J. Wolfrum

How many photons are necessary for fluorescence-lifetime measurements?

Abstract On planning time-correlated single photon counting experiments for fluorescence-lifetime measurements with minute samples, the question of ultimate sensitivity arises. To answer this question, we will find out how many detected photons one needs to achieve a certain accuracy in the lifetime estimate. Using least-squares or maximum-likelihood procedures and the multinomial distribution, the accuracy of the lifetime estimation can be expressed explicitly as a function of the signal strength. This allows one to determine the required number of detected photons.

Temperature and pressure dependences of the laser-induced fluorescence of gas-phase acetone and 3-pentanone

Laser-Induced Fluorescence (LIF) from the S1 state of acetone and 3-pentanone was studied as a function of temperature and pressure using excitation at 248 nm. Additionally, LIF of 3-pentanone was investigated using 277 and 312 nm excitation. Added gases were synthetic air, O2, and N2 respectively, in the range 0–50 bar. At 383 K and for excitation at 248 nm, all the chosen collision partners gave an initial enhancement in fluorescence intensity with added gas pressure. Thereafter, the signal intensity remained constant for N2 but decreased markedly for O2. For synthetic air, only a small decrease occurred beyond 25 bar. At longer excitation wavelengths (277 and 312 nm), the corresponding initial rise in signal with synthetic air pressure was less than that for 248 nm. The temperature dependence of the fluorescence intensity was determined in the range 383–640 K at a constant pressure of 1 bar synthetic air. For 248 nm excitation, a marked fall in the fluorescence signal was observed, whereas for 277 nm excitation the corresponding decrease was only half as strong. By contrast, exciting 3-pentanone at 312 nm, the signal intensity increased markedly in the same temperature range. These results are consistent with the observation of a red shift of the absorption spectra (≈9 nm) over this temperature range. Essentially, the same temperature dependence was obtained at 10 and 20 bar pressure of synthetic air. It is demonstrated that temperatures can be determined from the relative fluorescence intensities following excitation of 3-pentanone at 248 and 312 nm, respectively. This new approach could be of interest as a non-intrusive thermometry method, e.g., for the compression phase in combustion engines.

Lasers in combustion: From basic theory to practical devices

In recent years, a large number of linear and nonlinear laser-based diagnostic techniques for nonintrusive measurements of species concentrations, temperatures, and gas velocities in a wide pressure and temperature range with high temporal and spatial resolution were developed and have become extremely valuable tools to study many aspects of combustion. Modern “pump-probe” laser spectroscopic methods give direct insight into the microscopic dynamics, product channel distributions, and reaction rates of elementary combustion reactions over a wide range of temperatures and pressures. Nonlinear laser spectroscopic techniques using infrared-visible sum-frequency generation can now bridge the pressure and materials gap to provide kinetic data for catalytic combustion. Laminar flames are ideal objects to develop the application of laser spectroscopic methods for practical combustion systems and to test and improve gas-phase reaction mechanism in combustion models. Besides diagnostics, lasers can also provide well-defined starting conditions for detailed experimental and theoretical studies of ignition processes. Non-intrusive laser point and field measurements, especially joint velocity-scalar data at the same point in space and time, are of basic importance in the validation and further development of turbulent combustion models. As an are for the application of quanitative laser spectroscopy to practical combustion devices, investigations in internal combustion engines are described. Finally, the potential of laser techniques for active combustion control in various devices from laboratory burners to full-scale jet engines, municipal waste incinerators, and pressurized fluidized-bed reactors are illustrated.

New fluorescent dyes in the red region for biodiagnostics.

The increased sensitivity together with the advent of low-cost optical sources and detectors in the visible-near IR region has led us to current efforts to develop new efficient fluorescent labels for biodiagnostics with absorption and emission beyond 600 nm. In view of the general fluorescence decrease with increasing emission wavelength, we investigated the possibility to shift the absorption of rhodamine dyes toward the region 620–670 nm. The hydrophobic nature of all known long-wavelength dyes results in the tendency to form intra- and intermolecular aggregates in hydrophilic solvents, especially in aqueous environment. Due to the aggregation with biological materials, fluorescence quenching of the dyes is often observed. New strategies for prevention of these processes are considered.

DETECTION AND CHARACTERIZATION OF SINGLE MOLECULES IN AQUEOUS SOLUTION

Using a confocal microscope with a single-photon avalanche photodiode as detector, we studied photon bursts of single Rhodamine 6G (R6G) and Rhodamin B-zwitterion (RB) molecules in aqueous solution by excitation of the lowest excited singlet stateS1 with a frequency-doubled titanium: sapphire laser. Multichannel scaler traces, the fluorescence autocorrelation function and fluorescence decay times determined by time-correlated single-photon counting have been measured simultaneously. The time-resolved fluorescence signals were analyzed with a maximum likelihood estimator. Fluorescence lifetime patterns in steps of 100 ps were generated by convolution with the excitation pulse. The lifetime of theS1 state was derived from the Kullback-Leibler minimum discrimination information. We are able to demonstrate for the first time identification of two different single dye molecules via their characteristic fluorescence lifetimes of 1.79 ± 0.33 ns (RB) and 3.79 ± 0.38 ns (R6G) in aqueous solution.

Measurement of the instantaneous detailed flame structure in turbulent premixed combustion

Simultaneous two-dimensional UV-Rayleigh thermometry and 2D laser-induced fluorescence of OH radicals were used to analyze highly turbulent, premixed natural gas/air flames with up to 130-kW heat release, where the turbulence conditions are approximately homogeneous in the measurement area. The turbulent Reynolds number has been varied between 60 and 2500, and the turbulent Karlovitz numbers ( Ka ), between 0.03 and 14. The simultaneously recorded single-shot images resolve the instantaneous structure of turbulent premixed flames to a spatial resolution of about 0.1 mm. For flames with Ka Ka Ka . Obvious strongly localized deviations from a laminar-like flamelet structure were found for flames with Ka >5. In a few regions, temperatures above 1400 K, but no OH radicals, were observed, indicating regions with local extinction. A comparison of the size of small-scale eddies with the laminar eldovich flame-front thickness and the thermal thickness indicates that these eddies do have much smaller influence than has been expected by the Klimov-Williams criterion.

Simultaneous single-shot laser-based imaging of formaldehyde, OH, and temperature in turbulent flames

Single-shot formaldehyde (CH2O) laser-induced fluorescence (LIF) imaging measurements in turbulent flames were performed using XeF excimer laser excitation in the 410 transition of the A1A2-1A1 electronic system at 353 nm. Background contributions to the CH2O LIF detection were assessed in spectroscopic measurements. Simultaneous two-dimensional mapping of OH LIF, CH2O LIF, and temperature fields was carried out in a standard Bunsen flame. The zones of peak heat release rate localized via the product of OH and CH2O LiF intensities correlated with areas of intermediate temperatures. In addition, single-shot imaging of the transient formaldehyde distribution was performed in a 150 kW natural gas swirl burner. Formaldehyde distributions in strongly turbulent swirl, flames differed significantly from thin layers found in laminar and weak turbulent flames, indicating the presence of low-temperature chemistry in preheated gas pockets. The CH2O distribution measured in the swirl flame was compared with averaged fields of temperature, OH, and NO concentrations.

TIME-RESOLVED DETECTION AND IDENTIFICATION OF SINGLE ANALYTE MOLECULES IN MICROCAPILLARIES BY TIME-CORRELATED SINGLE-PHOTON COUNTING (TCSPC)

A PC plug-in card for on-line time resolved fluorescence detection of single dye molecules based on a new time-correlated single photon counting (TCSPC) module is described. The module contains all electronic components constant fraction discriminators (CFDs), time-to-amplitude converter (TAC), analog-to-digital converter (ADC), multichannel analyzer (MCA timers) on board required for TCSPC. A fast TAC design in combination with a fast flash ADC and an error-correcting ADC/MCA principle results in a maximum count rate of 8 MHz (dead time 125 ns). A dual memory architecture allows for unlimited recording of decay curves with collection times down to 150 μs without time gaps between subsequent recordings. Applying a short-pulse diode laser emitting at 640 nm with a repetition rate of 60 MHz in combination with a confocal microscope, we studied bursts of fluorescence photons from individual dye labeled mononucleotide molecules (Cy5-dCTP) in a cone shaped microcapillary with an inner diameter of 0.5 μm at the...

OH(X 2Π) state distribution from HNO3 and H2O2 photodissociation at 193 nm

The photodissociation of HNO3 and H2O2 at 193 nm is reported. The rotational vibrational and fine structure state distributions of OH radicals obtained in the photodissociation are described. (AIP)

Hot atom‐laser induced fluorescence experiments on the reaction of H(2S) with O2

A method has been developed to study the dynamics of reactions with substantial threshold energies by combining hot atom production by laser photolysis and fast product detection by laser induced fluorescence. The nascent OH rotational, vibrational, and fine structure state distributions produced in the endothermic reaction H+O2→OH(K,v, f )+O have been measured with hot hydrogen atoms from the photodissociation of HBr at 193 nm. OH rotational excitation is high and corresponds to the phase space statistical distribution over the measured range of rotational states. Also OH vibrational excitation is substantial. The OH spin doublets are produced statistically, but the partitioning in the λ doublets is very nonstatistical with a strong preference for the π+ component demonstrating the reaction to occur essentially in a plane at high collision energies. By measuring reactant and product densities at short times, an absolute total cross section of 0.42±0.40.2 (A2) has been obtained.