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Physical Review A

Multiphoton ionization of rubidium

C. B. Collins, Stephen M. Curry, B. W. Johnson, M. Y. Mirza, M. A. Chellehmalzadeh, Jon A. Anderson, D. Popscu, Iovitzu Popescu

Published Nov 1, 1976• 49 citations
Publisher

Abstract

The multiphoton excitation of rubidium has been investigated over the 4600-6500-\AA{} wavelength region with a tunable dye-laser source having a linewidth better than 0.1 \AA{} and a space-charge ionization detector sensitive to a few ions per second. Multiphoton transitions have been observed to occur both through intermediate atomic states and through intermediate continuum states of the rubidium molecule. In the former case two-photon transitions have been observed from the $5^{2}S$ ground state of atomic rubidium to higher-lying $n^{2}D$ levels for values of $n$ ranging from 9 to 34 and to $n^{2}S$ levels for values of $n$ from 11 through 20. The fine-structure intervals of the $n^{2}D$ levels for $n=9 \mathrm{to}c 13$ were measured together with the line-strength ratios and were found to be in good agreement with the predictions of a simple theoretical model. At the shorter wavelengths hybrid two-photon resonances were observed to be excited through resonant intermediate continuum states of ${\mathrm{Rb}}_{2}$. As a result the dispersion curve for two-photon absorption in rubidium showed what appears to be resonant intermediate $p\ensuremath{\pi}(^{3}\ensuremath{\Sigma}_{u1})$ and ${0}_{g}^{+}$ terms dissociating to give a $5{P}_{\frac{3}{2}}$ atom and resulting in the strong development of features corresponding to the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}D_{\frac{3}{2},\frac{5}{2}}$ part of the diffuse series, for $n\ensuremath{\le}50$ in absorption and the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}S_{\frac{1}{2}}$ part of the sharp series. Components to $32^{2}D$ and $30^{2}S$ were recorded to a precision of 0.3 ${\mathrm{cm}}^{\ensuremath{-}1}$ and quantum defects for these previously unobserved terms were derived. The corresponding hybrid two-photon resonances involving intermediate states dissociating to give a $5^{2}P_{\frac{1}{2}}$ atom were not observed in the wavelength interval available in this experiment.

DOI
10.1103/physreva.14.1662

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Experimental and theoretical results are presented for resonantly enhanced three-photon ionization of cesium atoms in the wavelength range from 830 to 950 nm. The accidental near resonance of the $6p^{2}P_{\frac{3}{2},\frac{1}{2}}$ levels at the one-photon energy and the near resonance at the two-photon level with the $6d^{2}D_{\frac{3}{2},\frac{5}{2}}$ states gives rise to large ionization signals and interesting anomalies in the relative intensities of the fine-structure levels. "Hybrid resonances" are observed which involve the production of $6p ^{2}P_{\frac{1}{2}}$ atoms from molecular cesium. One hybrid resonance involves ionization of the $6p ^{2}P_{\frac{1}{2}}$ state via an electric quadrupole transition ($6p ^{2}P_{\frac{1}{2}}\ensuremath{-}7p ^{2}P_{\frac{3}{2}}$). Both experimental and theoretical photoelectron angular distributions are compared for all of the observed resonances except for the $6p ^{2}P_{\frac{3}{2}}$ state where the signals were too weak for such a measurement. Agreement between experiment and theory is fair for three-photon ionization via the $6p^{2}P_{\frac{1}{2}}$ state. Comparison of experiment and theory for the case of the $6d^{2}D_{\frac{3}{2},\frac{5}{2}}$ states indicates that hyperfine coupling during the laser-pulse duration may be important. The angular distributions for the hybrid resonances show the general behavior predicted by theory but the features appear broadened.

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John A. Yeazell, Michelle S. Malcuit, Carlos R. Stroud +1 more

We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.

Annual Meeting Optical Society of America•Jan 1, 1985•0 citations

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

O. Gobert, Bernard Dubreuil, R.L. Inglebert +2 more

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.

Physical Review A•Jun 1, 1990•9 citations

Two-photon ionization of metastable helium

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Laser-induced resonant multiphoton and collisional ionizations of rubidium atoms

С. А. Бахрамов, E. V. Vaganov, А. М. Коххаров +1 more

Rubidium vapor (1011 - 1014 cm-3) was irradiated by a dye-laser beam (10 kW(DOT)cm-2 - 2 MW(DOT)cm-2, pulse duration approximately 30 nsec, linewidth <EQ 0.25 cm-1), which was tuned to the D1,2-resonance lines. The mutual competition of the two photon ionization, associative ionization and Penning ionization of the laser excited rubidium atoms was investigated. The data of laser intensities and a rubidium vapor concentrations, which results in domination of the each ionization channels were founded. On the basis of the received experimental results, the cross-section values for the multiphoton and collisional ionization channels of laser-excited atoms were determined.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 30, 2002•3 citations
The multiphoton excitation of rubidium has been investigated over the 4600-6500-\AA{} wavelength region with a tunable dye-laser source having a linewidth better than 0.1 \AA{} and a space-charge ionization detector sensitive to a few ions per second. Multiphoton transitions have been observed to occur both through intermediate atomic states and through intermediate continuum states of the rubidium molecule. In the former case two-photon transitions have been observed from the 52S5^{2}S52S ground state of atomic rubidium to higher-lying n2Dn^{2}Dn2D levels for values of nnn ranging from 9 to 34 and to n2Sn^{2}Sn2S levels for values of nnn from 11 through 20. The fine-structure intervals of the n2Dn^{2}Dn2D levels for n=9toc13n=9 \mathrm{to}c 13n=9toc13 were measured together with the line-strength ratios and were found to be in good agreement with the predictions of a simple theoretical model. At the shorter wavelengths hybrid two-photon resonances were observed to be excited through resonant intermediate continuum states of Rb2{\mathrm{Rb}}_{2}Rb2​. As a result the dispersion curve for two-photon absorption in rubidium showed what appears to be resonant intermediate p\ensuremathπ(3\ensuremathΣu1)p\ensuremath{\pi}(^{3}\ensuremath{\Sigma}_{u1})p\ensuremathπ(3\ensuremathΣu1​) and 0g+{0}_{g}^{+}0g+​ terms dissociating to give a 5P325{P}_{\frac{3}{2}}5P23​​ atom and resulting in the strong development of features corresponding to the 52P32\ensuremath→n2D32,525^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}D_{\frac{3}{2},\frac{5}{2}}52P23​​\ensuremath→n2D23​,25​​ part of the diffuse series, for n\ensuremath≤50n\ensuremath{\le}50n\ensuremath≤50 in absorption and the 52P32\ensuremath→n2S125^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}S_{\frac{1}{2}}52P23​​\ensuremath→n2S21​​ part of the sharp series. Components to 322D32^{2}D322D and 302S30^{2}S302S were recorded to a precision of 0.3 cm\ensuremath−1{\mathrm{cm}}^{\ensuremath{-}1}cm\ensuremath−1 and quantum defects for these previously unobserved terms were derived. The corresponding hybrid two-photon resonances involving intermediate states dissociating to give a 52P125^{2}P_{\frac{1}{2}}52P21​​ atom were not observed in the wavelength interval available in this experiment.
The two-photon excitation and three-photon ionization of atomic cesium is investigated over the 6550-6950-\AA{} wavelength region with a tunable-dye-laser source having a 0.06-0.08-\AA{} linewidth and a space-charge ionization detector sensitive to a few ions per second. An online data-acquisition computer provides significant signal-to-noise recovery. The resulting dispersion curve for photoionization is interpreted in terms of the two-photon transitions from the ground 62S126^{2}S_{\frac{1}{2}}62S21​​ level to resonant n2Dn^{2}Dn2D and n2Sn^{2}Sn2S intermediate states, and represents, to the authors' knowledge, the first such two-photon absorption spectrum of Cs. Transitions from n=9to13n=9\mathrm{to}13n=9to13 have been recorded for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}D_{\frac{5}{2},\frac{3}{2}}$ series and from n=11to14n=11\mathrm{to}14n=11to14 for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}S_{\frac{1}{2}}$ series.
Experimental and theoretical results are presented for resonantly enhanced three-photon ionization of cesium atoms in the wavelength range from 830 to 950 nm. The accidental near resonance of the 6p2P32,126p^{2}P_{\frac{3}{2},\frac{1}{2}}6p2P23​,21​​ levels at the one-photon energy and the near resonance at the two-photon level with the 6d2D32,526d^{2}D_{\frac{3}{2},\frac{5}{2}}6d2D23​,25​​ states gives rise to large ionization signals and interesting anomalies in the relative intensities of the fine-structure levels. "Hybrid resonances" are observed which involve the production of 6p2P126p ^{2}P_{\frac{1}{2}}6p2P21​​ atoms from molecular cesium. One hybrid resonance involves ionization of the 6p2P126p ^{2}P_{\frac{1}{2}}6p2P21​​ state via an electric quadrupole transition ($6p ^{2}P_{\frac{1}{2}}\ensuremath{-}7p ^{2}P_{\frac{3}{2}}$). Both experimental and theoretical photoelectron angular distributions are compared for all of the observed resonances except for the 6p2P326p ^{2}P_{\frac{3}{2}}6p2P23​​ state where the signals were too weak for such a measurement. Agreement between experiment and theory is fair for three-photon ionization via the 6p2P126p^{2}P_{\frac{1}{2}}6p2P21​​ state. Comparison of experiment and theory for the case of the 6d2D32,526d^{2}D_{\frac{3}{2},\frac{5}{2}}6d2D23​,25​​ states indicates that hyperfine coupling during the laser-pulse duration may be important. The angular distributions for the hybrid resonances show the general behavior predicted by theory but the features appear broadened.
We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.
Two-photon-resonant three-photon ionization of atomic phosphorus is studied in the 298--306-nm wavelength range. P atoms are produced by Ar+{\mathrm{Ar}}^{+}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{\mathit{p}}^{4}p4S3/20{\mathit{S}}_{3/2}^{0}S3/20​--4 4S3/20^{4}\mathrm{S}_{3/2}^{0}4S3/20​ two-photon excitation and one-photon ionization of 4p4{\mathit{p}}^{4}p4S3/20{\mathit{S}}_{3/2}^{0}S3/20​ phosphorus is determined in the 10--200 MW/cm2{\mathrm{cm}}^{2}cm2 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{}${10}^{\mathrm{\ensuremath{-}}18}$ cm2{\mathrm{cm}}^{2}cm2. 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 ${\mathrm{\ensuremath{\Omega}}}_{\mathit{R}}$(${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$)=8IL{\mathit{I}}_{\mathit{L}}IL​(W/cm2{\mathrm{cm}}^{2}cm2). This value compares relatively well with the theoretical estimate ${\mathrm{\ensuremath{\Omega}}}_{\mathit{R}}$=5.5IL{\mathit{I}}_{\mathit{L}}IL​ obtained by limiting the perturbation-theory summation to the dominant intermediate states. The combination of well-characterized Ar+{\mathrm{Ar}}^{+}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.
There have been relatively few investigations of multiphoton ionization from metastable helium. Of particular interest has been the work of Haberland et al. 1987 and Haberland and Oschwald 1988. In both the 1987 and 1988 papers they have described the two photon ionization of metastable helium. In each of these studies they have reported the occurrence of unexplained structure along the wings of their resonance profiles. Upon the performance of similar measurements by this study, the unexplained structure is not seen and the agreement of the experiment's measurements with the theoretical shape of the resonance curves has been good. To experimentally verify these resonance effects, we have used a tunable dye laser in conjunction with a time of flight mass spectrometer to create and detect ions from metastable helium by two-photon absorption. The use of a metastable state instead of the ground state is advantageous because of its proximity to the ionization continuum and its extended lifetime. Using a metastable state as a starting point for multiphoton absorption requires fewer photons to reach the ionization threshold. The extended lifetime of the state also makes it easy to access experimentally. For helium the singlet metastable state 2$\sp1$S lies at 20.61 eV above the ground level with a natural lifetime of close to a millisecond. Two photons of 501.7 nm and 504.35 nm are required for the ionization processes in resonance with the 3$\sp1$P and the 3$\sp1$D states. This thesis is the accounting of the experimental process involved in the measurement of the dipole and quadrupole resonances of two photon ionization from singlet metastable helium. The study includes the description of the laser, electron gun assembly for metastable helium creation, and the time of flight mass spectrometer. A discussion of the theory of multiphoton processes is included along with the discussion of the data, its reduction and analysis, and a comparison with theoretical prediction. This study has not only involved the acquisition of definitive atomic spectra data sets, but has also established a facility for resonant ionization spectroscopy at The American University.
Rubidium vapor (1011 - 1014 cm-3) was irradiated by a dye-laser beam (10 kW(DOT)cm-2 - 2 MW(DOT)cm-2, pulse duration approximately 30 nsec, linewidth <EQ 0.25 cm-1), which was tuned to the D1,2-resonance lines. The mutual competition of the two photon ionization, associative ionization and Penning ionization of the laser excited rubidium atoms was investigated. The data of laser intensities and a rubidium vapor concentrations, which results in domination of the each ionization channels were founded. On the basis of the received experimental results, the cross-section values for the multiphoton and collisional ionization channels of laser-excited atoms were determined.