(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 ${\mathrm{Ar}}^{+}$ ion sputtering of an InP solid sample and the photoions are detected by a quadrupole mass spectrometer. The laser power dependence of the 3${\mathit{p}}^4$ ${\mathit{S}}_{3/2}^0$–4 ${}_{}{}^4{}_{}{}^{}\mathrm{S}_{3/2}^0{}_{}{}^{}$ two-photon excitation and one-photon ionization of 4${\mathit{p}}^4$ ${\mathit{S}}_{3/2}^0$ phosphorus is determined in the 10–200 MW/${\mathrm{cm}}^2$ range. The cross section for the ionization step is estimated in the frame of the quantum-defect theory: σ∼3×${10}^{\mathrm{-}18}$ ${\mathrm{cm}}^2$. The two-photon Rabi frequency ${\mathrm{\Omega }}_{\mathit{R}}$ 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 ${\mathrm{\Omega }}_{\mathit{R}}$(${\mathrm{s}}^{\mathrm{-}1}$)=8${\mathit{I}}_{\mathit{L}}$(W/${\mathrm{cm}}^2$). This value compares relatively well with the theoretical estimate ${\mathrm{\Omega }}_{\mathit{R}}$=5.5${\mathit{I}}_{\mathit{L}}$ obtained by limiting the perturbation-theory summation to the dominant intermediate states. The combination of well-characterized ${\mathrm{Ar}}^{+}$ 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 μm.