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Physical Review A

Quantum interferences in the collisional excitation of a structured continuum

Roberto Buffa

Published Dec 1, 1993• 2 citations
Publisher

Abstract

This Brief Report reports a study of coherent phenomena occurring in the collisional excitation of an atomic continuum of ionization structured by intense laser radiation. Two interfering pathways to the continuum, induced by interatomic interaction processes (collision-induced ionization and radiative-collisional excitation followed by photoionization), give rise to Fano-type line shapes for the cross section of the process.

DOI
10.1103/physreva.48.4797

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This Brief Report reports a study of coherent phenomena occurring in the collisional excitation of an atomic continuum of ionization structured by intense laser radiation. Two interfering pathways to the continuum, induced by interatomic interaction processes (collision-induced ionization and radiative-collisional excitation followed by photoionization), give rise to Fano-type line shapes for the cross section of the process.
States behaving like autoionizing states can be selectively induced by laser radiation into one of the continuum spin channels in the photoionization of polarized excited alkaki atoms. As a result of destructive or constructive interference between the direct ionization channel and those introduced by the dressing laser radiation, the cross section of this specific spin component is completely suppressed or enhanced, while leaving the other spin channel unaffected. The qqq parameter which determines the line shape of the Fano-type resonance can be resonantly tuned as a function of the dressing laser frequency.
The ordinary Fano effect occurs in many-electron atoms and requires an auto-ionizing state.With such a state, photo-ionization may proceed via pathways that interfere, and the characteristicasymmetric resonance structures appear in the continuum. Here we demonstrate that Fano structure15may also be induced without need of auto-ionization, by dressing the continuum with an ordinarybound state in any atom by a coupling laser. Using multi-photon processes gives complete, ultra-fastcontrol over the interference. We show that a line-shape indexqnear unity (maximum asymmetry)may be produced in hydrogenic silicon donors with a relatively a weak beam. Since the Fanolineshape has both constructive and destructive interference, the laser control opens the possibility20of state-selective detection with enhancement on one side of resonance and invisibility on the other.We discuss a variety of atomic and molecular spectroscopies, and in the case of silicon donors weprovide a calculation for a qubit readout application
Laser induced continuum structures (LICS) can be produced by a strong laser embedding an excited atomic bound state into a flat atomic continuum, leading to a tunable resonance of adjustable width that can be probed by a second laser. If this second laser is of similar intensity to the embedding laser then the distinction between the two becomes artificial and saturation effects become important. In this paper the effect on LICS of transitions caused by both lasers between the original atomic continuum and a second atomic continuum are studied in the Markoff approximation by means of Laplace transform methods and allowing for the ionization of each atomic state by both laser fields. Formal results correct to all orders are also given in terms of a T-matrix approach. Numerical calculations are presented showing the effects on the LICS resonant structures of continuum-continuum coupling processes. Significant changes to the Fano profiles (second laser weak) and to coherence holes (second laser strong) occur. Analytical results are also given.
Coherent control provides a quantum interference based method for controlling atomic and molecular dynamics. We report two experimental observations of different ways to achieve such a control in atomic photoionization continuum. One is the photoionization of sodium atoms, initially prepared in the 3P1/2 excited state, by a non resonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The second experiment is the observation of the modification of the ionization continua of xenon through laser induced continuum structure (LICS). The multiphoton radiative decay of the xenon ground state into the two electronic continua, that correspond to the two fine structure levels of the ground state of the xenon ion, is varied by electromagnetically embedding a bound state of the atom into the two continua. Both experiments are performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons.
Collisionally aided radiative excitation in a three-level atomic system is investigated with use of a stationary-phase method for the case of large atom-field detunings and weak incident fields. It is found that collision-induced coherent phase-interference effects can give rise to oscillatory structure in the total absorption cross section as a function of relative speed (energy). An example is given for a specific interatomic potential, indicating that experimental observation of such an effect is feasible.