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Physica Scripta

Light induced modification of continua and ionization dynamics

Dimitrios Charalambidis, O. Faucher, Natalia E. Karapanagioti, Yun Shao, Cornelis J. G. J. Uiterwaal, D. Xenakis

Published Jan 1, 1997• 5 citations
Publisher

Abstract

We review experimental results on the electromagnetically induced modification of atomic continua and ionization characteristic. Results are presented on laser induced continuum structure (LICS) in smooth and structured continua, in A and ladder type of couplings. Coherent control of the ionization rates is demonstrated in four photon resonant, five photon ionization schemes in rare gases.

DOI
10.1088/0031-8949/1997/t72/005

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We review experimental results on the electromagnetically induced modification of atomic continua and ionization characteristic. Results are presented on laser induced continuum structure (LICS) in smooth and structured continua, in A and ladder type of couplings. Coherent control of the ionization rates is demonstrated in four photon resonant, five photon ionization schemes in rare gases.
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We report on the observation of the modification of the two spin-orbit ionization continua of Xe through laser-induced continuum structure (LICS). The radiative decay of the Xe ground state into the two electronic continua that correspond to the two fine-structure levels of the ground state of the Xe+{\mathrm{Xe}}^{+}Xe+ ion can be varied by electromagnetically embedding a bound state of the atom into the two continua. The dressed 2P1/2{}^{2}{P}_{1/2}2P1/2​ continuum, probed from the atomic ground state through three-photon absorption, exhibits a clear induced structure, while the 2P3/2{}^{2}{P}_{3/2}2P3/2​ continuum possesses no observable structure under the present experimental conditions, as manifested through high-resolution photoelectron spectra. The modified ionization ratio is controlled by the wavelength of the dressing field.
We report on the observation of the modification of the final status products in a photoionization of the Xe atom through the coherent process of laser-induced continuum structure (LICS). The photoionization decay of the Xe ground state into the two electronic continua, corresponding to the two fine structure levels of the ground state of the Xe ion, is varied by embedding a bound state into the two continua through a laser field (dressing laser). The dressed continua, probed from the atomic ground state through three-photon absorption, exhibit different induced structures. The experiment is performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons. The modified final state amplitudes are controlled by the wavelength of the dressing laser. No significant modification of the 2P3/2 continuum has been observed, in the limit of the detection efficiency. The effect is characterized by a window resonance in the slow electron continuum as the dressing laser is tuned over the LICS resonance.
We report the observation of pronounced, spectrally narrow laser-induced continuum structures (LICS) in the two ionization continua of xenon. A nanosecond laser pulse with nearly transform-limited bandwidth induces a LICS by coupling the initially unpopulated 5p59p[1/2]0{5p}^{5}9p[1/2{]}_{0}5p59p[1/2]0​ state to the continuum. The LICS is probed by one-photon ionization of the 5p56p[1/2]0{5p}^{5}6p[1/2{]}_{0}5p56p[1/2]0​ state. This state is populated in an independent excitation step by an additional laser pulse acting prior to the probe and dressing pulses. The two ionization channels that leave the xenon ion in either the 2P1/2{}^{2}{P}_{1/2}2P1/2​ or the 2P3/2{}^{2}{P}_{3/2}2P3/2​ fine-structure state are detected separately by monitoring the kinetic energy of the photoelectrons in a time-of-flight electron spectrometer. A suppression of the photoelectron signal of up to 80% as compared to the far off resonance value is achieved. The line shapes reveal a Fano parameter close to zero. The LICS is identical for the coupling into the two continua. Therefore the branching ratio into the two ionization channels does not vary with the dressing-laser detuning for the coupling scheme implemented here.
We present experimental and theoretical results of a detailed study of laser-induced continuum structures (LICS's) in the photoionization continuum of helium out of the metastable state 2s1S0.2s{}^{1}{S}_{0}.2s1S0​. The continuum dressing, with a 1064-nm laser, couples the same region of the continuum to the 4s1S04s{}^{1}{S}_{0}4s1S0​ state. The experimental data, presented for a range of intensities, show pronounced ionization suppression (by as much as 70% with respect to the far-from-resonance value) as well as enhancement, in a Beutler-Fano resonance profile. This ionization suppression is a clear indication of population trapping mediated by coupling to a continuum. We present experimental results demonstrating the effect of pulse delay upon the LICS's, and for the behavior of LICS's for both weak and strong probe pulses. Simulations based upon numerical solution of the Schr\"odinger equation model the experimental results. The atomic parameters (Rabi frequencies and Stark shifts) are calculated using a simple model-potential method for the computation of the needed wave functions. The simulations of the LICS profiles are in excellent agreement with experiment. We also present an analytic formulation of pulsed LICS's. We show that in the case of a probe pulse shorter than the dressing one, the LICS profile is the convolution of the power spectra of the probe pulse with the usual Fano profile of stationary LICS's. We discuss some consequences of deviation from steady-state theory.
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.