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Physical Review A

Controlling ionization products through laser-induced continuum structure

Stefano Cavalieri, R. Eramo, Lorenzo Fini, Marzia Materazzi, O. Faucher, Dimitrios Charalambidis

Published Apr 1, 1998• 31 citations
Publisher

Abstract

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 ${\mathrm{Xe}}^{+}$ ion can be varied by electromagnetically embedding a bound state of the atom into the two continua. The dressed ${}^{2}{P}_{1/2}$ continuum, probed from the atomic ground state through three-photon absorption, exhibits a clear induced structure, while the ${}^{2}{P}_{3/2}$ 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.

DOI
10.1103/physreva.57.2915

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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.

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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.
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.
We present experimental results on the modification of a molecular ionization continuum through a laser-induced continuum structure. The effect is demonstrated in nitric oxide where the resonant 2 + 1 multiphoton-ionization process of the X 2 3/2 electronic ground state is modified by dressing the continuum with the electromagnetically embedded M 2 + Rydberg state. Through selection of a two-photon rovibronic transition via the D electronic state, a single rotational ground state level is excited to the continuum with one laser field. By adding an extra dressing field, a coupling is then established between this three-photon excited ground state level and one of the M state rotational levels. The modification due to the dressed continuum results in a modification of the ground state level ionization signal in one of the decay channels where an asymmetric line-shape resonance is electromagnetically induced. Limiting effects due to ac-Stark shifts are discussed.
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.