Resonant Excitation Conditions Research Articles

Exciton Spin Dynamics in Self-Organized InAs/GaAs Quantum Dots

The spin dynamics in self-organized InAs/GaAs quantum dots have been studied by time-resolved photoluminescence performed under strictly resonant excitation conditions. It is demonstrated that the carrier spins in these nanostructures are totally frozen on the scale of the exciton lifetime.

Single molecule photo-bleaching observed by surface enhanced resonant Raman scattering (SERRS)

Single molecule photo-bleaching of Rhodamine 6G (RH6G) dye molecules is observed by surface enhanced resonant Raman scattering in gold (Au) colloids under double resonant excitation conditions. The effect is revealed through a breakdown in the normal exponential decay of the dye population at extremely low concentrations. Step-like structures in the decay of the Raman signals show that individual molecules are being destroyed as a function of time. This arrangement allows a study of photochemical degradation at single molecular level, and creates the possibility for the determination of absolute enhancement factors in the Raman cross-sections.

Ion parking during ion/ion reactions in electrodynamic ion traps.

Under appropriate ion density conditions, it is possible to selectively inhibit rates of ion/ion reactions in a quadrupole ion trap via the application of oscillatory voltages to one or more electrodes of the ion trap. The phenomenon is demonstrated using dipolar resonance excitation applied to the end-cap electrodes of a three-dimensional quadrupole ion trap. The application of a resonance excitation voltage tuned to inhibit the ion/ion reaction rate of a specific range of ion mass-to-charge ratios is referred to as "ion parking". The bases for rate inhibition are (i) an increase in the relative velocity of the ion/ion reaction pair, which reduces the cross section for ion/ion capture and, at least in some cases, (ii) reduction in the time of physical overlap of positively charged and negatively charged ion clouds. The efficiency and specificity of the ion parking experiment is highly dependent upon ion densities, trapping conditions, ion charge states, and resonance excitation conditions. The ion parking experiment is illustrated herein along with applications to the concentration of ions originally present over a range of charge states into a selected charge state and in the selection of a particular ion from a set of ions derived from a simple protein mixture.

Halogen Counter-Ion Effect on the Dissociation of Monosaccharide—iron Complexes Generated by Electrospray Ionization Combined with an Ion Trap Mass Spectrometer

A primary focus of research in our laboratory has been to determine the stereochemistry of four isomeric monosaccharides using the electrospray ionization/desorption mode combined with an ion trap mass spectrometer. α/β-Anomers of glucose, mannose, galactose and talose differ stereochemically at the C2 and C4 positions; previous work described an efficient system able to easily differentiate these four isomeric hexoses when combined with the iron(II)ion. This approach was based upon interaction between the sugar analyte and iron(II)yielding cationized [MFeIICl]+ monosaccharides which were then studied by sequential tandem mass spectrometry. The daughter ions of these clusters were produced using the resonant excitation mode and are characterized by variable rate constants related to the hydroxyl stereochemistry. When submitted to resonant excitation conditions, the [MFeIICl]+ cluster ions undergo processes that can be either stereospecific in certain cases or, more usually, kinetically controlled by stereochemical effects. It is, therefore, possible to distinguish stereoisomeric sugars by examining the variation in relative abundance of diagnostic fragment ions. The present work reports data on the influence of counter-ions associated with iron(II)(viz. FeCl2, FeBr2, FeI2 and FeF2) on desorbed species obtained by electrospray desorption/ionization. The cationized monomeric [MFeIIX]+ and dimeric [M2FeIIX]+ ions were studied by collision-induced dissociation (CID)to demonstrate a significant halogen effect from the resonant excitation experiments and on the extent of the differentiation of the diastereomers. The data presented demonstrate the influence of thermochemical properties (i.e. acidity and basicity), mainly related to the halide size, on the dissociation of cationic [MFeIIX]+ and [M2FeIIX]+ sugar–metal complexes in CID experiments. Only weak effects due to the halide were observed during desorption itself, when comparison of the latter cluster with the former shows a relatively enhanced desorption yield of the monomer with an iodide counter-ion. However, the more significant counter-ion effects concern the cationized dimeric species, which present spectacular product-ion abundance changes in CID spectra. In all cases, it has been possible to distinguish the diastereomers.

Internal symmetries in conical diffraction from metallic gratings

Previous investigations in conical diffraction have shown that for a metallic grating illuminated under conditions of resonant excitation of surface plasmons, the maximum efficiency of polarization conversion is obtained when the plane of incidence forms an angle φ = 45% with the main section of the grating. This fact has been understood as a consequence of the evident symmetry imposed by the incident wave and it has not been questioned up to now. Here we show that, apart from this highly symmetrical configuration, the polarization conversion efficiency has local maxima also in other cases which do not exhibit such an evident symmetry. We demonstrate that the occurrence of both the local maxima for the new configurations and the maximum corresponding to φ 45% are a consequence of internal symmetries imposed by the propagation direction of the resonantly excited surface plasmons.

Enhancement of Secondary Radiation of Chromophores Adsorbed on the Rough Surface of Thin Silver Films under Conditions of Resonance Excitation

The spectra of giant Raman scattering of light and surface‐enhanced fluorescence of certain chromophores absorbing light in the visible spectrum region are investigated. The factors that have an effect on the enhancement of secondary radiation of chromophores adsorbed on the surface of rough silver films under conditions of resonance excitation are determined. The spectra of mirror reflection of light by the substrates used are considered. The coefficients of diffuse and mirror reflection, absorption, and transmission of light by thin silver films at wavelengths of 488 and 633 nm are determined. The scheme of an experimental system for observation of the spectra of giant Raman scattering and surface‐enhanced fluorescence on the source side of a transparent substrate is proposed. Conclusions on the predominant contribution of the resonance excitation of surface plasmons to the enhancement of the signal of the giant Raman scattering and surface‐enhanced fluorescence of molecules adsorbed on the rough metal surface are drawn.

Full color light emission from amorphous SiCx:H with organic–inorganic structures

Hydrogenated amorphous silicon carbide films with an organic–inorganic compound structure were fabricated by using organic carbon source xylene (C8H10), in a traditional radio frequency plasma-enhanced chemical vapor deposition system. Strong full-color light emission ranging from 630 to 420 nm was observed at room temperature. The results of photoluminescence and photoluminescence excitation experiments showed the emission was originated from the radiative recombination at band tails of the inorganic SiC matrix. Moreover, luminescence from the transition between discrete energy levels, which were associated with organic π-conjugated carbon systems, was also observed at resonant excitation conditions.

Charge dependence of protonated insulin decompositions

Ion trap collisional activation is used to study the effects of charge state on protonated insulin decompositions for three forms of insulin: bovine, porcine, and human. Tandem mass spectrometry data are presented for ions with one to five protons dissociated under identical resonance excitation conditions. The (M+5H) 5+ and (M+4H) 4+ ions fragment exclusively by peptide bond cleavage of bonds outside the cycles formed by the disulfide linkages present in the insulin molecule, whereas the (M+3H) 3+ and (M+2H) 2+ ions appear to show a mixture of peptide bond cleavage and fragments arising from mechanisms associated with disulfide bond cleavage. The (M+H) + ion fragments almost exclusively by way of disulfide bond cleavage, with the only major exception being cleavage on the C-terminal side of glutamic acid residues external to the cycles formed by the disulfide linkages.

Continuous wave observation of massive polariton redistribution by stimulated scattering in semiconductor microcavities

A massive redistribution of the polariton occupancy to two specific wave vectors, zero and approximately 3.9x10(4) cm(-1), is observed under conditions of continuous wave excitation of a semiconductor microcavity. The "condensation" of the polaritons to the two specific states arises from stimulated scattering at final state occupancies of order unity. The stimulation phenomena, arising due to the bosonic character of the polariton quasiparticles, occur for conditions of resonant excitation of the lower polariton branch. High energy nonresonant excitation, as in most previous work, instead leads to conventional lasing in the vertical cavity structure.

Stereochemical Differentiation of Four Monosaccharides Using Transition Metal Complexes by Electrospray Ionization/Ion-Trap Mass Spectrometry

Several approaches may contribute to the structural elucidation of carbohydrates. The distinction of diastereoisomers by mass spectrometry is often a tedious task and, ultimately, the stereochemistry of monosaccharides must be determined. α/β anomers of glucose, mannose, galactose and talose stereochemically differ at the C2 and C4 positions and this work describes an efficient system used to differentiate four isomeric monosaccharides easily. The approach is based on mass spectrometry and it uses the gas-phase interaction between a sugar molecule and the iron(II) transition metal ion for differentiation of the species. Electrospray ionization allows the generation of cationized [M + FeCl]+ monosaccharides. Such ions have been prepared in high abundance in the external source of an electrospray ionization/ion-trap mass spectrometer instrument. Resonant excitation of these selected ions gives rise to dissociative collisions. Very likely, the strong interaction of ironII with nucleophilic sites (i.e. hydroxyl groups) seems to favor regioselective and stereoselective ironII ion attachment to each epimer. Consequently, when submitted to resonant excitation conditions, the cluster [M + FeCl]+ ions undergo fragmentations that can either be stereospecific in certain cases or, more usually, kinetically orientated (i.e. stereochemical effects on the rate constants of competitive unimolecular processes). Thus, by scrutinizing the variation in the relative abundances of diagnostic fragment ions, it is possible to distinguish stereoisomeric sugars. The origin of some product ions was proved by MS2 and MS n experiments through 13C and 2H-labeling that may enlighten the mechanisms.

Excitonic coherent gain induced by giant Zeeman splitting in Cd 1− xMn xTe quantum wells

We have observed that anomalously large optical gain emerges at the spectral position of the lowest heavy-hole exciton transition in Cd 1− x Mn x Te/Cd 1− y Mg y Te quantum wells ( x=0.02, y=0.33) with the resonant excitation condition by 200 fs laser pulse at 7 T and 5 K. The gain is enhanced by increasing the applied field. The maximum gain value with the injection energy of 6.0 μJ/cm 2/pulse reaches 3.5×10 4 cm −1 which corresponds to 13 of the linear absorption maximum of the exciton transition at 7 T. Further, the gain grows rapidly along with the injection of pump pulse and relaxes with the decay time less than 1 ps. These experimental results suggest that the gain is induced by the optical polarization of excitons which is created by the optical field of pump pulse and also that gain relaxation is connected to the dephasing time of the exciton coherence. Further, we have observed that the exciton absorption band splits into three bands before the gain appears, indicating the splitting is induced by the optical Stark effect. The results are discussed in terms of the dressed-atom model.

Ultrafast excitonic optical gain induced by giant-Zeeman splitting in Cd 1− xMn xTe quantum wells

We observed anomalously large optical gain at the spectral region of the lowest heavy-hole exciton transition in Cd 1− x Mn x Te/ Cd 1− y Mg y Te quantum wells ( x=0.022, y=0.33 ) with the resonant excitation condition by 200 fs laser pulse at 7 T and 5 K. The gain is enhanced with increasing magnetic field. The maximum gain value with the injection energy of 6.0 μJ/cm 2/pulse reaches 3.5×10 4 cm −1 which corresponds to 1 3 of the linear absorption maximum of the exciton transition at 7 T. Further, the gain grows rapidly along with the injection of pump pulse and relaxes with decay times of 0.18 and 0.98 ps. The gain induced by the magnetic field is always observed at the spectral position of the lowest exciton transition. These experimental results suggest that the gain is induced by the optical polarization of exciton which is created by the optical field of pump pulse and gain relaxation is connected to the dephasing time of the exciton coherence.

Collision-induced dissociation breakdown surfaces for n-alkylbenzene molecular ions in a quadrupole ion trap mass spectrometer

Complete collision-induced dissociation (CID) breakdown surfaces showing the 91 +/92 + ratios arising from the molecular ions (M +) of n-butylbenzene, n-pentylbenzene, n-hexylbenzene, and n-octylbenzene have been obtained on a quadrupole ion trap mass spectrometer (QITMS). The 91 +/92 + ratios are plotted as a function of both resonant excitation time and voltage at a q z (M +) = 0.300 and at a constant He buffer gas pressure of 1.0 × 10 −4 Torr (uncorrected ion gauge reading). Because the 91 +/92 + ratios reflect the average internal energy of the M + ions, the surfaces provide a complete map of the change in ion energy over a wide range of resonant excitation conditions. Comparisons of the 91 +/92 + ratios of the n-alkylbenzenes with those obtained using charge-exchange mass spectrometry (CEMS) indicate that the amount of internal energy deposited upon resonant excitation varies from ∼1.5 eV for the M + ion of n-butylbenzene using 100 mV excitation, to ∼7.5 eV for the M + ion of n-octylbenzene using 500 mV excitation. Moreover, calculations of the amount of internal energy deposited, as well as the percent energy transferred (% E), are found to increase as the size of the n-alkyl group of the M + ion is increased, paralleling the trend seen for the same M + ions following low-energy CID in a triple quadrupole mass spectrometer (TQMS) under single-collision conditions. This trend in the % E as a function of the size of the n-alkyl group parallels that found for the low-energy CID studies performed on a triple quadrupole mass spectrometer and thus indicates that the mechanism of CID in the QITMS is similar to that thought to occur in rf-only quadrupole collision cells.

Carrier dynamics and recombination processes of charged excitons in a GaAs/AlGaAs quantum well

We report a time-resolved photoluminescence study of neutral (X) and negatively charged (X −) excitons in GaAs/AlGaAs quantum wells under resonant excitation conditions. For X the temporal evolution of luminescence is dominated by three decays attributed to resonant Rayleigh scattering, resonantly created cold excitons, and an exciton population thermalized to the lattice temperature. By contrast, for X − we find only one temperature-dependent decay. We discuss these results in terms of the polariton description of radiative recombination.

Deep Ultraviolet Raman Scattering for the Monitoring of High-Temperature Processing of AlGaN

Deep ultraviolet micro-Raman scattering was employed to monitor high-temperature processing of AlGaN films under resonant excitation conditions, giving rise to enhanced first and second-order Raman scattering. High-temperature treatments at 1100 °C result in changes in the second-order Raman scattering signal and monitor the emergence of microscopic defects during the high-temperature processing. The second-order Raman spectrum was analyzed to gain insight into the AlGaN phonon density of states. For annealing temperatures higher than 1150 °C, the Al0.72Ga0.28N film decomposes: a low- and a high-aluminum composition AlxGa1—xN phase emerge. At 1100 °C, prior to the Al0.72Ga0.28N decomposition, deep UV Raman scattering detects the built-up of strain in the Al0.72Ga0.28N film.

Optical nutation at a Raman-active transition

Optical nutation at the Raman-active transition 6P1/2−6P3/2 of thallium atoms (ω R /2πc=7793 cm −1) under resonant Raman excitation by a biharmonic picosecond pulsed field, giving rise to substantial motion of the population, is detected. Optical nutation appears as an oscillatory behavior of the energy of the anti-Stokes scattering of probe pulses, which follow with a fixed delay, as a function of the product of the energies of the excitation pulses. As a result of the dynamic Stark effect, which decreases the frequency of the transition under study, resonance excitation conditions are satisfied for negative initial detunings of the Raman excitation frequency from resonance. The Raman scattering cross section for the transition under study is estimated by comparing the experimental data with the calculations.

Ion internal temperature and ion trap collisional activation: protonated leucine enkephalin

Protonated leucine enkephalin has been used as a prototypical high-mass ion to yield a quantitative estimate of the relationship between the amplitude of the resonance excitation voltage used in an ion trap collisional activation experiment, and the internal temperature to which an ion can be elevated over the bath gas temperature. The approach involves the measurement of the ion dissociation rate as a function of resonance excitation voltage, and the correlation of dissociation rate with ion internal temperature. The relatively high ion trap dissociation rates observed under typical resonance excitation conditions preclude the direct application of the Arrhenius equation to derive internal temperatures. An empirical determination of the relationship between ion internal temperature and dissociation rate over the rate range of interest here was made via the systematic variation of bath gas temperature. The data suggest a very nearly linear relationship between ion internal temperature and resonance excitation voltage, at least under conditions in which ion ejection is minimal. It is shown that protonated leucine enkephalin ions can be elevated by about 357 K over the bath gas temperature using a monopolar resonance excitation voltage of 540 mV p − p( q z = 0.163) without significant ion ejection. It is also demonstrated that ion internal temperature can be readily increased by increasing the bath gas temperature, by accelerating the ions in the presence of a room temperature bath gas (i.e. conventional ion trap collisional activation), or by a combination of the two approaches.

Magnetization of FeBO3 by microwave pumping

Changes in the magnetic moment of crystalline FeBO3 when an antiferromagnetic resonance is excited in it are studied. This done using a SQUID magnetometer in combination with a microwave spectrometer. At temperatures T>15 K a reduction in the total magnetic moment of the sample is observed when an antiferromagnetic resonance is excited in it. At liquid helium temperatures, an induced rise in the sample magnetic moment was observed. This type of magnetization of the sample can be explained qualitatively if it is assumed that under antiferromagnetic resonance excitation conditions, magnetoelastic modes with high wave numbers are excited along with magnons. Efficient excitation of magnetoelastic modes under antiferromagnetic resonance conditions is confirmed by the experimental observation of a size effect in thin, high quality single crystal slabs of FeBO3.

Excitonic absorption of GaN epilayers on sapphire: Dynamics, intensity, and temperature dependence

Using test-pump measurements, we studied the excitonic absorption of GaN epilayers on sapphire substrate under nanosecond and picosecond pulsed excitation conditions. We measured a short exciton lifetime of 25 ps, which is consistent with results obtained from degenerate four-wave mixing measurements. Since the radiative lifetime should be much longer, this short characteristic time shows the dominance of nonradiative processes. Under nonresonant excitation condition, we observed an intensity dependent shift and damping of the exciton resonance, attributed to an inhomogeneous temperature distribution. This thermal effect was not observed under resonant excitation conditions. This behavior is tentatively explained by the presence of shallow centers that have a rather long lifetime and recombine nonradiatively.

Cavityless optical bistability in a thin semiconductor film upon the resonant excitation of excitons and biexcitons

The bistable properties of a thin semiconductor film under conditions of resonant excitation of coherent excitons and biexcitons by photons in one or two different pulses are studied theoretically.