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Physical Review A

ac Stark effect in multiphoton ionization

Stephen E. Moody, Melissa Lambropoulos

Published Apr 1, 1977• 46 citations
Publisher

Abstract

Multiphoton ionization of atomic sodium has been studied experimentally for near resonance between the radiation field and several bound-bound transitions of the atom. It has been found that the atom-field interaction must be treated in a highly nonlinear way to describe successfully the observations, even when the absolute spectral power densities are relatively modest. Rabi frequencies as high as 10 GHz are found to be directly observable with simple flashlamp-pumped tunable dye lasers.

DOI
10.1103/physreva.15.1497

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Multiphoton ionization of atomic sodium has been studied experimentally for near resonance between the radiation field and several bound-bound transitions of the atom. It has been found that the atom-field interaction must be treated in a highly nonlinear way to describe successfully the observations, even when the absolute spectral power densities are relatively modest. Rabi frequencies as high as 10 GHz are found to be directly observable with simple flashlamp-pumped tunable dye lasers.
An intense laser and a probe laser in near resonance with the sodium $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ and $3{P}_{\frac{1}{2}}\ensuremath{\rightarrow}4{D}_{\frac{3}{2}}$ transitions, respectively, produce a three-photon ionization current used to study the broadening and Rabi splitting of the 3P123{P}_{\frac{1}{2}}3P21​​ intermediate state as a function of probe-laser tuning. We find a narrow, intense-laser detuning region where the peak-heights ratio of the Rabi-split 3P123{P}_{\frac{1}{2}}3P21​​ doublet is reversed from that predicted for a monochromatic laser field. This is attributed to the finite (\ensuremath{\sim} GHz) laser linewidth.
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 5p5p5p state is measured.
Multiphoton ionization (MPI) spectra of sodium and cesium were obtained in a crossed N2N_{2}N2​ pumped dye laser and alkali beam. Hybrid resonance were observed at the cesium 7d2D3/2−6p2P1/27d^{2}D_{3/2} -6p^{2}P_{1/2}7d2D3/2​−6p2P1/2​ and 7p2P3/2−6p2P1/27p^{2}P_{3/2}- 6p^{2}P_{1/2}7p2P3/2​−6p2P1/2​ transitions and the sodium 4d2D3/2,5/2−3p2P1/2,3/24d^{2}D_{3/2, 5/2} - 3p^{2}P_{1/2, 3/2}4d2D3/2,5/2​−3p2P1/2,3/2​ 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.
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 study the interaction of a two-level atom with two lasers of different frequencies and amplitudes: a strong laser of Rabi frequency $2{\ensuremath{\Omega}}_{1}$ on resonance with the atomic transition, and a weaker laser detuned by subharmonics $(2{\ensuremath{\Omega}}_{1}/n)$ of the Rabi frequency of the first. We find that under these conditions the second laser couples the dressed states created by the first in an nnn-photon process, resulting in ``doubly dressed'' states and in a ``multiphoton ac Stark'' effect. We calculate the eigenstates of the doubly dressed atom and their energies, and illustrate the role of this multiphoton ac Stark effect in its fluorescence, absorption, and Autler-Townes spectra.