Possibility and current status of absolute XHV measurement by laser ionization

The current status and possibility of XHV measurement by laser ionization using second harmonics of a picosecond YAG laser are discussed. Under the condition that incident laser energy was 30 mJ/pulse and the laser beam was focused with a spherical lens (focal length f= 250mm) to a power density sufficient for the saturation Of H₂ ionization at the focal point, the minimum pressure in an XHV chamber was measured. From the minimum pressure estimated by extrapolation (5 × 10^-11 Pa) and the ion counting rate at that pressure (one count during 500 laser shots), the volume of the ionization region was expected to be 1.7 × 10^-4 mm³. The ionization region was then measured experimentally under the same laser beam conditions. It was found that the ionization region has a spindle shape with maximum diameter of 45 μm and the length of 1.2mm. The estimated volume is larger than the expected volume by a factor between 4 and 12. The ion detection efficiency of the electron multiplier is one of the major factors affecting the difference, and it can be reduced to within a factor of 3. On the basis of the results, the possibility of absolute measurement of XHV pressure by laser ionization is discussed.