The variations of the ion yield of laser-produced metallic plasma have been investigated in the background pressure between 7×10^-7 and 3×10^-4 Torr. The yields of singly charged ion species increased monotonously with pressure and those of multiple-charged ions (up to z=4) showed maxima at 8×10^-5 or 2×10^-4 Torr depending on the target material and ionic charge.
The microwave measurements of expanding plasma indicated that the maximum charge per ion obtained in the initial stage of expansion would not change up to the pressure of 1×10^-4 Torr.
From the results, it is concluded that the pressure dependence is caused by the charge transfer between plasma and background gas molecule during expansion.