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Journal of Physics B Atomic Molecular and Optical Physics

Laser-induced structure in the continuum of sodium: a weak dressing field measurement

Stefano Cavalieri, R. Eramo, Lorenzo Fini

Published May 14, 1995• 26 citations
Publisher

Abstract

We report an experimental study of a laser-induced resonance in the continuum of sodium, performed by measuring the modified atomic susceptibility. The sensitivity of the set-up has allowed the observation of the induced structure in conditions of relatively weak intensity of the dressing laser field, where the line shape is independent of the field amplitude and the comparison with theory is strict and direct. We have found a good agreement with theoretical predictions and measured the Fano parameter of the resonance.

DOI
10.1088/0953-4075/28/9/016

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We report an experimental study of a laser-induced resonance in the continuum of sodium, performed by measuring the modified atomic susceptibility. The sensitivity of the set-up has allowed the observation of the induced structure in conditions of relatively weak intensity of the dressing laser field, where the line shape is independent of the field amplitude and the comparison with theory is strict and direct. We have found a good agreement with theoretical predictions and measured the Fano parameter of the resonance.
Observation of laser-induced continuum structure (LICS) in sodium due to the coupling of a bound state to the continuum by a strong laser field has been reported elsewhere. Such experiments have mostly employed a single laser photon to probe the structured continuum from the ground state. Other experiments have used multiphoton (third harmonic) probe schemes, but only one of these has successfully observed LICS, in agreement with theoretical predictions. We report here a very similar experiment generating third-harmonic radiation in sodium to probe the anticipated LICS produced by coupling the 5s state to the p continuum. However, under no conditions was any continuum structure observed, in agreement with in a number of other experiments, but at variance with theoretical predictions.
We report an experimental analysis of a structure induced by a Nd:YAG laser embedding the sodium excited state in the ionization continuum. The induced structure is probed from the ground state via a one-photon ionization. The experiment is performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analysing the emitted photoelectrons. The selectivity of this technique provides the best measurement of a laser-induced resonance in the photoionization spectrum reported to date. A good agreement is found between the theory and experimental results.
We present a theoretical study of the influence of laser-induced continuum structure on the third-harmonic generation in atomic gases. We employ the density-matrix formalism including the spatiotemporal structure of laser pulses. The theoretical model is then applied to third-harmonic generation in sodium experiments. Extensive numerical calculations using realistic atomic parameters in combination with the parameters in actual experimental situations demonstrate the subtle interplay of various effects and pinpoint causes of possible complications. We obtain good agreement with the few existing experimental data.
We present an experimental study of laser-induced continuum structure in cesium, performed by observing the birefringence and dichroism induced in the atomic medium by a circularly polarized laser embedding the 8s 2^{2}2S1/2{\mathit{S}}_{1/2}S1/2​ state into the ionization continuum. The optimization of the sensitivity for the detection of the polarization rotation has allowed a high-accuracy measurement of the resonance profile. A quantitative comparison with theoretical predictions, taking into account the hyperfine structure of atomic levels, shows a clear evidence of a line-broadening effect due to cesium resonant collisions.
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.
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.