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Physical Review A

Effects of positive-ion resonances in photoionization of neutral atoms

Stanley L. Haan, Martin Bolt, H. Nymeyer, R. Grobe

Published Jun 1, 1995• 10 citations
Publisher

Abstract

Single photoionization of neutral atoms is examined theoretically for a situation in which the photoionizing laser is tuned close to the resonant frequency for transitions between the ground and first-excited states of the positive ion. The resonant coupling in the positive ion is shown to have a dramatic peak-splitting effect on the kinetic-energy spectrum of the ionized electron. An essential-states analysis of the photoionization process is presented, and a comparison is made with numerical results for one-dimensional helium. The agreement between the results establishes the validity of the essential-states approach. The possibility of superposing this core-coherence effect with laser-induced autoionization effects is considered.

DOI
10.1103/physreva.51.4640

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Two-step resonant photoionization of free Ni atoms is studied by one-color and two-color processes, using pulsed lasers with photon energies around half the ground-state ionization energy. Two-color two-step resonance ionization from the ground state a${\mathrm{}}^{3}$${\mathit{F}}_{4}$ and the first metastable state a${\mathrm{}}^{3}$${\mathit{D}}_{3}$ via the triplet states z${\mathrm{}}^{3}$${\mathit{G}}_{3,4,5}^{0}$ was used to probe the autoionizing states just above the ionization limit. Several strong lines in the one-color ionization spectrum are explained as direct two-photon transitions to autoionizing states. Photoionization cross sections were deduced from saturation measurements. The large cross sections observed for specific one-color two-step transitions are due to the near double resonant excitation into autoionizing states. \textcopyright{} 1996 The American Physical Society.

Physical Review A•Sep 1, 1996•26 citations
Single photoionization of neutral atoms is examined theoretically for a situation in which the photoionizing laser is tuned close to the resonant frequency for transitions between the ground and first-excited states of the positive ion. The resonant coupling in the positive ion is shown to have a dramatic peak-splitting effect on the kinetic-energy spectrum of the ionized electron. An essential-states analysis of the photoionization process is presented, and a comparison is made with numerical results for one-dimensional helium. The agreement between the results establishes the validity of the essential-states approach. The possibility of superposing this core-coherence effect with laser-induced autoionization effects is considered.
Photoionization processes are considered for situations in which the laser frequency is close to a resonant frequency of the final positive ion core. It is predicted that resonant coupling within the ionic core can split photoelectron spectral peaks into doublets, with components separated by the effective Rabi frequency of the core. The possibility of superposing this final-state effect with previously studied effects involving laser-induced autoionizing states is considered. Specific predictions are made for the splitting of photoelectron peaks in photoionization of strontium.
The ionization spectrum of the He atom under double-resonance irradiation is studied theoretically. A previously developed theory is extended and adapted to treat this process. The first source (synchrotron) is tuned around the transition from the ground state to the 23sp-- 1Po^{1}P^{o}1Po autoionizing state, and the second source (laser) is put in resonance with the transition from 23sp-- 1Po^{1}P^{o}1Po to the higher-lying quasibound state 2p3p-- 1Pe^{1}P^{e}1Pe (or 2p4p, 2p5p). Accurate ab initio calculations have been performed to obtain all the necessary parameters. It is found that the single-field ionization spectrum is strongly modified, exhibiting coherent trapping and ac Stark splitting. These results are in consonance with previous model calculations as well as with other ab initio calculations on a different zone of the He continuum. The results show that for the resonances considered these strong effects can be observed at megawatt laser intensities.
A theoretical model of two-step resonant ionization which includes an autoionizing state is developed. The effects examined include interactions with a multimode laser and Zeeman and hyperfine multiplets. We approach this problem starting from first principles by treating the atom-radiation system as a unit. Theoretical analysis and numerical calculations of ionization probabilities are given. The intermode scattering is investigated in detail, and a simple physical interpretation of the dynamical effect is given. Comparisons of ionization probabilities for two-step processes with first-step excitations by single-mode or two-mode lasers are made. It is found that, if the mode spacing in a multimode-laser beam is less than the Doppler width of the atom, the multimode laser can more effectively produce ions than a single-mode laser does. This leads to the conclusion that the resonance effect is more important than the power-broadening effect. This is shown to be also true when Zeeman and hyperfine multiplets are included.
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.
Atoms in doubly excited states above the first ionization limit can decay via autoionization in which an electron is emitted leaving an ion, or by photoemission which leaves the atom in a singly excited state. In this paper, it is demonstrated that interaction between the atoms and a laser pulse that is short compared to the autoionization lifetime can lead to large enhancement of the photoemission process by stimulating the atoms to emit a photon. Since the resultant singly excited atoms do not autoionize, this process can be viewed as an enhancement of the stabilization of the doubly excited atoms against autoionization. A simple theoretical model is outlined that shows good agreement with the experimental results.
Two-step resonant photoionization of free Ni atoms is studied by one-color and two-color processes, using pulsed lasers with photon energies around half the ground-state ionization energy. Two-color two-step resonance ionization from the ground state a3{\mathrm{}}^{3}3F4{\mathit{F}}_{4}F4​ and the first metastable state a3{\mathrm{}}^{3}3D3{\mathit{D}}_{3}D3​ via the triplet states z3{\mathrm{}}^{3}3G3,4,50{\mathit{G}}_{3,4,5}^{0}G3,4,50​ was used to probe the autoionizing states just above the ionization limit. Several strong lines in the one-color ionization spectrum are explained as direct two-photon transitions to autoionizing states. Photoionization cross sections were deduced from saturation measurements. The large cross sections observed for specific one-color two-step transitions are due to the near double resonant excitation into autoionizing states. \textcopyright{} 1996 The American Physical Society.