PARTIAL-PRESSURE MEASUREMENT BY MEANS OF INFRARED-LASER ABSORPTION-SPECTROSCOPY

Compared with the conventional method of using a quadrupole mass spect rometer, partial pressure measurement by absorption spectroscopy has t he advantage of being non-intrusive: molecules are not cracked at a ho t cathode, no gas molecules are produced by the instrument itself etc. We have applied tuneable diode laser absorption spectroscopy in the m id-infrared to determine absolute partial pressures of CO in the high vacuum. The sensitivity was increased by use of a Herriott multi-refle ction cell. First of all, line strengths of several rotational-vibrati onal transitions of CO were measured in high vacuum with a relative st andard uncertainty of about 2%. The measured values agree with previou sly published data within their combined uncertainties. Based on these line strength values, partial pressure measurements were possible wit h a relative uncertainty of 3%. Our present experimental set-up allows to measure CO partial pressures down to 10(-5) Pa. At a total pressur e of 1kPa, the highest pressure we used, this corresponds to 1 part, i n 10(8) or 10 ppb. (C) 1998 Elsevier Science Ltd. All rights reserved.