Fred E. Lytle

Pump/probe method for fast analysis of visible spectral signatures utilizing asynchronous optical sampling.

We report the results from a new pump/probe spectrometer for potential use in combustion diagnostics that employs asynchronous optical sampling. The instrument consists of two frequency-doubled mode-locked Nd:YAG lasers operating at slightly different repetition rates, synchronously pumping two dye lasers (rhodamine 6G) to generate the pump and probe beams. The spectral and temporal capabilities of the instrument are examined by obtaining a spectrum and an excited state decay of rhodamine B. The instrument response is shown to be proportional to pump power, probe power, and sample absorptance. Different frequency synthesizers and different modes of triggering are used to study their effect on signal stability.

Pump/Probe Spectroscopy by Asynchronous Optical Sampling

We report the first results from a new pump/probe technique called asynchronous optical sampling (ASOPS). The method employs a mode-locked, frequency-doubled Nd:YAG laser operating at a repetition rate of 81.5970000 MHz as the pump laser, and a synchronously pumped dye laser (R6G) operating at a repetition rate of 81.5870000 MHz as the probe laser system. The 10-kHz beat frequency produces a repetitive relative phase walk-out of the pump and probe pulses which replaces the optical delay line used in conventional instruments. Studies of rhodamine B in methanol demonstrate that the instrument response is proportional to pump power, probe power, and sample absorptance. The fluorescence lifetime of 4 × 10−5 M rhodamine B is determined to be 2.3 ns.

SOLUTION LUMINESCENCE OF METAL COMPLEXES.

An over-all view of the solution luminescence of metal complexes is given. The three major classes of emission are examined in moderate detail including brief discussions of theoretical considerations. These are attuned to the problem of predicting which metal ion—ligand combinations should form luminescent compounds. Finally, the elements are covered group by group and as many examples of emitting compounds are given as possible.

Source-Corrected Two-Photon Excited Fluorescence Measurements between 700 and 880 nm

Passively mode-locked titanium:sapphire (Ti:S) lasers are capable of generating a high-frequency train of transform-limited subpico-second pulses, producing peak powers near 105 W at moderate average powers. The low energy per pulse (<20 nJ) permits low fluence levels to be maintained in tightly focused beams, reducing the possibility of saturating fluorescence transitions. These properties, combined with a wavelength tunability from approximately 700 nm to 1 μm, provide excellent opportunities for studying simultaneous two-photon excitation (TPE). However, pulse formation is very sensitive to a variety of intracavity parameters, including group velocity dispersion compensation, which leads to wavelength-dependent pulse profiles as the wavelength is scanned. This wavelength dependence can seriously distort band shapes and apparent peak heights during collection of two-photon spectral data. Since two-photon excited fluorescence is proportional to the product of the peak and average powers, it is not possible to obtain source-independent spectra by using average power correction schemes alone. Continuous-wave, single-mode lasers can be used to generate source-independent two-photon data, but these sources are four to five orders of magnitude less efficient than the mode-locked Ti:S laser and are not practical for general two-photon measurements. Hence, a continuous-wave, single-mode Ti:S laser has been used to collect a source-independent excitation spectrum for the laser dye Coumarin 480. This spectrum may be used to correct data collected with multimode sources; this possibility is demonstrated by using a simple ratiometric method to collect accurate TPE spectra with the mode-locked Ti:S laser. An approximate value of the two-photon cross section for Coumarin 480 is also given.

Excited singlet‐state lifetimes of hydrated chlorophyll aggregates

A time‐correlated single photon counting technique, coupled with a red sensitive photomultiplier tube, was used to determine the singlet‐state lifetimes and energy transfer mechanisms of Chl a⋅H2O, (Chl a⋅H2O)2, and (Chl a⋅2H2O)n. The use of low concentrations and low incident light fluxes permitted measurements in a regime where nonlinear quenching effects are absent. The lifetimes exhibit a pronounced dependence on aggregate size. The results are examined in terms of exciton interactions in the excited singlet state of the Chl a aggregate. It is shown that in the absence of exciton annihilation effects, the rate of singlet‐state decay via radiative coupling and intersystem crossing of a Chl a aggregate containing n equivalent monomeric units is equal to n times that of monomeric Chl a.

Asynchronous optical sampling: a new combustion diagnostic for potential use in turbulent, high-pressure flames

Asynchronous optical sampling (ASOPS) is a pump-probe method that has strong potential for use in turbulent, high-pressure flames. We show that rapid measurement of species number density can be achieved by maintaining a constant beat frequency between the mode-locking frequencies of the pump and probe lasers. We also describe the instrumental timing parameters for ASOPS and consider the optimization of these parameters. Measurement of the nanosecond decay for electronically excited sodium in an atmospheric flame demonstrates the viability of the ASOPS technique in highly quenched flame environments.

Measurements of atomic sodium in flames by asynchronous optical sampling: theory and experiment

Asynchronous optical sampling (ASOPS) is a pump-probe method for the measurement of species concentrations in turbulent high-pressure flames. We show that rapid measurement of species number density can be achieved in a highly quenched environment by maintaining a constant beat frequency between the mode-locking frequencies of the pump and the probe lasers. A model for the ASOPS method based on rate equation theory for three- and four-level atoms is presented. A number of improvements are made to the basic ASOPS instrument, which result in a greatly enhanced signal-to-noise ratio. Atomic sodium is aspirated into an atmospheric pressure C(2)H(4)/O(2)/N(2) flame and detected with the ASOPS instrument. When excited-state lifetimes are fitted by using the ASOPS theory, a 3P((1/2),3/2) ? 3S((1/2)) quenching-rate coefficient of 1.72 x 10(9) s(-1) and a 3P(3/2) ? 3P((1/2)) doublet-mixing rate coefficient of 3.66 x 109 s(-1) are obtained, in excellent agreement with literature values. ASOPS signals obtained over a wide range of pump and probe beam powers validate the rate equation theory. Improvements are suggested to improve the signal-to-noise ratio, since the present results are limited to laminar flows.

Determination of polycyclic aromatic hydrocarbons in biomass gasifier effluents with liquid chromatography/diode array spectroscopy.

Liquid chromatography/diode array spectroscopy (LC/DAS) is used for the determination of polycyclic aromatic hydrocarbons (PAH) isolated from the emissions of biomass gasifiers. The spectrometer is capable of obtaining complete absorption spectra of components as they elute from the chromatograph, thus confirming the identities of pure peaks and mixed or poorly resolved peaks. This technique is especially well suited for the determination of polynuclear aromatics, where many isomers difficult to resolve chromatographically are easily distinguished by their electronic spectra. The instrument is also capable of differentiating between unsubstituted PAH and their methylated analogues, even though methylation does not greatly perturb the electronic nature of the parent hydrocarbon.

High repetition rate subnanosecond gated photon counting

A gated photon counting instrument which is capable of operation at the full 100‐MHz repetition rate of mode‐locked ion lasers is described. This instrument provides a high throughput alternative to more complex and costly instrumentation commonly used in time‐correlated single photon counting. Applications to the measurement of subnanosecond fluorescence lifetimes at a 96.8‐MHz laser repetition rate and to the reduction of matrix quenching effects in fluorometry at a 5‐MHz laser rate are presented.

A SIMPLIFIED, RELIABLE, INEXPENSIVE METHOD FOR ROUTINELY MEASURING FLUORESCENCE LIFETIMES

THE RECENT interest in the study of excited-state processes (see, for example, Schlag et af., 1971), coupled with advances in instrumentation (see, for example, Ware, 1971), has increased the frequency of fluorescence lifetimes reported in the literature. However, the necessary equipment, both commercial and homemade, has really been plagued by expense, and a kick of simplicity and reliability. These in turn affect the ability of this parameter to be used in a routine fashion by those not particularly well versed in the art. What we have accomplished is a vast improvement in the above categories by utilizing a relatively inexpensive assemblage of commercially available components. The instrument in use is essentially a modified TRW Nanosecond Spectral Source System (TRW Instruments, Inc., 139 Illinois Street, El Segundo, Calif. 90245). The ultimate performance of the instrument is limited only by the characteristics of the gas discharge lamps. The nitrogen lamp emits 140 W into a line spectrum extending from 297-8 nm to 399-8 nm and has a 5 nsec rated decay time. The deuterium lamp emits 18 watts into a continuous spectrum extending from 200.0 nm to 350-0 nm and has 1.6 nsec rated decay time. It should be noted that the observed decay time will depend upon the convolution of all of the components in the system, including the coaxial cable. For our instrument, the measured deuterium decay is about 10 nsec. The first modification was the elimination of the oscilloscope as the display device yielding the lifetime. This was accomplished by utilizing an inexpensive Tektronix oscilloscope with a 1S1 Sampling Unit (Tektronix, Inc., P. 0. Box 500, Beaverton, Oregon 97005), instead of the suggested Tektronix 556 oscilloscope with a 1Al Unit. This plug-in has a risetime of 350 psec with a fixed input impedance of 50 0. It is attached via 50 SZ coaxial cable to the anode of an RCA 93 1 A photomultiplier with a 1 -6 nsec risetime (RCA Electron Tube Division, Hamson, N. J.). However, the most useful feature of the 1 S 1 is its ability to provide outputs to an x,y-recorder at a rate quite few orders of magnitude slower than the actual event. This permits the decays to be permanently recorded without the use of photographic film. Thus, the oscilloscope is simply used as a monitor to initially adjust the optical parameters. In actual operation, it was found to be advantageous to utilize the TRW sync output in conjunction with the 1S1 external trigger mode, instead of an internal triggering scheme. This avoids the problem of readjusting the trigger level for weak signals or the total disappearance of the trace during certain procedural operations. However, if a direct electrical connection were to be attempted, transistors in the triggering circuit