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Physical Review A

Resonance, antiresonance, and decoherence in the excitation of channeled ions in crystals

A. Salin, A. Arnau, Pedro Miguel Echenique

Published Apr 1, 1998• 15 citations

Abstract

We discuss general criteria differentiating incoherent excitation processes from coherent processes allowing the production of resonant coherent excitation (RCE) of channeled ions in crystals. We give special consideration to the ionization of C${}^{5+}$ ions traveling with energies between 15 and 25 MeV along the $〈100〉$ direction of a gold crystal. We show that incoherent ionization of the $n=2$ state by the valence electrons partially destroys the coherence producing resonant coherent excitation of the $n=\stackrel{\ensuremath{\rightarrow}}{1}n=2$ transition by the target static potential. It thus causes a considerable broadening of the resonance signal, as observed in the experiments. Finally we show that far from resonance, coherence quenches projectile excitation by the target static potential (anti-Okorokov effect).

DOI: 10.1103/physreva.57.2772

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We discuss general criteria differentiating incoherent excitation processes from coherent processes allowing the production of resonant coherent excitation (RCE) of channeled ions in crystals. We give special consideration to the ionization of C

{}^{5+}

ions traveling with energies between 15 and 25 MeV along the

〈100〉

direction of a gold crystal. We show that incoherent ionization of the

〈100〉

state by the valence electrons partially destroys the coherence producing resonant coherent excitation of the

n=\stackrel{\ensuremath{\rightarrow}}{1}n=2

transition by the target static potential. It thus causes a considerable broadening of the resonance signal, as observed in the experiments. Finally we show that far from resonance, coherence quenches projectile excitation by the target static potential (anti-Okorokov effect).

We report our observation of the resonant fluorescence from highly charged uranium ions. Using the resonant coherent excitation (RCE) technique, the

2s

-

2{p}_{3/2}

transition in 191.68 MeV/u Li-like U

{}^{89+}

ions was excited at 4.5 keV with a resonance width of 4.4 eV. The result demonstrated that the RCE can be applied to resonant fluorescence spectroscopy of high-

Z

ions up to uranium with high efficiency and resolution.

Two-color excitation by spatially coherent and incoherent laser beams has been used to study three-photon-resonant excitation and subsequent ionization of xenon in conditions, when internally generated sum-frequency field plays an important role in excitation of atomic resonances through interfering one-photon excitation pathway. We show that the incoherence in one of the pumping fields reduces the efficiency of generated sum-frequency field, and thus suppresses the interference between the three- and the one-photon excitation channels. The degree of suppression is controlled by varying the crossing angle between coherent and incoherent laser beams. We show that ionization profiles can be analyzed on the basis of the well-studied interference of one- and three-photon transition amplitudes, but with pumping field decomposed into multiple small-scale uncorrelated domains where coherent process of four-wave mixing occurs. The gain length for a coherent process in these domains depends on the coherence degree and excitation geometry. It gives a possibility of controlling the contribution of coherent processes to the excitation of multiphoton resonances.

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.

For the first time, associative ionization has been observed in an atomic beam of excited sodium atoms in both the and the state. Relative values for the partial cross sections were obtained specified according to the different magnetic substate combinations of the two fine-structure states. These were found to be comparable to those for associative ionization between two atoms. However, the effect of the excitation on the ion production rate was found to be considerable and to depend strongly on the relative polarizations of the laser beams producing the two populations due to the redistribution of the involved magnetic sublevels. The results indicate that the initial orientation of the electron spin S does not significantly influence the associative ionization reaction at the mean collision energy of 25 meV.