A HIGH SPEED, HIGH RESOLUTION NEAR IR FOURIER MULTIPLEX SPECTROMETER SYSTEM

Abstract

This system has been assembled for in-situ analyses of trace atmospheric gases. It comprises an Eocom Corp. Model 7001 laser controlled scanning Michelson interferometer and a Digital Equipment Corp, PDP 11/40 dedicated computer. The interferometer has an 8 cm. stroke, KBr beam-splitter, and 2 in. aperture. A glower source and $LN_{2}$ cooled HgCdTe detector are used, providing response between 2.$8\mu$ and $19\mu$. The computer features dual disk data storage, 16 K of core memory, a storage tube display scope, and an x-y point/line plotter. An extensive set of programs has been developed, all written and optimized in machine language. Full length interferogrsms (128 K words) digitized to 15 bits with less than 1/2 bit of system noise are recorded in 6 sec. Using single-word precision, two such are on-line averaged and transformed (in 3 minutes) to yield a faithful power spectrum with ful$l 1/16 cm^{-1}$ resolution and having a $S/N_{pp}$ of $> 50:1$ at $\lambda = 10\mu$. Precision and reproducibility are excellent, allowing virtually perfect reference-spectra subtraction, providing the spectrum used for background ratioing is updated every 30 min. Double precision programs have also been developed for generation of higher quality spectra. A library of reference spectra has been collected that enable the analysis-by-synthesis of the spectra of gas phase chemical reaction products. Examples taken from a laboratory study of the reactions of the $NO_{x}-HC-O_{3}$ system will be shown.