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 las er was imaged with a new detection system. Ions were observed as brigh t spots on a microchannel plate (MCP) intensifier, and they appeared o n a straight line, reflecting the ion generation along a laser beam pa th. The size of the ionization region, which has a spindle shape, was estimated from the image. The maximum diameter of the ionization regio n was approximately 60 mum, and the length was 3.6 mm for a Xe gas pre ssure 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 energ y, and on the lens assembly used for the focusing of the laser beam. T he possibility of relative pressure measurement by counting of the bri ght spot on the MCP intensifier was also investigated.