(2+1) RESONANT MULTIPHOTON IONIZATION OF SPUTTERED P-ATOMS - APPLICATION TO THE DETECTION OF PHOSPHORUS IN SILICON SAMPLES

Two-photon-resonant three-photon ionization of atomic phosphorus is studied in the 298306-nm wavelength range. P atoms are produced by Ar+ ion sputtering of an InP solid sample and the photoions are detected by a quadrupole mass spectrometer. The laser power dependence of the 3p 4S3/204 S3/204 two-photon excitation and one-photon ionization of 4p 4S3/20 phosphorus is determined in the 10200 MW/cm2 range. The cross section for the ionization step is estimated in the frame of the quantum-defect theory: 3×10-18 cm2. The two-photon Rabi frequency R is deduced from the comparison of our experimental results with the extended two-level model developed by Eberly [Phys. Rev. Lett. 42, 1049 (1979)] to describe (m+n) resonant multiphoton ionization processes and with the rate equation approximation analysis. The best fit gives R(s-1)=8IL(W/cm2). This value compares relatively well with the theoretical estimate R=5.5IL obtained by limiting the perturbation-theory summation to the dominant intermediate states. The combination of well-characterized Ar+ ion sputtering of a solid sample with resonant photoionization is used to perform actual trace analysis of materials. For example, 0.5 ppm of phosphorus in a silicium sample was measured by this method with a lateral resolution of 300 m. © 1990 The American Physical Society.