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Physical Review Letters

Observation of Electric Quadrupole Transitions in Multiphoton Ionization

Melissa Lambropoulos, Stephen E. Moody, Stuart James Smith, W. C. Lineberger

Published Jul 21, 1975• 41 citations
Publisher

Abstract

Two flashlamp-pumped, tunable, dye lasers have been utilized to study three-photon ionization in atomic sodium. The resulting ion yield shows large peaks at laser frequencies which correspond to electric quadrupole transitions. This is the first direct observation of electric quadrupole effects in multiphoton ionization. The $3p^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}4f$ matrix element is determined, and the fine-structure splitting of the $5p$ state is measured.

DOI
10.1103/physrevlett.35.159

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William R. Christian, R. N. Compton, John A. D. Stockdale +4 more

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.

Physical Review A•Oct 1, 1984•11 citations
Two flashlamp-pumped, tunable, dye lasers have been utilized to study three-photon ionization in atomic sodium. The resulting ion yield shows large peaks at laser frequencies which correspond to electric quadrupole transitions. This is the first direct observation of electric quadrupole effects in multiphoton ionization. The 3p2P32\ensuremath→4f3p^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}4f3p2P23​​\ensuremath→4f matrix element is determined, and the fine-structure splitting of the 5p5p5p state is measured.
Photoelectron angular distributions have been measured for two-photon ionization of rubidium atoms both when the first photon was in resonance with the 4d 2{\mathrm{}}^{2}2D state via an electric quadrupole transition and when it was detuned from it. Angular distributions have also been obtained for the first and second above-threshold ionization (ATI) photoelectron peaks. Tuning to the 4d 2{\mathrm{}}^{2}2D resonance creates a large enhancement of the two-photon ionization photoelectron signal but causes only a modest increase in the ATI signal.
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 $p\ensuremath{\pi}(^{3}\ensuremath{\Sigma}_{u1})$ 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 $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}D_{\frac{3}{2},\frac{5}{2}}$ 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 322D32^{2}D322D and 302S30^{2}S302S 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 52P125^{2}P_{\frac{1}{2}}52P21​​ atom were not observed in the wavelength interval available in this experiment.
We report measurements of photoelectron angular distributions via electric quadrupole transitions in cesium atoms using multiphoton excitation techniques. Experimental results are presented for the cases of one-photon resonant, two-photon ionization via the 6d 2{\mathrm{}}^{2}2D3/2{D}_{3/2}D3/2​ and 6d 2{\mathrm{}}^{2}2D5/2{D}_{5/2}D5/2​ electric quadrupole transitions and for one-photon resonant, three-photon ionization via the 5d 2{\mathrm{}}^{2}2 D3/2{\mathrm{D}}_{3/2}D3/2​ and 5d 2{\mathrm{}}^{2}2D5/2{D}_{5/2}D5/2​ transitions. The ratios of the total ionization rates for linearly and circularly polarized light have also been measured for the 6d 2{\mathrm{}}^{2}2D3/2{\mathrm{D}}_{3/2}D3/2​ and 6d 2{\mathrm{}}^{2}2D5/2{D}_{5/2}D5/2​ states. A theoretical analysis and detailed comparison with the experimental results are given.
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.
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.