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MULTIPHOTON IONIZATION STUDIES OF SODIUM AND CESIUM ATOMS

William R. Christian, John C. Miller, R. N. Compton, John A. D. Stockdale

Published Jan 1, 1982• 0 citations
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Abstract

Multiphoton ionization (MPI) spectra of sodium and cesium were obtained in a crossed $N_{2}$ pumped dye laser and alkali beam. Hybrid resonance were observed at the cesium $7d^{2}D_{3/2} -6p^{2}P_{1/2}$ and $7p^{2}P_{3/2}- 6p^{2}P_{1/2}$ transitions and the sodium $4d^{2}D_{3/2, 5/2} - 3p^{2}P_{1/2, 3/2}$ transitions together with the more common MPI via intermediate S, P and D states. Power dependences for these processes are presented together with and interesting case of a switch from red to blue Stark shifting in the sodium MPI spectrum.

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Multiphoton ionization (MPI) of alkali atoms has played a pivotal role in our understanding of the interaction of intense electromagnetic radiation with matter. The "hydrogen-like" energy levels and absence of low-lying autoionizing states make the alkali atom theoretically tractable. The low ionization potential of these atomic species and the ease with which one can produce atomic beams of alkali atoms facilitate the experimental studies. Many of the early MPI studies involved high-powered fixed-frequency lasers. Presently, the availability of tunable dye lasers with wavelength extension (i.e., frequency doubling, Raman shifting, etc.) makes it possible to study one- to six-photon ionization of the alkali atom with continuously tunable radiation. Also, harmonic generation in these systems which may be present during ionization can be recorded more easily than in other systems since the radiation generated is in the visible or near ultraviolet spectral region. The ability to tune the laser to high Rydberg states with even or odd numbers of photons accesses electronic states of both parities. Stepwise excitation is capable of selectively producing states of high orbital angular momentum. Experiments in which highly excited states are produced in external electric or magnetic fields allow one to test fundamental ideas about the electron–Coulomb system in the quasi–continuum.3
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.
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.
Multiphoton ionization (MPI) of barium atom using an excimer-pumped pulsed dye laser has been studied in the wavelength range 570-608 nm at a laser intensity approximately 10.0 GW cm -2 . Resonances in MPI were recorded by monitoring either the ions produced in the multiphoton process using a thermionic detector or fluorescence from the ions formed in their various excited states of Ba + . The observation of photoion and fluorescence spectra were made on Ba vapour in the heat-pipe oven in the moderate-density regime ( approximately 10 13 atoms/cm 3 ) so that the collisional effects are negligible. Fine-structure resolution of the final ionic state (6p 2 P 1/2,3/2 ), produced after four-photon autoionizing decay was obtained. The decay of the Ba atom after four-photon absorption preferentially populates the 6p 2 P 3/2 state relative to the 6p 2 P 1/2 state.
Two independently tunable, linearly polarized dye lasers are counterpropagated through a heat pipe containing sodium vapor. The frequency of the first laser ${\ensuremath{\omega}}_{1}$ is incremented through both 3p2{p}^{2}p2P1/2{P}_{1/2}P1/2​,3/2 fine-structure levels while the second laser ${\ensuremath{\omega}}_{2}$ is scanned through the energy increment necessary to reach ns 2S1/2^{2}S_{1/2}2S1/2​ and nd2{\mathrm{nd}}^{2}nd2D3/2{D}_{3/2}D3/2​,5/2 levels (n=5--35). Following excitation, ionization occurs through chemi-ionizing collisions. The polarization (defined as the ionization intensity measured when the two planes of polarization of the laser beams are parallel minus the intensity when perpendicular divided by the sum of the intensities) P=(I?{I}_{?}I?​-${I}_{\ensuremath{\perp}}$)/(I?{I}_{?}I?​+${I}_{\ensuremath{\perp}}$) goes from +1 to -1 as ${\ensuremath{\omega}}_{1}$ is tuned between the 3p2{p}^{2}p2P1/2{P}_{1/2}P1/2​,3/2 fine-structure levels. This novel polarization effect is shown to be due to an interference between the two pathways to the ns 2S1/2^{2}S_{1/2}2S1/2​ levels. Upon exciting the unresolved nd 2D3/2^{2}D_{3/2}2D3/2​,5/2 levels, the polarization is very close to zero when ${\ensuremath{\omega}}_{1}$ is at or near the 3p 2P3/2^{2}P_{3/2}2P3/2​,1/2 levels. A simple few-level atom theory reproduces the experimental observations except for the case when both lasers are in resonance.