Characterization of laser ablation and ionization in helium and argon: A comparative study by time-of-flight mass spectrometry

Laser ablation and ionization in ambient helium and argon gases were studied by multiple-stage time-of-flight mass spectrometry. Measurements made at different gas pressures indicated that there exists an optimal pressure for adequately cooling energetic ions and reducing multiply charged ions that are higher for He than for Ar. The temporal distributions of ions were compared at various laser fluences and gas pressures, and the broad distributions for He could be ascribed to elastic scattering and thermodynamic processes. The diffusion of ions in He resulted in a longer delay before the instrument registered its maximal signal. Ions with different masses were observed to have the same kinetic energies in He, which was confirmed using the SIMION software, while ion movement was hydrodynamically controlled in Ar. The velocities of singly and doubly charged ions were also studied, and doubly charged ions showed much higher kinetic energy because of their frontal location in the plasma expansion.