Possibility and current status of absolute XHV measurement by laser ionization
Abstract
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 = 250 mm) to a power density sufficient for the saturation of H 2 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 mm 3. 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 1.2 mm. 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.
- Publication:
-
Vacuum
- Pub Date:
- January 1996
- DOI:
- 10.1016/0042-207X(96)00017-6
- Bibcode:
- 1996Vacuu..47..545I
- Keywords:
-
- XHV;
- laser ionization;
- ionization volume;
- molecular density imaging