Time-of-flight mass spectrometry of aluminium plasma: Investigation of multiply charged ions and clusters

Abstract

Ionic species in laser-produced plasma plume are generally analyzed by the time-of-flight method using a charge collector placed near to the target surface. One of the shortcomings of these methods is a sacrifice of the important information for large mass cluster ions and the different charge states of atomic ions. In the present manuscript, design, development, and optimization procedure of a modified Wiley-McLaren type time-of-flight mass spectrometer for the diagnostics of ions and cluster distribution in laser-produced plasma plume is presented. Second harmonic (532~nm) from a pulsed Nd: YAG laser is used to ablate aluminium disk in a vacuum chamber evacuated to base pressure of $7\times10^{-8}$~mbar. The mass spectrometric results of aluminium (Al) plasma are presented and discussed in detail. To minimize the degradation in detection and mass resolution of spectrometer due to the wide angular and energy spared of the ions in the plasma plume, optimization of the TOF spectrometer in different configurations, single as well as a double field, is also discussed. Cluster ions as large as Al$_{32}^{+}$ and Multiply Charged Ions (MCI) up to Al$^{11+}$ are observed in the mass spectra. The possible pathways for the production of MCI and cluster ions are discussed on the basis of the Coulomb explosion mechanism. The dependence of ion distribution on the laser fluence is also discussed.