Photofragment Fluorescence as a Sensitive Probe for Gas-Phase Alkali Compounds and Their Photochemistry

摘要

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 KCl. The KCl vapor was contained in a 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 (4 exp 2 P) 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 KCl density over at least 5 orders-of-magnitude. As little as 4 x 10 exp 7 KCl molecules/cc, or a 0.4 part-per-billion KCl 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. 14 refs., 7 figs. (ERA citation 10:028236)