Partial pressure measurement by means of infrared laser absorption spectroscopy

Abstract

Compared with the conventional method of using a quadrupole mass spectrometer, partial pressure measurement by absorption spectroscopy has the advantage of being non-intrusive : molecules are not cracked at a hot cathode, no gas molecules are produced by the instrument itself etc. We have applied tuneable diode laser absorption spectroscopy in the mid-infrared to determine absolute partial pressures of CO in the high vacuum. The sensitivity was increased by use of a Herriott multi-reflection cell. First of all, line strengths of several rotational-vibrational transitions of CO were measured in high vacuum with a relative standard uncertainty of about 2%. The measured values agree with previously published data within their combined uncertainties. Based on these line strength values, partial pressure measurements were possible with a relative uncertainty of 3%. Our present experimental set-up allows to measure CO partial pressures down to 10 −5 Pa. At a total pressure of 1kPa, the highest pressure we used, this corresponds to 1 part in 10 8 or 10 ppb.