Time-correlated Photon Counting Technique Research Articles

Time-Correlated Photon Counting for Estimating the Duration of Na-Line and Continuum Emission Flashes in the Spectra of Multi-Bubble Sonoluminescence

The time-correlated photon counting technique using software is used to estimate the duration and succession of Na-line and continuum emission flashes upon irradiation of aqueous solutions of NaCl and Na dodecyl sulfate with ultrasound at a frequency of 20 kHz. The duration of Na flashes was determined as 5.1, 8.0, and 11.7 ns for 5 M NaCl, 0.5 M NaCl, and 5 mM Na dodecyl sulfate solutions, respectively. The duration of a continuum emission flash varied insignificantly within 2.1–2.9 ns for all solutions. The close correlation revealed between the Na-line and continuum emission flashes allows us to conclude that bubbles producing both types of flashes emit them simultaneously with a measured delay of ~0.5 ns for Na.

Realization of a time-correlated photon counting technique for fluorescence analysis.

An ultralow level light detection module, the time-correlated photon counter, is proposed and evaluated for fluorescence analysis. The time-correlated photon counter employs a silicon photomultiplier as a photon counting sensor in conjunction with a Poisson statistics algorithm and a double time windows technique, and therefore it can accurately count the photon number. The time-correlated photon counter is compatible with the time-correlated single photon counting technique and can record the arrival time of very faint light signals. This low-cost and compact instrument was used to analyze the intensity and lifetime of fluorescein isothiocyanate; a limit of detection of 16 pg/ml with a large linear dynamic range from 2.86 pg/ml to 0.5 µg/ml was obtained, and the lifetime of fluorescein isothiocyanate was measured to be 3.758 ns, which agrees well with the results of a sophisticated commercial fluorescence analysis instrument. The time-correlated photon counter may be useful in applications such as point-of-care testing.

Time-resolved double-resonance spectroscopy: Lifetime measurement of the electronic state of molecular sodium.

We report on the lifetime measurement of the state of Na2 molecules, produced in a heat-pipe oven, using a time-resolved spectroscopic technique. The level was populated by two-step two-color double resonance excitation via the intermediate state. The excitation scheme was done using two synchronized pulsed dye lasers pumped by a Nd:YAG laser operating at the second harmonics. The fluorescence emitted upon decay to the final state was measured using a time-correlated photon counting technique, as a function of argon pressure. From this, the radiative lifetime was extracted by extrapolating the plot to collision-free zero pressure. We also report the calculated radiative lifetimes of the ro-vibrational levels in the range of = 0-200 with J = 1 and J = 31 using the LEVEL program for bound-bound and the BCONT program for bound-free transitions. Our calculations reveal the importance of the bound-free transitions on the lifetime calculations and a large difference of about a factor of three between the J = 1 and J = 31 for the = 40 and = 100, respectively, due to the wavefunction alternating between having predominantly inner and outer well amplitude.

Investigating protein-protein interactions in the plant endomembrane system using multiphoton-induced FRET-FLIM.

Real-time noninvasive fluorescence-based protein assays enable a direct access to study interactions in their natural environment and hence overcome the limitations of other methods that rely on invasive cell disruption techniques. The determination of Förster resonance energy transfer (FRET) by means of fluorescence lifetime imaging microscopy (FLIM) is currently the most advanced method to observe protein-protein interactions at nanometer resolution inside single living cells and in real-time. In the FRET-FLIM approach, the information gained using steady-state FRET between interacting proteins is considerably improved by monitoring changes in the excited-state lifetime of the donor fluorophore where its quenching in the presence of the acceptor is evidence for a direct physical interaction. The combination of confocal laser scanning microscopy with the sensitive advanced technique of time-correlated single photon counting allows the mapping of the spatial distribution of fluorescence lifetimes inside living cells on a pixel-by-pixel basis that is the same as the fluorescence image. Moreover, the use of multiphoton excitation particularly for plant cells provides further advantages such as reduced phototoxicity and photobleaching. In this protocol, we briefly describe the instrumentation and experimental design to study protein interactions within the plant endomembrane system, with a focus on the imaging of plant cells expressing fluorescent proteins and acquisition and analysis of fluorescence lifetime resolved data.

Photosynthetic Antenna−Reaction Center Mimicry: Sequential Energy- and Electron Transfer in a Self-assembled Supramolecular Triad Composed of Boron Dipyrrin, Zinc Porphyrin and Fullerene

A self-assembled supramolecular triad, a model to mimic the photochemical events of photosynthetic antenna-reaction center, viz., sequential energy and electron transfer, has been newly constructed and studied. Boron dipyrrin, zinc porphyrin, and fullerene respectively constitute the energy donor, electron donor, and electron acceptor segments of the antenna-reaction center mimicry. For the construction, first, boron dipyrrin was covalently attached to a zinc porphyrin entity bearing a benzo-18-crown-6 host segment at the opposite end of the porphyrin ring. Next, an alkyl ammonium functionalized fullerene was used to self-assemble the crown ether entity via ion-dipole interactions. The newly formed supramolecular triad was fully characterized by spectroscopic, computational, and electrochemical methods. Selective excitation of the boron dipyrrin moiety in the dyad resulted in energy transfer over 97% efficiency creating singlet excited zinc porphyrin. The rate of energy transfer from the decay measurements of time-correlated singlet photon counting (TCSPC) and up-conversion techniques agreed well with that obtained by the pump-probe technique and revealed efficient photoinduced energy transfer in the dyad (time constant in the order of 10-60 ps depending upon the conformer). Upon forming the supramolecular triad by self-assembling fullerene, the excited zinc porphyrin resulted in electron transfer to the coordinated fullerene yielding a charge-separated state, thus mimicking the antenna-reaction center functionalities of photosynthesis. Nanosecond transient absorption studies yielded a lifetime of the charge-separated state to be 23 micros indicating charge stabilization in the supramolecular triad. The present supramolecular system represents a successful model to mimic the rather complex "combined antenna-reaction center" events of photosynthesis.

Femtosecond Fluorescence Studies of Self-Assembled Helical Aggregates in Solution

For the diamino-bipyridine based C(3)-symmetrical disk molecule, TAB, (sub)picosecond fluorescence transients have been observed by means of femtosecond fluorescence upconversion and picosecond time-correlated photon counting techniques. The dodecyl peripheral side chains of the synthetic compound are large enough to allow, in apolar solvents, self-assembling of the discotic molecules to helical aggregates. In polar solvents, the hydrogen bonding and pi-pi interactions pertaining to the chiral aggregation are compensated by solvation and self-assembling of the disklike molecules is disrupted. For comparison, time-resolved fluorescence measurements have been performed for the subgroup molecule, DAC, which is the (asymmetric) building block for TAB. It is concluded that, after pulsed photoexcitation, TAB and DAC exhibit excited-state intramolecular double proton transfer (ESIDPT) with a typical time of approximately 200-300 fs, irrespective of the degree of aggregation. Picosecond components in the fluorescence of TAB and DAC, ranging from 3 to 25 ps, are representative of vibrational cooling effects in the excited product state. Only aggregated TAB shows a rapid ( approximately 1 ps) decay of its fluorescence anisotropy. This component is attributed to excited-state energy transfer within the aggregate. Finally, the excited-state lifetime of TAB in the aggregated form is found to be an order of magnitude longer than that for TAB in its nonaggregated form. It is inferred that aggregation diminishes the influence of low-frequency twisting motions in the radiationless decay of the excited state.

Surface Acoustic Wave-Induced Electroluminescence Intensity Oscillation in Planar Light-Emitting Devices

AbstractElectroluminescence (EL) emission controlled by means of surface acoustic waves (SAWs) in planar light-emitting diodes (pLEDs) is demonstrated. Interdigital transducers (IDTs) for SAW generation were integrated onto pLEDs fabricated following a scheme compatible with SAW propagation [1]. EL in presence of SAW was studied by time-correlated photon-counting techniques. We found intensity oscillation at the SAW frequency (˜1 GHz) demonstrating electron injection into the p-type region synchronous with the SAW wavefronts.

Luminescence induced by swift atomic ions in samples with nanoscale dimensions

Thin self-supporting foils with nanoscale dimensions (thickness: 40–1000 nm) consisting of a mixture of the polymer Formvar and the luminescent dye POPOP were irradiated with 12 C and 32 S atomic ions at energies from 2 to 36 MeV and 10 to 55 MeV, respectively. The relative luminescence yield originating from the impact of these ions was measured by applying the technique of time-correlated single photon counting. The 32 S induced yield increases nearly linearly with increasing ion energy in the whole energy range studied, whereas a constant value after an increase up to approximately 25 MeV is reached for the 12 C induced yield. In the energy range above the Bragg-maximum of the ion energy loss, the observed yield can be reproduced by the application of a model based on the energy density resulting from the secondary electrons which are set free in binary collisions with the projectiles. The luminescence yield measured at constant projectile energy increases linearly with foil thickness if the latter exceeds sufficiently the range of secondary electrons which are able to leave the sample. For thinner foils, a reduced luminescence yield was observed due to the larger fraction of secondary electrons which leaves the sample. The result is a smaller energy density along the ion track. The effect of escaping electrons is also shown to arise in microcrystalline samples, which have typical crystal dimensions in the nm-range, and leads to a new interpretation of earlier measurements.

Time-correlated photon counting technique robust against multiple photon events using a multianode photomultiplier tube

A time-correlated photon counting technique which is robust against multiple photon events has been developed. Utilizing a 16 channel multianode photomultiplier tube, both the time interval between the excitation and the first detection of a fluorescence photon and the number of detected fluorescence photons are measured simultaneously. Based on the time and the number data fluorescence decay parameters can be estimated by maximizing the log likelihood. Results of computer simulations and experiments show that this technique can be successfully applicable at high detection rates of >0.9 fluorescence photon detection event per excitation on average.

Photophysics of ethidium bromide complexed to ct-DNA: a maximum entropy study

Time-integrated and time-resolved fluorescence spectroscopies have been used to probe the photophysical properties of ethidium bromide (Eb) complexed to calf thymus DNA (ct-DNA). Fluorescence decay profiles are obtained using the technique of time-correlated single photon counting (TCSPC), and subsequently analysed using conventional sum-of-exponential (SOE) routines and also the maximum entropy method (MEM). Through use of these methods and simulated decay data, it is demonstrated that the kinetics of Eb in the presence of ds-DNA are best described by a generic model consisting of three exponential terms. At all DNA:Eb ratios and NaCl concentrations studied, free Eb is detected. Furthermore, Eb is found to interact with ds-DNA through two mechanisms, each distinguishable by its fluorescence decaytime. Eb is shown to interact with DNA through classic intercalation, and also through binding at secondary sites. The component decaytimes are shown to be a function of NaCl concentration but independent of DNA:Eb molar ratio.

Direct determination of kinetic parameters for diffusion-influenced reactions in solution by analysis of fluorescence decay curves

Fluorescence decay curves were synthesized using the Smoluchowski-Collins-Kimball (SCK) model for diffusion-influenced bimolecular reactions so as to simulate measurements using the technique of time-correlated single photon counting (TCSPC). The experimental conditions required for successful recovery of the parameters used to generate these decay curves were assessed by direct analysis using the decay function associated with the SCK model, where the term "direct analysis" refers to analysis in which all of the fundamental parameters associated with the SCK model are permitted to vary freely. Analysis of decay curves synthesized using an instrument response function measured using a flash-lamp TCSPC system and having a full-width at halfmaximum (fwhm) of approximately 2.8 ns was unsuccessful for the values of the parameters associated with the SCK model that were used in this work. However, analysis of these synthesized fluorescence decay curves using the long-time approximation to the SCK model was found to be valid. The results of analysis of fluorescence decay curves synthesized using a 37-ps fwhm instrument response function indicate that the long-time approximation becomes a poor description of the kinetics of diffusion on the time scale associated with these decay curves. Furthermore, direct analysis using the function associated with the SCK model of the synthesized fluorescence decay curves convoluted with this response function and containing 5.0× 10(4) or 1.0 × 10(5) counts in the channel of maximum intensity resulted in the recovery of parameter values that are in very good agreement with those used to generate these decay curves. The results obtained using the simple methodology developed in this work demonstrate for the first time that direct analysis of fluorescence decay curves measured using TCSPC according to the decay function derived from the SCK model can yield reliable estimates for values of the relevant parameters under suitable experimental conditions.

Time-resolved luminescence measurements in poly( p-phenylenevinylene)

We report time-resolved measurements of photoluminescence in poly( p-phenylenevinylene) (PPV) and related polymers, using the technique of time-correlated single photon counting. We find the photoluminescence is longer lived in less-conjugated samples, and that this accounts for the higher efficiency of light-emitting devices made from less-conjugated materials. At low temperatures, the luminescence is longer lived, and at 77 K there is an interesting red shift of the emission during the first nanoseconds after photoexcitation. We consider that this arises from migration of excitons to more-conjugated (and therefore lower energy) regions of the sample.

Homogeneous linewidths of Rhodamine 6G at room temperature from cavity-enhanced spontaneous emission rates

Fluorescence lifetimes of Rhodamine 6G in levitated micron-sized droplets have been measured using a time-correlated photon counting technique. The coupling of emission into spherical cavity modes of the droplet results in significant emission rate enhancements which allow estimation of the homogeneous linewidth at room temperature.

Electronic excitation transfer in concentrated micelle solutions

Electronic excitation transport among interacting clusters of chromophores is investigated as a function of chromophores is investigated as a function of chromophore and cluster concentration. The technique of time-correlated single photon counting is employed to obtain time-resolved fluorescence depolarization data on aqueous octadecylrhodamine B/triton X-100 micelle solutions. The time-dependent fluorescence anisotropy, the energy transport observable, is directly compared to a theory developed to model this system. The theory is based on a first-order cumulant approximation to the solution of the transport master equation. The model depicts the micelles as monodisperse hard spheres with chromophores (octadecylrhodamine B) distributed about their surfaces. At low micelle concentration, the dynamics of excitation transfer depend only on internal micelle structure. At high micelle concentration excitation transfer occurs among chromophores on different micelles in addition to intramicelle transfer. The theoretical treatment provides nearly quantitative descriptions of the time and concentration dependence of the excitation transport. It correctly predicts the concentration at which intermicelle transfer becomes significant. In the low micelle concentration limit (energy transport confined to isolated micelles) the model having a Poisson distribution of chromophores works well for small {nu} ([chromophores]/[micelle]), but progressively worse as {nu} is increased. Following the literature, a chromophere interaction parametermore » (in the form of a two dimensional second virial coefficient) is used to skew the probe distribution. This enables the transport theory to reproduce the data for all the values of {nu} investigated and provides a determination of the second virial coefficient. 34 refs., 4 figs., 2 tabs.« less

Picosecond carrier dynamics and tunneling-assisted recombination in photorefractive δ-doped superlattices

We report the first time-resolved photoluminescence of photorefractive δ-doped superlattices with picosecond resolution. The measurements were performed by the time-correlated photon counting technique. The subband luminescence shows that even at very high photogenerated carrier densities the screening of the internal electric fields remains incomplete. At short times (<500 ps) the dynamics of the structure is determined by recombination in flatband regions. On a longer time scale (1–100 ns) the structure returns to equilibrium via tunneling-assisted recombination. The corresponding lifetimes are consistent with estimates based on the calculated wave functions of the sawtooth structure.

High-accuracy picosecond characterization of gain-switched laser diodes

A unique combination of the time-correlated photon-counting technique and single-photon avalanche diode detectors gives an accurate characterization of gain-switched semiconductor lasers with picosecond resolution. The high sensitivity and the clean shape of the time response reveal even small features (reflections and relaxation oscillations), making a true optimization of the laser-diode operation possible. The technique outperforms the standard characterization with ultrafast p-i-n photodiodes and a sampling oscilloscope. In addition, compared with other methods, it has favorable features that greatly simplify the measurement.

Lifetime of F 2 and F +3 centers in LiF crystals doped with magnesium

The influence of Mg impurity in an LiF crystal on the lifetime of the relaxed excited state of the F 2 and F + 3 centers is investigated for the first time by using the time-correlated photon counting technique at room temperature (RT). It is found that the radiative lifetime of the excited F 2 centers is almost independent of the Mg impurity concentration, while that of the excited F + 3 centers varies strongly with the corresponding impurity concentration.

Time-resolved measurements of short-wavelength fluorescence from x-ray-excited ions

We demonstrate a novel technique for time-resolved spectroscopic studies of highly excited ions. The technique uses a laser-produced plasma as a short-pulse, soft-x-ray light source with a high repetition rate. A Nd:YAG laser with a pulse duration of 90 psec, a pulse energy of 70 microJ, and repetition rate of 10(4) pulses per second is focused onto a rotating metal target. Soft x rays from the resulting plasma photoionize a gas surrounding the target, and fluorescence from the gas is detected by using a spectrometer and a high-speed photodetector. Using the technique of time-correlated photon counting, we determined the radiative lifetime and collisional quenching rate of the Xe III 5s(0)5p(6)(1)S(0) state by observing its fluorescence at 108.9 nm. A time resolution of better than 400 psec was obtained. We also measured relative Auger decay yields of a core hole state in xenon using a higher-energy laser-produced plasma light source at a lower repetition rate.

Transient free-exciton luminescence and exciton—lattice interaction in pyrene crystals

Time-resolved spectra and luminescence decay time in pyrene crystals have been studied by a time-correlated photon-counting technique over the temperature range 295−136 K (above the phase-transition point). Transient free-exciton luminescence is observed as well as thermally activated free-exciton luminescence. The rate of decay of transient free-exciton luminescence σ is proportional to exp(− E B/ kT, where k is the Boltzmann constant and T the temperature. The ST barrier height E B is found to be 260 cm −1. The ratio of the ST barrier height against the exciton bandwidth is found to be 0.1. The lower bound of the time required for excitons to tunnel through the ST barrier is estimated to be 36 ps.

Time spectroscopy in the a 1Σ u+ states of Li 2 and Na 2. lifetimes and electronic dipole moment of the A-X transition

We have measured the radiative lifetimes of individual rotation-vibration levels of the A 1Σ u + stales of Li 2 and Na 2 using a synchronously pumped dye laser and a time-correlated photon-counting technique. In Li 2 we observe light variation of lifetime with increasing term energy. while in Na 2 lifetimes are almost constant with an average value of 12.45 ns. The dependence of the electronic transition moment on internuclear distance is inferred from the lifetime data and compared with theoretical results.