A pressure measurement method for xhv using laser ionization has been developed. By means of nonresonant multiphonon ionization, the proportional relation between the amount of laser-generated ions and the gas pressure is experimentally verified in the range 1 × 10-3-2 × 10-8 Pa. Also, a nonresonant 4-photon ionization of the He atom, which has the highest ionization potential of all atoms and molecules, and saturations of ionization for Xe, Kr, O2 and especially for H2, which is a main residual molecule in uhv and xhv, are successfully observed. In addition, a high-sensitivity ion-detection system for the exact measurement of several atoms and molecules in xhv is prepared. Noise due to scattered laser light is discussed.
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