Sub-nanosecond Resolution Research Articles

Luminescence of F and F + centers in corundum upon excitation in the interval from 4 to [formula omitted

Time-resolved excitation spectra of luminescence from F +- and F-centers in α-Al 2O 3 were analyzed with subnanosecond resolution in the interval from 4 to 40 eV at different orientations of the C 3 optical axis relative to the intensity vector of an electric field E of the exciting light. A new excited state of the F +-center near 6 eV was examined. The time-resolved excitation spectra had a doublet structure at the edge of the fundamental absorption. The structure was interpreted as the manifestation of a Frenkel exciton. It was found that the luminescence decay kinetics of F +-centers changed considerably with increasing excitation energy ( E⩾ E g, where E g is the band gap energy).

Self-trapped excitons in LiB3O5 and Li2B4O7 lithium borates: Time-resolved low-temperature luminescence VUV spectroscopy

Results are reported of a coordinated investigation of the dynamics of electronic excitations in LiB3O5 and Li2B4O7 crystals by low-temperature luminescence VUV spectroscopy performed with subnano-second resolution under synchrotron photoexcitation. Data on the photoluminescence (PL) decay kinetics, time-resolved PL and PL excitation spectra, and reflectance spectra obtained at 295 and 9.6 K are reported for the first time; the PL of the borates in the 3.5-eV region caused by radiative annihilation of self-trapped excitons (STE) has been established to have an intrinsic nature; the σ and π STE luminescence bands originating from singlet and triplet radiative transitions have been isolated; the shift of the STE σ band relative to the π band has been interpreted; the LBO recombination luminescence band has been isolated; and the creation and decay channels of relaxed and unrelaxed excitons in lithium borates are discussed.

The need of sub-nanosecond resolution to reveal new features during laser induced solidification

This work reports the achievement of real-time optical reflectivity measurements with sub-nanosecond time resolution during pulsed laser induced structural transformations. The experimental set-up uses a streak camera and provides both, excellent time resolution in single exposure and high versatility. This is illustrated through the results obtained upon picosecond laser pulse melting of thin amorphous Ge films, which leads to processes that are completed in a time window of a few tens of nanoseconds. By means of a novel set-up we could optically resolve film recalescence after solidification and demonstrate the presence of surface initiated solidification. The fact that both processes could not be resolved by measurements performed with nanosecond resolution confirms the need of sub-nanosecond resolution.

Time Resolved Electron Motion on Isolated Polymer Chains in Solution

Using a novel development of the Auston switch technique, ultrafast intramolecular photocurrents are for the first time directly observed on a collection of isolated polymer chains in solution with subnanosecond resolution. The polydiacetylenes used may undergo conformational changes with temperature which have a profound effect on the ability of the polymer to transport charge. The carrier range on the chain is obtained and the carrier velocity deduced from the shape of the transient.

Analysis of the fast recovery dynamics of a semiconductor laser with feedback in the low-frequency fluctuation regime

We experimentally investigate the temporal evolution of the power of an external cavity semiconductor laser in the low-frequency fluctuation regime with subnanosecond resolution. We show, for the first time to our knowledge, that generally the laser power drops to a value significantly different from the solitary laser power. We demonstrate the analogy between the recovery of the laser intensity and the turn-on transient of a semiconductor laser.

Surface photovoltage in semiconductors under pulsed optical excitation, and its relevance to synchrotron radiation spectroscopy

Abstract We used the SuperACO Free Electron Laser (FEL) to excite photocarriers in semiconductors at room temperature, and synchrotron radiation photoemission spectroscopy to measure the consequently induced surface photovoltage (SPV). The natural synchronization between the two sources allowed us to study with sub-nanosecond resolution the temporal dependence of the non-equilibrium charge carrier distribution for two prototype systems, Ag/GaAs(110) and Si(111)2×1. We found that to correctly interpret the effects of the SPV on the band position, the interplay between instrumental factors (pulse duration and repetition rate) and semiconductor parameters (such as surface and bulk recombination rates) must be taken into account. Since the FEL has the typical temporal structure of a synchrotron radiation source and a flux comparable to the one obtainable at third generation storage rings, these results are of relevance to the most advanced spectroscopic techniques used to study the electronic structure of semiconductors; therefore, in this paper we discuss the possible effects of intense synchrotron radiation beams on position and lineshape of photoemission features at semiconductor surfaces.

Two color experiments combining Free Electron Laser and Synchrotron Radiation

We present the results of the first two color experiment coupling FEL and Synchrotron Radiation, where the SuperACO FEL was used to photoexcite carriers at semiconductor surfaces and the consequently induced surface photovoltage was studied with soft X-ray photoemission spectroscopy. Thanks to the subnanosecond temporal resolution and high surface sensitivity of our experimental setup, we were able to measure for the first time the effect of surface states on band bending dynamics. Based on our direct experience during these experiments, and on the future developments related to other two color experiments proposed on SuperACO, we will discuss the advantages and the technical aspects of this mode of operating the storage ring and the FEL.

Surface states and space charge layer dynamics on Si(111)2×1: A free electron laser-synchrotron radiation study

Combining the use of a UV storage ring free electron laser and of synchrotron radiation, a time resolved core level spectroscopy study has been performed on photoexcited Si(111)2×1 surfaces with subnanosecond resolution. This enabled us to measure band bending fluctuations, caused by surface carrier dynamics, during the first nanosecond after photoexcitation; differences in the Si2p core level lineshape, dependent on the pump-probe time delay, were also observed. The presence of defects was found to reduce the fluctuations and make the carrier recombination process faster.

Simultaneous measurement of the transient photoconductivity and photoluminescence in Alq 3

We present a unique and simple technique which allows one to measure the transient photoluminescence (PL) and transient photoconductivity (PC) of thin film materials simultaneously. This technique will be useful in characterizing thin film materials that have a relatively high PL quantum yield, such as those used in EL devices (i.e. metal chelates, polymers, dyepolymer composite, etc.). The photogeneration of charge carriers and radiative excited states in pristine tris-(8-hyaroxyquinoline) aluminum (Alq 3) were studied by measuring both its photoconductive and photoluminescence response simultaneously, with subnanosecond resolution. 25 psec laser pulses at 355nm were used for excitation of an Auston switch with a 2–4 μm thick film of Alq 3 vacuum-evaporated across the active region. Quantitative measurements of the dependence on laser intensity, and applied electric field were performed. A scheme for the photoprocess in Alq3 is presented.

Photooxidation effects on picosecond photoluminescence and photoconductivity in tris-(8-hydroxyquinoline) aluminum (Alq3).

The photogeneration of radiative excited states and charge carriers in pristine and photooxidized tris-(8-hydroxyquinoline) aluminum (Alq 3) was studied by measuring both its photoluminescence and photoconductive response simultaneously, with subnanosecond resolution. 25 psec laser pulses at 355nm were used for excitation of vacuum evaporated 2–4 μm thick films of Alq 3. We observed that photooxidation of Alq 3 with oxygen and UV light substantially decreases the fluorescence lifetime and quantum yield as well as the photoconductive response. This decrease is shown to be a result of quenching of singlet molecular excitations by products of photooxidation of Alq 3.

Observation of the Temperature Dependence of the Dynamics of Photoexcited States in Pristine Tris(8-Hydroxyquinoline) Aluminum (Alq3)

AbstractWe have investigated the temperature dependence of the dynamics of radiative excited states in pristine thin films of tris(8-hydroxyquinoline) aluminum (Alq3). By measuring the transient photoluminescence (PL) response with subnanosecond resolution, our results revealed an increase in the radiative excited state lifetime and fluorescence quantum yield with decreasing temperature from 300K to 77K. At low temperature we observed a decrease in the bimolecular recombination rate constant, singlet exciton diffusion coefficient and diffusion length. A singlet exciton trapping model is used to explain these results.

An integrated CMOS time interval measurement system with subnanosecond resolution for the WA-98 calorimeter

The time interval measurement system of the WA-98 calorimeter is presented. This system consists of a constant fraction discriminator (CFD), a variable delay circuit, a time-to-amplitude converter (TAC), and a Wilkinson analog-to-digital converter (ADC) all realized in a 1.2-/spl mu/m N-well CMOS process. These circuits measured the time interval between a reference logic signal and a photomultiplier tube (PMT) signal that had amplitude variations of 100:1 and 10-ns rise and fall times. The system operated over the interval range from 2 ns to 200 ns with a resolution of /spl sim//spl plusmn/300 ps including all walk and jitter components. The variable delay circuit allowed the CFD output to be delayed by up to 1 /spl mu/s with a jitter component of /spl sim/0.04% of the delay setting. These circuits operated with a 5-V power supply. Although this application was in nuclear physics instrumentation, these circuits could also be useful in other scientific measurements, medical imaging, automatic test equipment, ranging systems, and industrial electronics.

Nuclear Resonance Beamline at ESRF

The Nuclear Resonance Beamline at ESRF is dedicated to the excitation of nuclear levels by synchrotron radiation. The sources of radiation and optical elements are optimized to provide an intense, highly monochromatic, collimated and stable X-ray beam of small cross-section at the Mossbauer transition energies between 6 and 30 keV. The set-up of the beamline allows to perform studies in diffraction, small angle scattering, forward scattering and incoherent scattering. Equipment is available to maintain the sample at variable temperature and magnetic field. Fast detectors and timing electronics serve to separate the delayed nuclear scattering from the “prompt” electronic scattering and to measure the time spectra of nuclear radiation with sub-nanosecond resolution. The general lay-out and the parameters of the beamline are reported. Typical domains of applications are discussed and illustrated by first experimental results.

Time-resolved confocal fluorescence microscopy of porphyrins for phototherapy

The application of a novel time-resolved confocal fluorescence microspectrometer to studies of the distribution and speciation of porphyrin photosensitizers in rat C6 cerebral glioma cells is described. The instrument combines a mode-locked argon ion laser excitation source with time-correlated single photon counting fluorescence detection and has sub-micron spatial and sub-nanosecond temporal resolution. The porphyrins studied were haematoporphyrin derivative (HpD), haematoporphyrin IX (HP), porphyrinc (Pc) and the tetrakiscarborane carboxylate ester of 2,4-(α,β-dihydroxyethyl) deuteroporphyrin IX (BOPP). From the heterogeneous emission observed in vitro, assignments and spatial location of various porphyrin species are proposed.

High-energy passive-ionization electrons and holes in dielectrics with pulsed irradiation by high-density electron beams

This article is a survey of works by the author and colleagues on the investigation of charging and discharging dynamics in solid dielectrics exposed to dense electron beams with subnanosecond resolution. Small high-current electron accelerators of theDzhin type, which were developed and fabricated at the Nonlinear Physics Laboratory, were used as the source of the primary electron beam. The primary electron beam parameters were: 0.25–0.45 MeV, 1–30 nsec, 0.1–10,000 A/cm3. The dielectric is investigated experimentally with its surface covered by a metallic electrode and with critical electron emission into the vacuum eliminated. In this case, the total current in the dielectric consists of three components: the primary beam current, the displacement current, and the conduction current. The first and last are responsible for charging and discharging of the dielectric volume. It is shown that the bulk nonequilibrium radiation conduction mechanism depends greatly on the dose intensity. For small dose intensities, the principal current carriers are the low-energy electrons of the conduction band and holes of the valence band, which are in quasi-equilibrium with the lattice phonon field before capture by defects or merging into excitons preceding recombination. This type of conduction has been well studied in the physics of semiconductors and dielectrics. However, over a broad interval of intermediate and high dose intensities, another type of nonequilibrium conduction dominates — the high-energy, which was discovered and studied by the author and colleagues. The principal carriers are then passive-ionization electrons and holes with energies of 0.1–10 eV in the process of phonon relaxation in which phonon emission dominates absorption. The high-energy conduction differs considerably from the low-energy in many properties, which determines the unusual dynamics of dielectric charging and discharging with irradiation by the dense electron beam of a high-current accelerator.

Nuclear-resonance beamline at ESRF

The Nuclear-Resonance Beamline at ESRF is dedicated to the excitation of nuclear levels by synchrotron radiation. The source of radiation and optical elements are optimized to provide an intense, highly monochromatic, collimated and stable X-ray beam of small cross-section at the Mossbauer transition energies between 6 keV and 30 keV. The set-up of the beamline allows to perform studies in diffraction, small-angle scattering, forward scattering and incoherent scattering. Equipment is available to maintain the sample at variable temperature and magnetic field. Fast detectors and timing electronics serve to separate the delayed nuclear scattering from the «prompt» electronic scattering and to measure the time spectra of nuclear radiation with sub-nanosecond resolution. The general layout and the parameters of the beamline are reported. Typical domains of applications are discussed and illustrated by first experimental results.

Electron tunnelling rates as a function of intermolecular distance, measured in a Langmuir-Blodgett assembly

Results are presented that demonstrate an exponential dependence of the electron tunnelling rate on interlayer separation. Direct measurements of transient photocurrents with sub-nanosecond resolution have been made on a series of Langmuir-Blodgett multilayer structures assembled from two amphiphilic bis-phthalocyanine molecules. Using either homostructures of the two molecules or a heterodimer structure incorporating both molecules, three tunnelling gaps were attainable. The attempt rate, v 0, as determined by Franck-Condon factors, and the well depth in the three structures was identical for each, being determined by the phthalocyanine ring structure and electron affinity. It is thus possible to relate any change in measured tunnelling rate with the change in barrier width, b. The tunnelling rate, k ⊥, is related to the tunnelling barrier width by: k ⊥=v 0(E)exp( −2√2mA bh b) where A is the well depth (electron affinity of the conjugated rings) and m is an effective mass for the tunnelling carrier. A plot of ln( k ⊥) vs. b reveals a straight line whose gradient is −2 √2mA bh which intercepts the k ⊥ axis at k ⊥ = v 0( E). From the data a value of A = 2.4 eV and v 0 = 7 × 10 16 Hz at an electric field of 2 × 10 8 V m −1 are found. The well depth is in good agreement with expectations. The attempt rate is high but there is a large uncertainty in this value and it is in overall agreement with expectations. v 0 is expected to depend on electric field and this is the subject of a further study.

Laser Fluorometry Using a Visible Semiconductor Laser and an Avalanche Photodiode for Capillary Electrophoresis

A laser-induced fluorescence detection system for capillary electrophoresis using a visible semiconductor laser and an avalanche photodiode has been developed. These solid-state devices offer both reliability and compactness for instrumental design. A time-resolved photon-counting method with a subnanosecond resolution has reduced the large dark counts of the avalanche photodiode and the contribution from scattered light. The baseline fluctuation of the electropherogram is due to the shot noise of the avalanche photodiode. The detection limit (S/N=2) for a model dye compound, Oxazine 725, is 2.1 fmol as an injection amount. This value is comparable to that of the conventional laser fluorometry system with a gas laser and a photomultiplier.

Orthogonal strains and onset of plasticity in shocked LiF crystals.

We report experimental determinations of the elastic component of the transverse strain during passage of a shockwave. A 1 ns laser pulse drove an \ensuremath{\approxeq}60 kbar shock into an aluminum coated, radiation hardened, pure LiF single crystal. By diffracting a 0.25 ns pulse of x rays from a nearby high brightness, time-synchronized laser plasma, we measured the simultaneous compression of the lattice both parallel and transverse to the shock propagation direction with subnanosecond resolution. Definite compression was recorded in the inertially confined transverse direction, indicating the onset of plastic lattice response. Peak strains were of comparable magnitude in the orthogonal directions, but were not equal.

Nanosecond time-resolved circular polarization of fluorescence: study of NADH bound to horse liver alcohol dehydrogenase.

Circularly polarized luminescence (CPL) spectroscopy provides information on the excited-state chirality of a lumiphore analogous but complementary to information regarding the ground-state chirality derived from circular dichroism. The sensitivity of CPL spectra to molecular conformation makes this technique uniquely suited for the study of biomolecular structure, as extensively demonstrated in earlier studies. Unfortunately, the CPL spectra of many biomolecules often contain significantly overlapping contributions from emitting species either because multiple lumiphores are present (e.g., tryptophan residues in a protein) or because multiple conformations of the biomolecule simultaneously exist, each with a unique CPL spectrum. Increased resolution between individual contributions to the CPL may be achieved by time-resolving this signal, thus taking advantage of the fact that, as a rule, each of the emitting species also has a characteristic decay time associated with its electronically excited state. In addition, the time resolution provides information regarding dynamics associated with the different chiral states of the system. The present study describes an instrument for the determination of time-resolved CPL (TR-CPL) with subnanosecond resolution and its application to several chiral systems. The technique was first demonstrated on a model system with a strong time-dependent CPL signal. Subsequently, the circularly polarized component in the fluorescence of reduced nicotinamide adenine dinucleotide (NADH) bound to liver alcohol dehydrogenase was time-resolved. The CPL of NADH in the binary enzyme-coenzyme complex is time-dependent, reflecting structural differences around the reduced nicotinamide possibly due to a dynamic restructuring. In contrast, the CPL of the coenzyme in the ternary complex formed with enzyme and the substrate analog isobutyramide is essentially time-independent, likely reflecting a more rigid binding domain. Since the linear polarization of the fluorescence of the two complexes did not show any local flexibility of the NADH chromophore, the excited-state conformational rearrangement of the binary complex indicates a subtle change in its interactions with group(s) in direct contact with it.