(2+1) resonant multiphoton ionization of sputtered P atoms: Application to the detection of phosphorus in silicon samples.

Abstract

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: \ensuremath{\sigma}\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ ${\mathrm{cm}}^{2}$. The two-photon Rabi frequency ${\mathrm{\ensuremath{\Omega}}}_{\mathit{R}}$ 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 ${\mathrm{\ensuremath{\Omega}}}_{\mathit{R}}$(${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$)=8${\mathit{I}}_{\mathit{L}}$(W/${\mathrm{cm}}^{2}$). This value compares relatively well with the theoretical estimate ${\mathrm{\ensuremath{\Omega}}}_{\mathit{R}}$=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 \ensuremath{\sim}300 \ensuremath{\mu}m.