(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 298--306-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 3p⁴{S}3/2⁰$--4 $⁴S3/2⁰$ two-photon excitation and one-photon ionization of 4${p}⁴S3/2⁰ phosphorus is determined in the 10--200 MW/cm² range. The cross section for the ionization step is estimated in the frame of the quantum-defect theory: {σ}{~}3×{}10^-18 cm². The two-photon Rabi frequency ΩR 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 ΩR(s^-1)=8IL(W/cm²). This value compares relatively well with the theoretical estimate ΩR=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 {μ}m.