Tunable Diode Laser Quantification of Isomeric Partially Deuterated Cyclobutanes and Ethenes

Abstract

A specially designed tunable diode laser (TDL) absorption spectrometer which operates near 900 and 1300 cm⁻¹ has been constructed in order to identify and measure very low concentrations of product gases extracted from a single-pulse shock-tube experiment. The ultra-high-frequency resolution of the TDL spectrometer (5 to 10 MHz) shows the rotational fine structure in an absorption spectrum, clearly distinguishing between isomers of the same compound. The spectrometer features dual-beam detection in order to simultaneously acquire source and reference gas spectra for unambiguous labeling of the product gases, and rapid sweep averaging, which minimizes the effects of low-frequency mechanical vibrations and achieves very high signal-to-noise ratio. Cis and trans isomers of 1,2-d₂-cyclobutane were detected and measured in quantities of shock-tube products as small as 75 cm³-Torr. Several conformations of deuterated ethenes were also detected in samples smaller than 1 cm³-Torr.