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Physical Review A

Quantum beats in photoionization from a coherent superposition of fine-structure levels

A. T. Georges, P. Lambropoulos

Published Sep 1, 1978• 35 citations
Publisher

Abstract

We present a theory of quantum beats in photoionization from a coherent superposition of ${D}_{\frac{3}{2},5.2}$ levels in alkali-metal atoms. The semiclassical density-matrix formalism is used to describe both the excitation of the ${D}_{\frac{3}{2},\frac{5}{2}}$ levels by a two-photon resonant pulse and the subsequent photoionization by a delayed probe pulse. Quantum beats occur in the ionization signal as a function of the delay of the probe pulse. The depth of modulation is determined for different light polarizations.

DOI
10.1103/physreva.18.1072

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Journal of the Physical Society of Japan•May 1, 1981•0 citations
We present a theory of quantum beats in photoionization from a coherent superposition of D32,5.2{D}_{\frac{3}{2},5.2}D23​,5.2​ levels in alkali-metal atoms. The semiclassical density-matrix formalism is used to describe both the excitation of the D32,52{D}_{\frac{3}{2},\frac{5}{2}}D23​,25​​ levels by a two-photon resonant pulse and the subsequent photoionization by a delayed probe pulse. Quantum beats occur in the ionization signal as a function of the delay of the probe pulse. The depth of modulation is determined for different light polarizations.
Pulsed dye lasers are used to excite sodium atoms into a coherent superposition of high Rydberg nd(2D32,2D52)\mathrm{nd} (^{2}\mathrm{D}_{\frac{3}{2}},^{2}D_{\frac{5}{2}})nd(2D23​​,2D25​​) states. Analysis of the field ionization behavior of these atoms reveals the time evolution of the superposition state. Rapid application of a weak electric field projects different components of the superposition state onto states having identifiably different field ionization behavior. Analysis of the signal from subsequent field ionization as a function of the time of weak-field application reveals fine-structure quantum beats. These beats result from the time development of the original superposition state. Measurements of the fine-structure quantum beat frequency are presented for the principal quantum numbers n=32,34,35,36,38,and40n=32, 34, 35, 36, 38, \mathrm{and} 40n=32,34,35,36,38,and40.
An expression for the probability of time-resolved three-photon ionization via coherently excited resonant states of a molecular system is derived in the perturbative density matrix formalism with the aid of the Liouville space Feynman diagram. It is shown how the time evolution of the molecular coherence in the first resonant state can be detected as a function of the delay time between the pumping and probing lasers through the probing two-photon ionization process. The three-photon ionization process consists of the simultaneous and sequential processes which are classified by the Liouville space Feynman diagrams. The possibility of appearance of quantum beats in the pump–probe three-photon ionization is discussed by performing model calculations of the probability of ionization. The role of the direct and redistributed processes is discussed. The redistributed process is induced by intramolecular processes in the resonant state. The theoretical treatment is applied to the (1+2) three-photon ionization of pyrazine observed by Knee et al. It is demonstrated that the fast decay component (110 ps) of the pyrazine spectra originates from the intramolecular dephasing of the vibronic coherence created by the pumping pulse laser, and this component appears only when the redistributed process (the ionization process through triplet levels) is absent.
Photoionization processes involving polarized Xe atoms in the presence of an external magnetic field were analyzed in our previous studies. It was shown that the modulation of quantum beats in the observed electronic photo signals could not be explained by the Paschen-Back effect accounting for fine-structure levels only. In the current work, ionized Xe+ isotopes are used as the output channel of photoionization. By comparing the temporal partial signals in the mass spectra of the Xe+ isotopes, it was possible to explain the modulation of quantum beats in ion fluxes as a consequence of the presence of the hyperfine structure in the Zeeman components of the 129Xe and 131Xe isotopes. Keywords: quantum beats, hyperfine structure, Zeeman effect, two-photon excitation, femtosecond ionization, "pump-probe"-experiment.