Picosecond time-resolved four-wave mixing experiments in sodium-seeded flames

Double-crossed polarization transient grating for distinction and characterization of coherences

Coherent phenomena have been widely suggested to play a role in efficient photosynthetic light harvesting and charge separation processes. To substantiate these ideas, separation of intramolecular vibrational coherences from purely electronic or mixed vibronic coherences is essential. To this end, polarization-controlled two-dimensional electronic spectroscopy has been shown to provide an effective selectivity. We show that analogous discrimination can be achieved in a transient grating experiment by employing the double-crossed polarization scheme. This is demonstrated in a study of bacterial reaction centers. Significantly faster acquisition times of these experiments make longer population time scans feasible, thereby achieving improved frequency resolution and allowing for accurate extraction of coherence frequencies and dephasing times. These parameters are crucial for the discussion on relevance of the measured coherences to energy or electron transfer phenomena.

Quantitative concentration measurements of atomic sodium in an atmospheric hydrocarbon flame with asynchronous optical sampling

We report the development of a pump-probe instrument that uses a high-repetition-rate (82-MHz) picosecond laser. To maximize laser power and to minimize jitter between the pump- and the probe-pulse trains, we choose the asynchronous optical sampling (ASOPS) configuration. Verification of the method is obtained through concentration measurements of atomic sodium in an atmospheric methane-air flame. For the first time to our knowledge, ASOPS measurements are made on a quantitative basis. This is accomplished by calibration of the sodium concentration with atomic absorption spectroscopy. ASOPS measurements are taken at a rate of 155.7 kHz with only 128 averages, resulting in a corresponding detection limit of 5 × 10(9) cm(-3). The quenching-rate coefficient is obtained in a single measurement with a variation of ASOPS, which we call dual-beam ASOPS. The value of this coefficient is in excellent agreement with literature values for the present flame conditions. Based on our quantitative results for detection of atomic sodium, a detection limit of 2 × 10(17) cm(-3) is predicted for the Q(1) (9) line of A (2)Σ(+) (v = 0)-X(2)II (v = 0) hydroxyl at 2000 K. Although this value is too large for practical flame studies, a number of improvements that should lower the ASOPS detection limit are suggested.

Picosecond degenerate four-wave mixing on potassium in a methane-air flame

We demonstrate that picosecond mode-locked laser-based degenerate four-wave mixing can be detected with good signal-to-noise ratios in an optically thin flame and that detailed turbulence statistics can be acquired by use of this technique. A regeneratively mode-locked Ti:sapphire laser was tuned to the 4(2)S((1/2))-4(2)P degrees ((1/2)) transition in atomic potassium (which was doped into the flame) at 769.9 nm. Using the all-forward degenerate four-wave mixing geometry, we achieved signal-to-noise ratios of 70:1 without the use of a spatial filter. A sensitivity curve and a method for acquiring turbulence statistics are presented.

Suppression of stimulated Raman scattering from microdroplets by seeding with nanometer-sized latex particles

Stimulated Raman scattering (SRS) in laser-irradiated microdroplets is suppressed by the addition of nanometersized latex particles. The microdroplets consist of either pure ethanol or a solution of Rhodamine 6G dye in ethanol, seeded with latex particles having diameters of 50 < d < 500 nm. SRS emission occurs at droplet morphology-dependent resonances (MDR's) after either direct pumping by the incident 532-nm laser or indirectly whereby the pump laser first initiates dye lasing, which in turn pumps SRS. For large latex, we observe SRS suppression at a near-coincident threshold concentration independent of the presence of dye, whereas, for small latex, adding dye reduces the threshold concentration by more than an order of magnitude. These findings are consistent with the interpretation that for large latex ~1 particle must occupy the MDR mode volume at threshold, whereas for small latex the addition of particles facilitates Förster-assisted annihilation of both 532-nm and dye-lasing MDR pump photons.

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.