MEASUREMENT OF CO PRESSURES IN THE ULTRAHIGH-VACUUM REGIME USING RESONANCE-ENHANCED MULTIPHOTON-IONIZATION TIME-OF-FLIGHT MASS-SPECTROSCOPY

An evaluation is made of measurements of CO pressures in the UHV regim e using resonance-enhanced multiphoton ionization coupled with time-of -flight mass spectroscopy (REMPI-TOFMS). It has been found that once t he REMPI-TOFMS system has been calibrated, quantitative measurement of CO pressures as low as 10(-10) Pa is possible, even in overwhelming N 2 backgrounds. With compensation for laser pulse energy variations, we find measurements with uncertainties of 10%-15% are possible for pres sures down to 10(-7) Pa, and an ultimate detection limit for CO pressu res 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 23 0 nm radiation to excite the B 1SIGMA+ state of CO at 10.8 eV via a tw o-photon absorption and then ionizing some of the excited state molecu les by the absorption of an additional photon from the laser beam.