Character and energy spectrum of surface recombination centers in silicon

We have investigated changes in the rates of surface recombination and capture by fast states in silicon during adsorption and dehydration of the surface. The data are interpreted by treating the surface as a disordered system.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 94–97, September, 1982.     

The influence of thin helium films on the phonon spectrum of optically excited silicon

We have studied the phonon spectra resulting from laser pulse excitation of a silicon surface. We used a phonon spectrometer based on pressure tuned boron levels in silicon (Si: B-spectrometer). A new design allowed us to use the spectrometer in vacuum for the first time. We observed phonon frequencies up to 1 THz. The pulse shapes indicated both, quasidiffusion and a long exciton lifetime. On adding helium films to the surface we have found dramatic changes of the spectrum. Full thermalisation was reached already at a thickness of 1.5 monolayers. This indicates a strong anharmonic interaction in the very first monolayers at these high frequencies and, in addition, a strong confinement of the phonons to the surface by quasidiffusion.     

Fluorescence decay time measurements in tetracene crystals excited with synchrotron radiation

Fluorescence decay times from tetracene single crystals excited at room temperature with synchrotron radiation have been recorded as a function of the excitation wavelength (in the 400–500 nm range). A non-exponential decay with two decay rates is observed. The analysis of our data shows that the first singlet exciton level of tetracene (single crystal) decays radiatively mainly through, as we call it, channel 1, with a lifetime of 0.200 ± 0.020 ns. About 10% of the emitted fluorescence transits through channel 2 with a lifetime of 1.7 ± 0.2 ns. These results do not agree with previously published decay data obtained when tetracene is excited by means of powerful lasers. Thus there is experimental evidence to believe that the decay properties of condensed materials can be very dependent on the excitation density. Because synchrotron radiation compared to lasers is a very weak source, and therefore secondary effects are minimized in our experimental conditions, the decay values reported in the present work are the true lifetimes of the tetracene single crystal.     

Lysine IR emission spectrum excited by moderately intense visible radiation

The excitation of IR emission is observed for thin films illuminated by continuous visible light in the absence of light absorption in the sample. The effect is demonstrated by the example of the lysine molecule and explained by the resonant combined action of optical photons with frequencies ω_i and ω_j, whose differences ω_i?ω_j=Ω coincide with the frequencies of the lysine vibrational modes.     

Intrinsic recombination radiation from GaP

Intrinsic recombination radiation has been detected in GaP crystals grown by vapour transport on GaAs and GaP substrates and in the form of needles, using 50 KeV electron beam excitation in the temperature range 25°K to 80°K. The three major components of the radiation are associated with the decay of free excitons with the emission of the transverse acoustic, longitudinal acoustic and transverse optic phonons which conserve momentum for transitions across the indirect energy gap. Structure has also been observed associated with the emission of two phonons, and for T > 77 >K a phonon absorption component can also be detected. In crystals grown on GaAs substrates, a no-phonon component has been observed with a threshold at the intrinsic exciton energy gap. This is thought to be associated with the presence of arsenic and there is a corresponding shift in the band gap to lower energy. close agreement is observed between the measured shapes of the emission components and those computed by the principle of detailed balance from the intrinsic edge absorption spectrum, when the broadening associated with the crystal imperfections and anomalous structure in the absorption spectrum are taken into account. The phonon energies derived from the emission spectra are in good agreement with the values determined from intrinsic edge absorption measurements.     

Resonant Auger decay in the L2,3-MM spectrum of Ar excited by electron bombardment

The L_2,3-MM Auger spectrum of argon was measured using electron beam excitation. A weak structure on the high energy side of the main transitions was identified as corresponding to the resonant Auger transitions by comparing these spectral structures with individual resonant Auger spectra excited by monochromatic synchrotron radiation.     

Influence of the medium on the electromagnetic radiation spectrum of highly excited atoms and molecules

The influence of neutral species in the E- and D-layers of the Earth’s upper atmosphere on the spectrum of the spontaneous emission (absorption) of Rydberg atoms and molecules for transitions that occur without changing the principal quantum number (Δn = 0) is examined. Along with the process of l-mixing, the splitting of orbitally degenerate states due to interaction with perturbing neutral species of the medium is taken into account. The possible types of radiative transitions between them are analyzed. It is demonstrated that, for principal quantum numbers of n = 10–30, decimeter-band radiation corresponds to transitions between the levels of split states, whereas meter-band radiation, to transitions between their individual components. It is established that, for these values of n, the ratios of the intensities of the decimeter and meter bands for Δn = 0 transitions to the intensity of IR radiation (Δn = 1) are 10⁻⁴ and 10⁻⁶, respectively. The issue of satellite signal phase shift because of multiple Raman scattering in the D-layer of the atmosphere is discussed.     

Emission spectrum of concentrated dye solutions excited by high-power laser radiation

Emission spectra of concentrated (~10¹⁸–10¹⁹ cm^–3) ethanol solutions of Rhodamine C excited by high-power (~10²⁵–10²⁶ cm^–2s^–1) laser radiation were measured. The emission spectrum consists of two narrow bands (with half-widths of ~200–300 cm^–1). The long-wavelength band was interpreted as cooperative spontaneous emission, and the short-wavelength band was assigned to the amplified spontaneous emission. The ratio of their intensities depends on the dye concentration and the pump power density. The ratio of band intensities is assumed to be determined by the dephasing rate of excited molecules responsible for cooperative spontaneous emission.     

Fission and decay of excited nuclei

The statistical theory of the decay of excited nuclei including the competition between particle emission and fission is considered. Fission barriers are given by a phenomenological formula which is an analog of the known Cameron formula and approximates well experimental data on nuclei up to transuranic elements. The contribution of shell corrections and odd-even effects is discussed. The calculated ratios of evaporation and fission widths Γ_n/Γ_f agree with experiment even in the region of nuclei with Z > 90. The $\text{Γ}{}_{\mathrm{<mtext>n</mtext>}}\text{Γ}{}_{\mathrm{<mtext>f</mtext>}}$ values depend weakly on excitation energy for nuclei with Z ≈ 90, While in other regions this dependence proves rather strong. The last conclusion does not contradict known experimental data since in experiments one usually measures some effective value $\text{〈}\text{Γ}{}_{\mathrm{<mtext>n</mtext>}}\text{Γ}{}_{\mathrm{<mtext>f</mtext>}}\text{〉}$, which is only indirectly related to the real $\text{Γ}{}_{\mathrm{<mtext>n</mtext>}}\text{Γ}{}_{\mathrm{<mtext>f</mtext>}}$, values. It is shown that the calculated values of $\text{〈}\text{Γ}{}_{\mathrm{<mtext>n</mtext>}}\text{Γ}{}_{\mathrm{<mtext>f</mtext>}}\text{〉}$ depend on excitation energy either. The effects of angular momentum and uncertainty in the choice of level density parameter are discussed.     

Temperature dependence of the spectrum of recombination radiation for indium phosphide n-p junctions

The spectrum of the recombination radiation from diffused indium phosphide n-p junctions was investigated at 77, 138, 210, and 298° K. The quantum energy in the maximum of the spectrum of recombination radiation differs from the corresponding energy gap values by 0.01 eV; this phenomenon is related to recombination processes at impurity levels.     

Gamma-ray spectrum from gravitino dark matter decay

Gravitinos are very promising candidates for the cold dark matter of the Universe. Interestingly, to achieve a sufficiently long gravitino lifetime, R parity conservation is not required, thus preventing any dangerous cosmological influence of the next-to-lightest supersymmetric particle. When R parity is violated, gravitinos decay into photons and other particles with a lifetime much longer than the age of the Universe, producing a diffuse gamma-ray flux with a characteristic spectrum that could be measured in future experiments, such as GLAST or AMS-02. In this Letter we compute the energy spectrum of photons from gravitino decay and discuss its main qualitative features.     

Effect of the in-plane magnetic field on the recombination radiation spectrum of spatially-separated electron-hole layers

Changes in the recombination radiation spectrum of spatially-separated electron-hole layers has been studied under variation of the in-plane magnetic field and interlayer distance. It has been found that a change in the spectral position of the luminescence line in the low-field limit is proportional to the square of the magnetic field with the proportionality coefficient depending on the interlayer distance. The observed dependence has been shown to agree with the theoretical conceptions, according to which the line shift is quadratic in the magnetic field and interlayer distance and inversely proportional to the sum of the electron and hole masses. This total mass obtained in the experiment has been found to depend on the electric field that separates the layers and may substantially differ from the expected value.     

Selective reflection of resonance radiation from excited media

According to quantum electrodynamics, the cross section for resonant scattering of radiation on an aggregate of excited atoms can be written as a sum of positive definite terms. This type of structure is not consistent with the Fresnel formulas for the reflection coefficient of radiation from thermally excited media. The difference shows up on a macroscopic level and indicates that semiclassical radiation theory cannot be used. A study of the correlation between elastic scattering and stimulated emission processes clarifies the reason for the discrepancies. The resulting singularities require summing of Feynman diagrams which appear beginning in the sixth order of perturbation theory. A lower bound estimate for the reflection coefficient from a plane layer is given, including processes which violate the statistics of radiation. The contribution of stimulated emission processes caused by the initially scattered photon are examined specifically. An experiment is proposed which would settle the choice of theories. Zh. éksp. Teor. Fiz. 113, 521–538 (February 1998)     

Coherence matrices of radiation from an excited system

A new basis for the calculation of the coherence matrices of the radiation emitted by a system is introduced. The utility of this representation is demonstrated by calculating the coherence matrices of the radiation in scattering problems as well as in transitions involving a multiplet of states.     

Investigation of the recombination radiation from layers of n-CdSiAs2

Complex investigations of the recombination radiation from monocrystals of p-CdSiAs₂, thermally processed together with pure indium, which is accompanied by a conversion to p n conductivity at a given depth, are reported. The effect of the thermal processing on the spectral shape and position of the maxima in the formed energy bands was determined. It was found that an increase in the time and temperature of the thermal processing is associated with a band shift in the direction of longer wavelength portion of the spectrum. The nature of the dominant radiative transitions is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 78–84, October, 1991.     

Gamma radiation from the ground state decay of95Tc

The 20 h^95gTc decay to levels of⁹⁵Mo has been investigated studyingγ-singles andγγ-coincidences. The first excited state of⁹⁵Mo at 204 keV, previously assumed to be populated in the^95m Tc decay only, is found to be fed by aγ-transition of 870 keV deexciting the level at 1074 keV which is populated in the^95g Tc decay. More over three newγ-transitions of 477, 786 and 1552 keV deexciting the level at 1552 keV have been found. For the levels at 1074 and 1552 keV the range of possibleJ ^π assignments could be reduced.     

Transition rates and sequential decay of excited nuclei

We present two possible sequential decay mechanisms which correspond either to an inverse fusion process (detailed balance) or to a generalised fission process (transition state theory). We compare the corresponding transition rates and try to understand the implications of these two different choices on the behaviour of mass and charge multiplicities. Deviations from percolation calculation predictions and the experiment are also analysed.