations leads to an enhancement of recombination glow. In the VUV luminescence spectra the existence of quick and slow emission was observed in irradiated and non-irradiated crystals. The luminescence bands with maximum at 3.8 eV are produced by F + centres. A new type of quick luminescence was established for the band at 4.6 eV. This is called cross-luminescence and is connected with the recombination of valence band electrons with holes in low-lying core levels. It was shown that the band at 3.0 eV is not due to anionic centres (F-centres), but is determined by a short lifetime emission center (F - -centre).

Luminescence Excitation of Y3Al5O12 Monocrystals in the Vacuum Ultraviolet Region

Luminescence excitation spectra of Y 3 Al 5 O 12 single crystals in the region from 4 to 30 eV at tentperatures of 300 and 90 K were investigated using synchrotron radiation. In the region of fundamental absorption, excitation bands corresponding to excitonic (S ex ) and recombination (S b + e) states as well as an UV band stipulated by doped Nd 3+ ions were revealed. In the high energy region (hv > 25 eV), peculiarities of the multiplication of electronic excitations in both pure crystals and those doped by different concentrations of Nd 3+ are discussed.

Reflectivity of Gallium Phosphide in VUV Spectral Region

Publisher Summary The chapter discusses the results of GaP crystal surface reflectivity measurement at room temperature and with the use of synchrotron radiation. Within 4 to 10 eV spectral region, the reflectivity measurements were carried out with the help of the vacuum monochromator VMR-2 that had been specially altered for the work on the spectroscopic synchrotron radiation channel of Yerevan Physics Institute Accelerator. For this purpose, the specimen surfaces were mechanically polished, and residual mechanical stress and oxides were removed chemically. After etching the samples were exposed to air for a minimum period of time required for a sample mounting into a camera. The GaP surface reflectivity measurements results are presented in the chapter. Results show that the reflectivity spectrum in the region of L 2 3 - L c 3 transitions has a complex structure. It comprises three outlines maxima e 1 , e 2 , and E ′ 1 . The obtained results cannot be considered as a consequence of L 2 3 or L c 3 band spin-orbit-split, because the energy distance among maxima is not correlated with the theoretical values of Δ ′ 1 . The detected structures e 1 and e 2 are probably because of transitions among surface exciton states of the crystal. Similar surface excitons are observed in GaAs and InAs crystals in the region of transitions from the d-electron shell of gallium and indium to L c 1 and X c 1 states. For example, surface exciton binding energy for In4d-X c 1 transition forms 0,5 eV.