The role of recombination in formation of charge composition of ions in a laser plasma upon mass spectrometric analysis of solids

Abstract

A numerical experiment is used to study expansion of a plasma cloud with complex composition into a vacuum. Under the assumption that the density and temperature of the laser plasma are homogeneous throughout the cloud, the system of differential equations describing the plasma cloud expansion with allowance for inelastic processes is reduced to a system of ordinary differential equations for ions of five elements (Fe, Mn, Cr, Ni, Mo) with the charges z = 1, 2, 3, 4. The velocity of the plasma cloud boundary and its temperature as functions of the expansion time are determined. It is demonstrated that, after recombination stops, thermodynamically nonequilibrium plasma is created as a result of “quenching,” and in this plasma the residual degree of ionization and ion charge distribution depend on the initial plasma parameters T₀, N₀, and R₀. In the course of recombination, the dependence of ion charge distribution on ionization potential weakens.