Measurement of CO pressures in the ultrahigh vacuum regime using resonance-enhanced multiphoton-ionization time-of-flight mass spectroscopy

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Abstract
An evaluation is made of measurements of CO pressures in the UHV regime using resonance-enhanced multiphoton ionization coupled with time-of-flight mass spectroscopy (REMPI-TOFMS). It has been found that once the REMPI-TOFMS system has been calibrated, quantitative measurement of CO pressures as low as 10−10 Pa is possible, even in overwhelming N2 backgrounds. With compensation for laser pulse energy variations, we find measurements with uncertainties of 10%–15% are possible for pressures down to 10−7 Pa, and an ultimate detection limit for CO pressures of 10−10 Pa for our measurement system. In this study, the REMPI-TOFMS system was calibrated using a pressure division technique along with a spinning rotor gage. The ionization of CO is achieved using 230 nm radiation to excite the B 1Σ+ state of CO at 10.8 eV via a two-photon absorption and then ionizing some of the excited state molecules by the absorption of an additional photon from the laser beam.

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