PRESSURE MEASUREMENT BY LASER IONIZATION - DIRECT COUNTING OF GENERATED IONS BY IMAGING OF THEIR SPATIAL-DISTRIBUTION

The spatial distribution of ions generated by nonresonant multiphoton ionization of Xe and H-2 with second harmonics of a picosecond YAG laser was imaged with a new detection system. Ions were observed as bright spots on a microchannel plate (MCP) intensifier, and they appeared on a straight line, reflecting the ion generation along a laser beam path. The size of the ionization region, which has a spindle shape, was estimated from the image. The maximum diameter of the ionization region was approximately 60 mum, and the length was 3.6 mm for a Xe gas pressure of 5 X 10(-5) Pa and an incident laser energy of 10 mJ/pulse for our measurement system. The size depended on the incident laser energy, and on the lens assembly used for the focusing of the laser beam. The possibility of relative pressure measurement by counting of the bright spot on the MCP intensifier was also investigated.