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Physical Review A

Resonant infrared two-photon ionization of D(n=8) atoms

J. E. Bayfield, David W. Sokol

Published Oct 1, 1986• 1 citation
Publisher

Abstract

Two-photon ionization of hydrogenic atoms with principal quantum number n=8 has been studied experimentally over the wavelength range 10.45--10.59 \ensuremath{\mu}m. The ionization was resonantly enhanced by the n=12 intermediate state, with a large n=8 to n=12 coupling responsible for a fractional resonance width of about 0.1%. On resonance, ionization rates of order 10 MHz were achieved with nominal 2-mJ, 40-nsec laser pulses, with 40 mJ being required at wavelengths 1% away from resonance. The data are interpreted in terms of the extended two-level model for multiphoton ionization. The peak laser electric field strength reached values where the top of the instantaneous potential energy barrier was near the intermediate-state energy value.

DOI
10.1103/physreva.34.2977

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Two-photon ionization of hydrogenic atoms with principal quantum number n=8 has been studied experimentally over the wavelength range 10.45--10.59 \ensuremath{\mu}m. The ionization was resonantly enhanced by the n=12 intermediate state, with a large n=8 to n=12 coupling responsible for a fractional resonance width of about 0.1%. On resonance, ionization rates of order 10 MHz were achieved with nominal 2-mJ, 40-nsec laser pulses, with 40 mJ being required at wavelengths 1% away from resonance. The data are interpreted in terms of the extended two-level model for multiphoton ionization. The peak laser electric field strength reached values where the top of the instantaneous potential energy barrier was near the intermediate-state energy value.
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 measured the resonance-enhanced four-photon ionization of atomic hydrogen. The degenerate four-photon process occurs via a three-photon resonance between the 1s and the 2p levels, with subsequent one-photon ionization near threshold. We have studied the highly asymmetric resonance-enhanced profile, i.e., the photoion yield as a function of laser detuning from three-photon resonance between the 1s and 2p levels. In particular, we have determined the width, shift, peak, and asymmetry of the profile as a function of laser intensity. The experimental results are compared to theoretical models. These models involve both the properties of the atom in an intense near-resonant radiative field, and a detailed model for the multimode laser field, particularly the field fluctuations due to mode beating. The asymmetric resonance-enhanced photoionization profile is well reproduced by both a random-phase and a chaotic-light model.
The resonantly enhanced multiphoton ionization of atomic hydrogen is studied for the case of four-photon ionization (three-photon resonant). The time-dependent probabilities, the shape of the resonance profile at different intensities, and the nonlinear index are computed for coherent light at frequencies around the resonance (365 nm). The systematic dependence of the cross section as a function of the principal quantum number of the resonant state (as this tends towards the Rydberg states) is investigated at zero detuning. The probabilities of absorption of one photon more than necessary for the four-photon ionization process (resonantly enhanced above-threshold ionization) is computed and the relevant branching ratios are examined. Then it is shown that the effect of the spatio-temporal structures of the laser pulse is to red shift the maximum of the resonance profile, whereas that of an ideal chaotic light is to blue shift and broaden the resonance peak. The shape, shift, and width of the profile resulting from both these effects are in very fair agreement with the experimental ones.
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 present both experimental and theoretical studies of resonantly enhanced multiphoton ionization of cesium atoms. Photoelectron angular distributions for two-photon ionization via the one-photon-allowed 7p2P12,327p^{2}P_{\frac{1}{2},\frac{3}{2}}7p2P21​,23​​ and 8p2P12,328p^{2}P_{\frac{1}{2},\frac{3}{2}}8p2P21​,23​​ intermediate states and three-photon ionization via the two-photon-allowed 8d2D52,328d^{2}D_{\frac{5}{2},\frac{3}{2}}8d2D25​,23​​ resonant intermediate states are reported. The photoelectrons are energy analyzed with a spherical-sector electrostatic energy analyzer with an energy resolution of ~ 0.1 eV and angular resolution of ~ \ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}. A straightforward calculation based upon a finestructure scheme gives excellent agreement with the measured angular distributions for the 2P12^{2}P_{\frac{1}{2}}2P21​​ states. The laser pulse duration is comparable to the hyperfine precession period which allows the hyperfine coupling to partially destroy the anisotropy initially produced in the 2P32^{2}P_{\frac{3}{2}}2P23​​ resonant intermediate states. Quantitative calculations including hyperfine coupling take this into account and provide an expression which gives a reasonable fit to the 2P32^{2}P_{\frac{3}{2}}2P23​​ data.