Kenneth R. Winn

Excited-state protonation kinetics of coumarin 102

Abstract The rate constant for protonation of the electronically excited state of coumarin 102 in water has been determined to be (1,4 × 0.4) × 10 10 mol −1 s −1 using picosecond spectroscopy. The laser pH jump technique is shown to be suitable for rapidly decreasing, as well as increasing, solution pH.

Picosecond time-resolved spectral shifts in emission: dynamics of excited state interactions in coumarin 102

Abstract The temporal behavior of the spectral emission from coumarin 102 in several liquids is studied as a function of temperature using picosecond techniques. Kinetics of the disappearance of a blue edge emission and the formation of a red edge emission are related. Strong correlations with viscosity suggest that the reorientation time is a key parameter.

CH3O+CO removal rate constant measurements over the 473–973 K temperature range

Abstract Removal rate constants for CH 3 O by CO have been measured over the temperature range 473–973 K using a laser photolysis/laser-induced fluorescence technique. For temperatures exceeding 773 K, the removal rate constant begins to exhibit a non-linear Arrhenius behavior suggesting that other removal processes, in addition to oxidation of CO by CH 3 O, are important at elevated temperatures.

Exciton annihilation versus excited state absorption and stimulated emission effects in laser studies of fluorescence quenching of chlorophyll in vitro and in vivo

Abstract The suggestion made by Hindman et al. that the decrease in fluorescence yield of chrorophyll a in vivo as a function of laser pulse intensity may be due to stimulated emission and/or excited state absorption effects, is examined critically. It is concluded that the contributions of these effects in previous picosecond pulse experiments is negligible.

Direct measurements of methoxy removal rate constants for collisions with CH4, Ar, N2, Xe, and CF4 in the temperature range 673–973 K

Removal rate constants for CH3O by CH4, Ar, N2, Xe, and CF4 were measured over a 300 K temperature range using a laser photolysis/laser-induced fluorescence technique. Rapid methoxy removal rates are observed for the non-reactive collision partners (Ar, N2, Xe, and CF4) at elevated temperatures showing that the dissociation and isomerization channels for CH3O are indeed important. The total removal rate constant (reaction + dissociation and/or isomerization) for CH4 follows Arrhenius behavior over the investigated temperature range and is well represented by the form. kr = (1.2 ± 0.7) × 10−8 exp [(−10170 ± 350)/T] cm3 molecule−1 s−1. Using an RRKM formalism to scale the removal rate constants for the non-reactive collision partners, the dissociation/isomerization portion of the total CH3O + CH4 removal rate constant has been estimated. Subtracting this estimate from the total removal rate constant yields a CH3O + CH4 reaction rate constant equal to k1 = (2.2 ± 2.0) × 10−10 exp [(−7585 ± 466)/T] cm3 molecule−1 s−1.

Photofragment Fluorescence As A Sensitive Probe For Gas-Phase Alkali Compounds And Their Photochemistry

Sensitive techniques are needed for the detection of highly corrosive gas-phase alkali compounds in coal gasifier gas turbine streams. We report on the use of photofragment fluorescence as a very sensitive, selective probe for alkali compounds. Photodissociation of a gas-phase alkali compound using a laser at suitably short ultraviolet (uv) wavelengths produces an electronically excited alkali atom. Detection of fluorescence from these excited atoms allows sensitive and quantitative density measurements of a compound while signal strength as a function of dissociation laser wavelength allows differentiation of compounds. We present here an evaluation of this approach based on the results of experiments to study the photodissociation of the alkali compound KC1. The KC1 vapor was contained in a heated quartz cell and irradiated at 193 nm with an ArF laser and at other Raman-generated wavelengths. Emission at 766 nm was observed from atomic potassium (42P°) produced in the photodissociation process. The spectral dependence for the production of excited potassium atoms is distinct enough that discrimination from other compounds, such as KOH, appears likely. The atomic emission intensity quantitatively tracks the KC1 density over at least 5 orders-of-magnitude. As little as 4 x 107 KC1 molecules/cc, or a 0.5 ppb KC1 concentration, can be measured on a single laser shot, making this a very sensitive diagnostic technique. Experiments on other alkali compounds are now in progress.

Production Of Gallium Atoms By Excimer Laser Photolysis Of Trimethylgallium

The gas phase kinetics of group III elements such as gallium are important in potential chemically-driven energy-transfer lasers and in chemical vapor deposition processes in the electronics industry. Excimer laser photodissociation of volatile gallium compounds via multiple-photon processes provides, in principle, a convenient room-temperature source of gallium atoms for study using laser photolysis/laser-induced fluorescence techniques. In this paper, we report preliminary results on the production of atomic gallium from the multiple-photon dissociation of trimethylgallium at 193 nm. Prompt emission from a number of excited gallium states (5 2S, 4 2D, 6 2S, 6 2P°, 5 2D, and 4 4P) is observed. The time histories of the ground state (4 2P°1/2) and the metastable state (4 2P°3/2) have been measured using laser-induced fluorescence. The resulting time profiles are complicated even in the absence of an added reactant gas by the apparent production of ground state gallium at relatively long times (-10 μs) after the excimer laser pulse. Possible mechanisms for this (i.e., radical reactions to produce gallium, energy transfer, cascading from high lying metastable states, ionic processes, etc.) are being investigated. These results indicate that the photodissociation of trimethylgallium at 193 nm is complex. Mechanistic considerations suggest that photolysis at other wavelengths and with other precursors may lead to a cleaner source of gas-phase atomic gallium for kinetic studies, and these studies are in progress.