Photoionization cross sections in resonance ionization mass spectrometry of sputtered metal atoms: Applications to material analysis

(1+1) resonant multiphoton ionization of several metal atoms was investigated in the 285–301-nm-wavelength range. Fe, Mg, Cr, Ti, and Ni free atoms were produced using Ar+-ion sputtering of different metal samples in ultrahigh-vacuum conditions. The photoionization yields, measured as function of the laser fluence, exhibit saturation or near saturation at the highest laser energy density available in the experiment (100 mJ/cm2). From these measurements the ionization cross sections of the intermediate excited states are deduced, assuming saturation of the excitation step. In this way values ranging between 0.7 and 4×10−17 cm2 are obtained. One can take advantage of the relatively large cross sections to detect these sputtered metal atoms by resonance ionization mass spectroscopy with great sensitivity. Some examples are given, demonstrating, on a single-laser-shot basis, the real analyses of Mg and Ti atoms in materials at the ppm level.