SURFACE-ANALYSIS BY PHOTOIONIZATION AT VERY HIGH LASER INTENSITIES

Photoionization at high laser intensities (approximately 10(14) W/cm2) has been used to examine gaseous and sputtered atoms and molecules together with reflecting time-of-flight mass spectrometry. Light from a 35-ps Nd:YAG laser pulse was focused with a 10-cm focal length lens. The wavelengths used were 1064, 532, 355, and 266 nm, at nominal pulse energies of 70, 40, 18, and 10 mJ, respectively. Significant amounts of multiple ionization were observed even for high ionization potential (IP) atoms. For open-shell heavy transition metal atoms, die amounts of multiple ionization exceed single ionization. Relative elemental sensitivity factors were determined for the compounds SiO2, GaAs, and GaN which cover a wide range of EPs. 532-nm light gave the largest amounts of multiple ionization and also relative sensitivity factors most approaching unity, for the conditions studied. From these observations it is concluded that such high intensity laser beams lead to saturation of photoionization as well as sharply defined ionization volumes, both of which can advance surface analytical quantitation under high sensitivity conditions.