Plasma dynamics in GaAs under strong picosecond surface excitation

We have investigated the time-resolved luminescence of GaAs in air at room temperature, under strong picosecond surface excitation. The energy density was in the range 4–40 $\text{mJ}\text{cm}{}^2$ and the excitation wavelength λ_ex ? 0.53 μ. The peak temperature reached by the generated electron-hole plasma at the end of the excitation pulse amounts to T_M ? 720 K at an energy density of about 10 $\text{mJ}\text{cm}{}^2$. Further increase of the power induces surface damage on the sample. The plasma relaxes its kinetic energy at a slow rate of the order of 10¹⁰$\text{eV}\text{s}$, supporting our previous calculations which concluded that electron-phonon interactions are strongly screened by intravalley free-carrier collisions at high plasma density, so that the plasma cooling slows down.     

Electron tunneling from a metallic TS2 layer underneath an ultra-thin MS layer with semiconducting properties for misfit-layer compounds

The effects of electron tunneling from the underlying TS₂ (H) layer on the scanning tunneling microscopy (STM) images of the uppermost MS (Q) layer have been studied for the misfit-layer compounds which are represented by the chemical formula {(MS)_1+x}_m{TS₂}_n. Systematic STM observations have been carried out under ultra-high vacuum (UHV) conditions for the 1Q/1H, 1.5Q/1H and 2Q/1H types of misfit-layer compounds. As Q layer thickness increases from about 6 to 12 Å while going from the 1Q/1H type to the 2Q/1H type, pseudo-tetragonal arrays of bright spots as expected from the atomic arrangement of a Q layer are observed more easily and more distinctly. It is found that tunneling electrons from the underlying H layer play an important role on the STM observations of the 1Q/1H and 1.5Q/1H types of compounds. Fast Fourier transform (FFT) analyses give clear evidences for electron tunneling from the underlying H layer and scattering by surface atoms of the uppermost Q layer and a mutual modulation structure peculiar to the compounds.     

The fluorescence spectra of pharmaceutical compounds under two-photon excitation

Spectra of two-photon-excited fluorescence in pharmaceutical crystalline compounds were obtained and studied as functions of the pump power and delay time. Stimulated fluorescence was observed in polycrystalline Novocaine under high-power pumping. The spectra of two-photon-excited fluorescence in pharmaceutical compounds are localized within the wavelength range of 270–480 nm. The most intense fluorescence was observed in the Novocaine polycrystals. The therapeutic effect of diphenylhydramine and sulfanilamide is attributed to the fact that the energy of optical quanta of the fluorescence emission in these substances is higher than the energy of excited π-electrons in tryptophan and nucleotides. The data obtained for the spectra of the pharmaceutical substances studied can be used to solve analytical problems related to identification of the composition and properties of composite pharmaceutical substances.     

Luminescence of some zirconium-containing compounds under vacuum ultraviolet excitation

Photoluminescence of compounds that contain stoichiometric zirconium has been studied under vacuum ultraviolet excitation. The compounds show emission peaking at 280-320 nm while the excitation spectra show some bands in 130-190 nm region. The ultraviolet emission is explained as Zr to O charge transfer transition. The luminescence result and structural information classify the studied compounds into two groups. The former group involves ZrP₂O₇, CaZr(PO₄)₂, NaZr₂(PO₄)₃, Ca₂ZrSi₄O₁₂, Ca₃ZrSi₂O₉ and SrZrSi₂O₇, which show rather intense luminescence and do not have any infinite Zr-O-Zr-O-3D chain or link in their structure. The latter group is CaZrO₃ and ZrSiO₄, which do contain infinite Zr-O-Zr-O-3D chain and show quite weak luminescence. Luminescence of Ca_1−xMn_xZr(PO₄)₂ has also been studied. By replacing a part of Ca with Mn, ultraviolet emission of the host weakens and visible emission peaking at 540 nm appears. It is claimed that transfer of absorbed energy from Zr to Mn occurs.     

Photoluminescence of aromatic compounds in photon traps under resonant pulse-periodic excitation

The photoluminescence spectra of aromatic compounds in a photon trap under ultraviolet pulse-periodic excitation are recorded. It is shown that the photoluminescence spectrum shape depends on the material layer thickness in a cell, physical state of the sample, and exciting radiation characteristics: as the layer thicknesses decreases, the intensity is significantly redistributed, which is explained by the transition from spontaneous luminescence to superluminescence.     

Theoretical investigation of photoprocesses in complex organic compounds upon high-power laser excitation

A procedure of theoretical investigations into the dynamics of energy level populations of organic molecules upon high-power laser excitation is developed that allows information on stimulated emission of molecules, gain and lasing spectra, dynamics of lasing pulses, populations of the ground and excited states, and influence of the induced absorption on lasing of the organic molecules to be obtained. Laws of changing of the above-indicated processes are established for two pyran substitutes. Results of theoretical and experimental investigations are well correlated. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 51–60, March, 2009.     

The nonlinear optical response of new organometallic compounds under picosecond excitation

The nonlinear optical response of three new organometallic compounds was studied by z-scan method under picosecond excitation at 1064 and 532 nm. The compounds showed self-focus effect at both wavelengths, and effective reverse saturation absorption at 532 nm and strong two-photon absorption at 1064 nm occurred. Through calculating and comparing the nonlinear optical indexes of the compounds, we found that the third-order nonlinear susceptibility was enhanced by two-fold for one photon transition resonant effect, and incorporating different heavy metal atoms in the delocalized organic system slightly influenced the nonlinear optical indexes.     

Energy yield of fluorescence of complex organic compounds upon electron excitation

With use of electron energy loss spectra for vapors of anthracene, perylene, 1,4[di(2,5-phenyloxazolyl)] benzene, and paratherphenyl the energy yields of fluorescence are calculated for electron collision. It is shown that they are considerably smaller than those for optical excitation conditions. Deceased Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 146–148, January–February, 1998.     

Auger parameters for sulfur-containing compounds using a mixed aluminum-silver excitation source

A mixed X-ray source (Al K-Ag L) has been used to obtain the precise Auger parameters (S2p_3/2-S KL₂₃L₂₃) for about sixty sulfur-containing compounds. A two-dimensional chemical state plot for the identification of the valence state of sulfur in a compound and of the specific sulfur-containing compound is presented.     

Angular and energy dependences of the surface excitation parameter for semiconducting III-V compounds

Sum-rule-constrained extended Drude dielectric functions were used to study surface excitations generated by energetic electrons moving across surfaces of semiconducting III-V compounds. Parameters in the dielectric functions were determined from fits to experimental optical data and electron energy-loss spectra. Electron inelastic mean free paths (IMFPs) in GaN, GaP, GaAs, GaSb, InAs and InSb were calculated for electron energies between 200 and 2000 eV, and the results were found to follow the simple formula, i.e., λ = kE^p, where λ is the IMFP and E is the electron energy. Surface excitation parameters (SEPs), which describe the total probability of surface excitations by electrons crossing the surface and travelling in vacuum, were also calculated for different electron energies and crossing angles. The SEP was found to follow the simple formula, i.e., , where P_s is the SEP and α is the crossing angle relative to the surface normal.     

Photoluminescence of traces of aromatic compounds in aqueous solutions upon excitation by a repetitively pulsed laser

A method is suggested for quantitative and qualitative analysis of aromatic compounds in water at extremely low concentrations (<1 ng/l). The method is based on excitation of luminescence by short (20 ns) UV laser pulses with a peak power of 10⁴ W at 255.3 nm and a pulse repetition rate of 10 kHz. The shape of luminescence spectra of benzene, benzoic acid, saccharin, aspirin, and L-tryptophan at low concentrations in water is analyzed. The luminescence kinetics of these compounds is studied at short delay times (10–20 ns). The lifetimes of the electronic excited states of the aromatic compounds are measured. The applications of the method for studies of low-concentration aqueous solutions of biological and medicine compounds are analyzed.     

The luminescence of II–VI compounds with blende type structure under high excitation

The luminescence of ZnSe is investigated as a function of excitation intensity for temperatures between 5 K and 300 K. At low excitation we observe emission due to free and bound excitons and due to donor-acceptor pair recombination. At higher excitation, the emission is dominated by inelastic exciton-exciton and exciton-free carrier scattering at lower and higher temperatures, respectively. A “M-band” observed in ZnSe and ZnTe is tentatively ascribed to a biexciton decay. The biexciton binding energies are 2±1 meV for ZnSe and 1,5±1 meV for ZnTe.