Liquid-Nitrogen-Cooled Optical Cell for the Study of Absorption Spectra in a Fourier Spectrometer
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TICAL INSTRUMENTATION
Liquid-Nitrogen-Cooled Optical Cell for the Study of Absorption Spectra in a Fourier Spectrometer V. I. Serdyukova, L. N. Sinitsaa, *, A. A. Lugovskoia, **, and N. M. Emel’yanova, *** aV.E.
Zuev Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences, Tomsk, 634055 Russia *e-mail: [email protected] **e-mail: [email protected] ***e-mail: [email protected] Received November 22, 2019; revised November 22, 2019; accepted December 11, 2019
Abstract—A low-temperature 17.5-cm long vacuum cell with removable quartz, ZnSe, and KBr windows has been designed for working with the high-resolution Bruker IFS 125M FT-IR spectrometer. The cell provides a threshold absorption sensitivity of about 10−6 cm−1. The cell makes it possible to record the absorption spectra of gases in the region 1000–20 000 cm−1 in the temperature range from 108 to 298 K with an error of ±0.1 K. The 12CH4 absorption spectra were recorded in the range from 9000 to 9200 cm−1 at a pressure of 300 mbar and a temperature of 298 and 108 K with a spectral resolution of 0.03 cm−1. The empirical values of the energy levels of lower states were calculated from the ratios of line intensities measured at different temperatures. Keywords: Fourier spectroscopy, absorption spectrum, methane DOI: 10.1134/S1024856020040144
INTRODUCTION The study of absorption spectra of greenhouse gases at low temperatures, which correspond to the conditions in the upper atmosphere, is of growing interest today, especially of highly symmetric molecules, such as CH4, NF3, CF4, SF6. High-resolution spectra recorded at low temperatures allow effectively separating rotational lines, resolving the multiplet structure of complex spectra, and calculating the quantum number J of the lower state from the temperature dependence of the line intensities. The temperature dependences of the broadening parameters of methane and other highly symmetric molecules are necessary for simulation of the atmosphere of the Earth and other Solar System planets, the temperature of which can drop below 100 K [1–7]. To study the molecular spectra at a low temperature, single-pass and multipass low-temperature cells were designed. For example, Sung and Mantz [8, 9] suggested single-pass cells operating at temperatures of 66–300 K in the spectral region ~1200–1800 cm–1, with paths 24.29 and 20.38 cm long. The cells were cooled using a closed-cycle helium cryostat. Small cell sizes (10–20 cm) do not allow their use in the highfrequency spectral region due to the low sensitivity. In addition, a closed-cycle refrigeration system produces significant mechanical vibrations of a cell [10, 11],
which deteriorate the spectra recorded by a Fourier spectrometer. Kassi with coauthors [12, 13] described a cell 1.4 m long and 2 cm diameter cooled by liquid nitrogen, designed for use in a laser spectrometer. The cell design does not allow smooth variations in the temperature. The spectra were recorded in the spectral regions 5850–6190 and 6700–7700 cm–1 at a liquid nitrogen temperature of (
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