Moderate Excitation Research Articles

Research NoteTh4-4: A Be star which has turned into a planetary nebula within 30 years

The rst optical spectra of Th4{4, obtained by the author in 1970, showed strong continuum and broad HI emission lines. In other words, at this time the object looked like a Be star. Some years later, (OIII) emission lines appeared in the spectrum indicating the existence of a gaseous envelope. Today the optical spectrum of Th4{4 is quite similar to that of a moderate excitation planetary nebula. The magnitude of the object has dropped from V =1 4: 4t o 16: m 6, and the eective temperature of the central star has increased from 22 000 K up to 40 000 K

Conjugated polymers: lasing and stimulated emission

The discovery of gain narrowing in electroluminescent conjugated polymers has launched a new field of research aiming towards polymer-based injection lasers. Especially poly( para-phenylene vinylene)- (PPV) and poly( para-phenylene)- (PPP) -type polymers have been applied as active materials. Time-resolved measurements have unravelled the photophysics for low and high excitation levels. If suited resonator configurations of the device are used (e.g. microcavity, microring, microdisc, distributed feedback and distributed Bragg resonators) optically pumped single mode laser emission can be observed for moderate excitation levels. These results represent an important step towards the realisation of future ‘all-plastics’ injection lasers. Conjugated polymer laser devices might lead to novel applications in measurement instrumentation and information technology which rely on large area emitters and geometries not accessible to conventional inorganic laser materials.

Optical coherent transient effects in semiconductor quantum wells exhibiting a two-hole species valence band

The paper deals with the detailed theoretical investigation of optical coherent transient processes in a narrow direct gap semiconductor quantum well structure (QWS), duly irradiated by a near band gap resonant ultrashort pulsed laser with moderate excitation intensity. The photoinduced band-to-band electronic transitions are considered from both the heavy-hole (hh) and light-hole (lh) valence bands to the lowest (1s) exciton state below the fundamental absorption edge. Since the hole populations in both hh and lh bands are nontrivial in the case of the transverse plane in a QWS, we have recognized that the hh and lh excitons participate in photoinduced transitions. The photoinduced electron density is chosen to be less than the Mott density such that various many-body processes, otherwise significant, can be neglected. The well-established time-dependent perturbation treatment of the semiconductor Bloch equations has been followed to calculate the induced polarization as well as the differential transmission spectra. We find from the numerical estimates made for a GaAs/AlGaAs single QWS shined by a femtosecond pulsed Ti : Sapphire laser that the transmission characteristics of the coherent transient processes are dominated by the lh species in the QWS. Rabi oscillation and Stark splitting as calculated for the two-hole species QWS agree qualitatively very well with recent experimental observations.

Investigation of the level density parameter using evaporativeα-particle spectra from the19F+181Tareaction

Light charged particles in coincidence with evaporation residues from the ${}^{19}\mathrm{F}{+}^{181}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{Ta}}}^{200}\mathrm{Pb}$ reaction have been studied at ${E}_{\mathrm{lab}}=121,$ 154, 179, and 195 MeV with the aim of determining, within the context of the statistical model, a possible dependence of the Fermi-gas level density parameter on the temperature/excitation energy of the emitting system. Angular distributions of evaporation residues and of fission fragments, along with fission fragment folding angle distributions, were also measured. A moderate excitation energy dependence of the level density parameter, as given by recent theoretical calculations, is consistent with the measured $\ensuremath{\alpha}$-particle spectral shapes. Although the spectra are reproduced best with a weakly varying level density parameter, a constant level density parameter of a=A/12 ${\mathrm{MeV}}^{\ensuremath{-}1}$ cannot be excluded.

Distribution of compound-nucleus shapes and its influence on evaporation

The tail of the equilibrium distribution of compound-nucleus shapes is shown to extend out to very deformed shapes even for a moderate excitation energy of 100 MeV. The standard Hauser-Feshbach formalism is extended to predict the decay of a deformed compound nucleus as a function of spin, spin projection, and excitation energy. The inclusion of the equilibrium distributions of shapes is found to have little effect on the predicted neutron and proton kinetic energy spectra, but for $\ensuremath{\alpha}$ particles, the low-energy ``sub-barrier'' region of the spectrum is enhanced, in agreement with experimental data. Langevin simulations, using dissipation given by the wall formula, predict that compound nuclei start evaporating before the shape distribution equilibrates for excitation energies above $\ensuremath{\sim}100\mathrm{MeV}.$

Laser-induced thermal effects on the optical properties of free-standing porous silicon films

A detailed study of the unique optical properties of free-standing oxidized porous silicon films has been performed. Under continuous-wave laser irradiation, a strongly superlinear light emission and a very large laser-induced absorption are observed. The nonlinear emission is characterized by a sharp intensity increase that is in proportion to at least the eighth power of the excitation intensity. This emission has a broad peak (600–1300 nm), slow time constant (⩾10 ms), and very low polarization memory (⩽0.01 near the emission peak). The induced absorption increases linearly with the pump laser intensity and can be as large as several times the linear absorption. The increase in the normalized absorption coefficient is almost independent of the oxidation temperature and emission wavelength, with essentially no polarization dependence. These experimental results are discussed in terms of laser-induced thermal effects. An evaluation of the temperature rise under the laser irradiation is performed both experimentally and theoretically. A remarkably high-temperature rise of ⩾700 °C is estimated for a moderate excitation intensity of 20 W/cm2.

Vibrational state-resolved study of the O−+H2 reaction: Isotope effects on the product energy partitioning

The deuterium isotope effect on product energy partitioning in the O−+H2 particle transfer reaction is investigated in a crossed molecular beam experiment. Vibrational-state-resolved angular distributions are measured at six collision energies between 0.20 and 0.77 eV for the O−+H2 reaction and at seven collision energies between 0.22 and 1.20 eV for the O−+D2 reaction. The fraction of the total available energy deposited into product vibration is significantly larger in the deuterium system than in the hydrogen system. This effect is greatest at the lowest collision energies where OD− products are formed with more than twice as much vibrational energy as OH− products. The isotopic systems display similar trends in the product angular distributions, which extend over the full range of scattering angles at low energies and shift towards the forward direction as the collision energy is increased. These observations are discussed in terms of a competition between reaction mechanisms. An insertion-migration mechanism, yielding products with moderate vibrational excitation, is especially important at the lower energies. The insertion process leads to the isotope effect in the product energy partitioning which is explained in terms of Franck–Condon factors. As the energy increases, larger impact parameter collisions are able to proceed through a direct mechanism, yielding more tightly forward-scattered, vibrationally excited products. Since direct mechanisms show isotopically independent energy partitioning, the overall isotope effect diminishes with increasing energy as more collisions become purely direct. Bimodal rotational state distributions help strengthen the claim that two distinct reaction mechanisms produce the particle transfer product.

Influence of pairing correlations on the excitation energy, angular momentum, and parity dependence of nuclear level densities

We have studied the effects of pairing correlations on the excitation energy ${(E}^{*})$ dependence of the total nuclear level density, parity-projected level density, and spin cutoff factor for two heavy systems ${}^{152}\mathrm{Sm}$ and ${}^{160}\mathrm{Yb},$ within a microscopic approach based on static path approximation to a grand canonical partition function. For both systems considered, we find that pairing has significant effects on the ${E}^{*}$ dependence of total level densities. At moderate excitation energies ${(E}^{*}\ensuremath{\sim}10 \mathrm{MeV})$ it supresses the total level density nearly by two orders of magnitude. By fitting the total level density to a Bethe-type formula with energy backshift $({\ensuremath{\Delta}}_{\mathrm{BS}}),$ we find that ${\ensuremath{\Delta}}_{\mathrm{BS}}\ensuremath{\approx}2.5 \mathrm{MeV}$ for the excitation energy beyond $15 \mathrm{MeV}.$ At lower excitation energies, however, ${\ensuremath{\Delta}}_{\mathrm{BS}}$ is strongly ${E}^{*}$ dependent. We find that the effects of pairing on the spin cutoff factor and the parity-projected level densities (or parity asymmetry) is somewhat less important. We also compare our results for the spin cutoff factor with the ones conventionally employed in statistical model codes.

Vibrational relaxation of NO(=1–3) and NO2(0,0,1) with atmospheric gases

Measurements are reported of the vibrational quenching of NO(v=1–3) by NO2 and O2, and of NO2(0,0,1) by NO, O2 and N2, close to room temperature. Vibrationally excited NO was formed from the photolysis of NO2 at 308 and 355 nm, and the kinetic behaviour of the different levels was followed by wavelength resolved FTIR emission. The rate constant for the removal of NO(v=1) by NO2, (1.9±0.2)×10-12 cm3 molecule-1 s-1, is in excellent agreement with previous measurements. For v=2 and 3 the rate constants showed a marked increase, with values of (2.9±0.3) and (4.8±0.7)×10-12 cm3 molecule-1 s-1 respectively, and the relaxation process was found to proceed dominantly through single quantum transitions in NO. However, simultaneous observation of emission from the (0,0,1) level of NO2 revealed that although single quantum exchange between NO and NO2(0,0,1) is close to resonance it took place with less than 50% quantum efficiency. The results are discussed in terms of formation of a N2O3 complex in which free flow of energy is incomplete. For quenching by O2, energy transfer again was found to be dominated by single quantum loss in NO, with rate constants of (2.4±0.2), (5.3±2.6) and (12±4)×10-14 cm3 molecule-1 s-1 for v=1, 2 and 3, respectively, in good agreement with previously reported values. Vibrationally excited NO2 was produced by the reaction of NO with NO3, and its quenching kinetics studied by observation of time resolved emission. Rate constants were found to be (4.0-1.5+1.9)×10-12, (1.8-0.8+1.1)×10-13 and (3.1-1.0+1.3)×10-14 cm3 molecule-1 s-1 for quenching by NO, O2 and N2 respectively. The results show that for the first two species the rate constants are similar to those previously reported for quenching of NO2 with moderate but unspecified excitation in the ν1 and ν3 modes, but for N2 the present value is a factor of four lower. As quenching rate constants for the ν1+ν3 modes have been used for estimations of the atmospheric quenching rate of NO2(0,0,1), the present results suggest that these estimates need downward revision by approximately a factor of 2.5.

Mechanisms of Optical Gain in Cubic GaN and InGaN

The epitaxial growth of zinc-blende (cubic) GaN and InGaN on GaAs with a common cleavage plane and readily high-quality, low-cost wafers may be considered as an alternative approach for the future realization of cleaved laser cavities. To obtain detailed information about the potential of cubic GaN and InGaN for device applications we performed optical gain spectroscopy accompanied by time-integrated and time-dependent photoluminescence measurements at 2 K and 300 K. From intensity-dependent gain measurements, the identification of the gain processes was possible. For moderate excitation levels, the biexciton decay is likely to be responsible for a gain structure at 3.265 eV in cubic GaN [10]. For the highest pump intensities, the electron- hole-plasma is the dominant gain process, providing gain values up to 200 cm −1. Furthermore cubic GaN samples with different cavity lengths from 250 to 600 µm were cleaved to investigate the influence of the sample geometry on the gain mechanisms. In these samples increased gain values up to 150 cm −1 as well as lower threshold excitation densities were observed, indicating the potential of cubic GaN for device applications. The results of GaN will be compared with intensity-dependent gain measurements on InGaN samples, grown on GaAs with varying In-content. The observed gain mechanisms in cubic InGaN will be discussed in detail.

Photoluminescence from Si/Si1−xGex single quantum wells in high magnetic fields: localized and free exciton recombination

We have performed photoluminescence measurements on three Si/Si0.76Ge0.24 single quantum wells with well widths of 2.5, 4.8 and 8.5 nm in magnetic fields up to B=20 T. For the excitation and detection of the photoluminescence a large core (0.6 mm) glass fiber was used, which provides sufficient signal intensities at moderate excitation power densities. The magnetic field leads to a luminescence peak splitting and rather large diamagnetic shifts in the wider quantum wells. The increase of the diamagnetic shift with well width indicates a strong reduction of the exciton binding energy, which is consistent with the previously proposed type II band alignment in these quantum wells, and we interpret the peak splitting in terms of localized and free exciton recombination.

Reaction dynamics of Cl $\mathsf{+}$ O $\mathsf{_{3}\rightarrow}$ ClO $\mathsf{+}$ O $\mathsf{_{2}}$

Applying the two photon laser induced fluorescence technique for nascent state resolved ClO( ) detection, the reaction dynamics of Cl+O ClO+O2 is investigated. The ClO product is formed in its electronic ground state ClO( ). A complete product state analysis in terms of vibration, rotation, spin-orbit and -states indicates that nascent ClO radicals are formed in v =0-6 vibrational states peaking at v =3. The ClO fragment shows a moderate rotational excitation, described by a Boltzmann distribution with a temperature parameter of 1300 K 200 K. The spin orbit ratio of : . Most of the excess energy is released as translational energy or as internal energy of the O2 product. By comparing our results with the trajectory studies of Farantos and Murrell, we favour a reaction mechanism, where the transition complex is planar containing an essentially linear OOCl group. In order to determine the possible influence of vibrationally excited ClO on other trace components of the atmosphere, especially the reaction ClO(v >0)+ O3, a rough estimate of the vibrational relaxation rate of ClO with the major atmospheric collision partner, N2, has been performed. A measurement of the vibrational distribution of ClO at different N2 pressures indicates a mean vibrational relaxation rate of .

Approach to characterize the sound field of pulse-excited ultrasonic sensors using a Laser-Doppler vibrometer

A new efficient approach to make visible sound fields in front of acoustic sensors in liquids will be presented. The goal is to investigate the sound field characteristic of real manufactured acoustic transmitters particularly by discontinuous excitation like pulse, burst, and other. Another point is the analysis of the influence of materials and sensor design on the final acoustic behavior. In comparison to the Schlieren technique it is possible to use moderate excitation of transmitters (only a few volts) and more variable signal shapes too. The experimental setup consists of a Laser Doppler vibrometer and a high-resolution three-dimensional positioning system. The control of the positioning system as well as the data acquisition and processing will be fully automatic realized by a personal computer. A software tool archives the data and calculates the complex sound field characteristic in dependence on time or place. The result is an interesting animation representing the real sound field characteristic of acoustic sensors. The advantages and limits of the approach and developed experimental setup will be described. Some examples will show the sound fields in front of cylindrical focusing transmitters. The influence of different excitation signal shapes on the final sound field will be discussed.

Mechanisms of optical gain in cubic gallium nitrite

We report on the mechanisms of optical gain in cubic GaN. Intensity-dependent gain spectra allow a distinction of the processes involved in providing optical amplification. For moderate excitation levels, the biexciton decay is responsible for a gain structure at 3.265 eV. With increasing excitation densities, gain is observed on the high energy side of the cubic band gap due to band filling processes. For the highest pump intensities, the electron-hole plasma is the dominant gain process. Gain values up to 210 cm−1 were obtained, indicating the high potential of cubic GaN for device applications. The observed gain mechanisms are similar to those of hexagonal GaN.

Mechanisms of Optical Gain in Cubic GaN and Ingan

AbstractThe epitaxial growth of zinc-blende (cubic) GaN and InGaN on GaAs with a common cleavage plane and readily high-quality, low-cost wafers may be considered as an alternative approach for the future realization of cleaved laser cavities. To obtain detailed information about the potential of cubic GaN and InGaN for device applications we performed optical gain spectroscopy accompanied by time-integrated and time-dependent photoluminescence measurements at 2 K and 300 K. From intensity-dependent gain measurements, the identification of the gain processes was possible. For moderate excitation levels, the biexciton decay is likely to be responsible for a gain structure at 3.265 eV in cubic GaN [10]. For the highest pump intensities, the electron- hole-plasma is the dominant gain process, providing gain values up to 200 cm-1. Furthermore cubic GaN samples with different cavity lengths from 250 to 600 mim were cleaved to investigate the influence of the sample geometry on the gain mechanisms. In these samples increased gain values up to 150 cm-1 as well as lower threshold excitation densities were observed, indicating the potential of cubic GaN for device applications. The results of GaN will be compared with intensity-dependent gain measurements on InGaN samples, grown on GaAs with varying In-content. The observed gain mechanisms in cubic InGaN will be discussed in detail.

Influence of carrier relaxation on the dynamics of stimulated emission in microcavity lasers

The influence of carrier relaxation on the emission dynamics of a semiconductor microcavity laser is investigated using femtosecond optical excitation. For moderate excitation intensities, the dynamics of the output laser pulse becomes significantly slower when the photon energy of the pump laser is tuned from the quantum well band-gap energy towards higher energies. Theoretical calculations reproduce this trend only if the interaction-induced dephasing of the polarization driven by the pump pulse, the formation, and relaxation of the nonequilibrium carrier distribution as well as the chirp of the excitation pulse are taken into account. Additionally, band-structure effects such as excitation of light holes influence the thermalization dynamics and lead to discontinuities in the general trend.

Neutrons in coincidence with fission of238U induced by stopped antiprotons

Neutron multiplicities after antiproton-induced fission have been determined as a function of the total mass of the fission fragments. Pre- and postscission neutron multiplicities of the reaction $\mathrm{p\ifmmode\bar\else\textasciimacron\fi{}}$+${}^{238}$U have been deduced using the moving-source parametrization of the neutron energy spectra by Maxwellian distributions. A postscission multiplicity of 6.3(6) neutrons out of 20.0(10) in total emitted neutrons in fission indicates that fission is a rather slow process. The increase of the evaporative prescission neutron multiplicity is about 80% of the total increase of evaporative neutrons as a measure of excitation energy. The postscission multiplicity gets only 20% of the total increase, thus indicating that the nucleus has only a moderate excitation energy at the moment of scission. The results are compared with standard cascade-evaporation calculations predicting a much lower number of evaporated prescission neutrons as the experiment shows. This is a strong indication of the importance of dissipative effects in the fission process. Application of dissipative effects like a saddle-to-scission delay time and nuclear viscosity in the model calculation describes the experimental results much better.

Recombination statistics involving inter-trap recombination

The usual Shockley - Read - Hall statistics is generalized to involve recombination between two different traps A and B in distributed non-interacting trap-pairs. Characteristics of inter-trap (A to B) and band-to-trap recombinations are numerically surveyed as regards their dependence on the thermal equilibrium Fermi level, the excitation intensity and the interaction between A and B. The inter-trap A to B recombination rate peaks if the Fermi level is near the energy level of trap A or B in the case of moderate excitation and moderate interaction. The inter-trap recombination rate saturates at characteristic values when these parameters become large. We find that for strong interaction between the traps the inter-trap recombination saturates. The reason is that the recombination between the upper trap and the valence band as well as the recombination between the lower trap and the conduction band can then be neglected. Saturation occurs also for strong excitation since then the occupation probabilities approach unity for the upper trap and zero for the lower trap.

Photoconductivity study of crescent-shaped GaAs/GaAlAs quantum wires grown by flow rate modulation epitaxy

We present photoconductivity data on GaAs quantum wires grown on a V-grooved substrate by flow rate modulation epitaxy. They show that a moderate excitation power density, ∼1 W/cm2, allows the observation of the absorption structure of a single GaAs quantum wire embedded in a p-i-n diode. Furthermore, by increasing the number of active wires inside a diode, the photoconductivity signal is enhanced and additional details of the absorption structure are evidenced. And, finally, a rough quantitative agreement is obtained between the experimental absorption transitions and a simple calculation of the one-dimensional excitonic states using the envelope function approximation.

Time-resolved photoluminescence investigations of cubic GaN layers and crystals up to room temperature

We compare the recombination dynamics of GaN single crystals to that of epitaxial cubic GaN layers on GaAs(001) using picosecond photoluminescence (PL) spectroscopy. At low temperatures, the band-edge PL decay time of the layer is much shorter than that of the crystals, evidencing the importance of nonradiative processes in the case of the layer. However, at room temperature both the emission spectra and their decay times of layer and crystals are almost identical. At 300 K the decay times are short (10 ps) for low excitation density and increase to values of 100 ps at moderate excitation density reflecting the saturation of nonradiative recombination channels.