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Progress in theoretical chemistry and physics

QED Approach to Atoms in a Laser Field: Multi-Photon Resonances and Above Threshold Ionization

Alexander V. Glushkov, О. Yu. Khetselius, Andrey V. Loboda, А. А. Свинаренко

Published Sep 11, 2008• 31 citations
Publisher
DOI
10.1007/978-1-4020-8707-3_25

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We describe experimental studies of resonantly enhanced multi‐photon ionization (MPI) of cesium atoms in the presence and absence of an external electric field. In the zero‐field studies, photo‐electron angular distributions for one‐ and two‐photon resonantly enhanced MPI are compared with the theory of Tang and Lambropoulos. Deviations of experiment from theory are attributed to hyperfine coupling effects in the resonant intermediate state. The agreement between theory and experiment is excellent. In the absence of an external electric field, signal due to two‐photon resonant three‐photon ionization of cesium via np states is undetectable. Application of an electric field mixes nearby nd and ns levels, thereby inducing excitation and subsequent ionization. Signal due to two‐photon excitation of ns levels in field‐free experiments is weak due to their small photoionization cross section. An electric field mixes nearby np levels which again allows detectable photo‐ionization signal. For both ns and np states the ‘‘field induced’’ MPI signal increases as the square of the electric field for a given principal quantum number and increases rapidly with n for a given field strength.Finally, we note that the classical two‐photon field‐ionization threshold is lower for the case in which the laser polarization and the electric field are parallel than it is when they are perpendicular.

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Two-photon above-threshold ionization of atoms

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Bulletin of the American Physical Society•May 19, 2006•0 citations
We describe experimental studies of resonantly enhanced multi‐photon ionization (MPI) of cesium atoms in the presence and absence of an external electric field. In the zero‐field studies, photo‐electron angular distributions for one‐ and two‐photon resonantly enhanced MPI are compared with the theory of Tang and Lambropoulos. Deviations of experiment from theory are attributed to hyperfine coupling effects in the resonant intermediate state. The agreement between theory and experiment is excellent. In the absence of an external electric field, signal due to two‐photon resonant three‐photon ionization of cesium via np states is undetectable. Application of an electric field mixes nearby nd and ns levels, thereby inducing excitation and subsequent ionization. Signal due to two‐photon excitation of ns levels in field‐free experiments is weak due to their small photoionization cross section. An electric field mixes nearby np levels which again allows detectable photo‐ionization signal. For both ns and np states the ‘‘field induced’’ MPI signal increases as the square of the electric field for a given principal quantum number and increases rapidly with n for a given field strength.Finally, we note that the classical two‐photon field‐ionization threshold is lower for the case in which the laser polarization and the electric field are parallel than it is when they are perpendicular.
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.