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Journal of Physics B Atomic Molecular and Optical Physics

Laser-induced continuum structure in helium:ab initionon-perturbative calculations

Niels J. Kylstra, Emmanuel Paspalakis, P. L. Knight

Published Sep 28, 1998• 25 citations
Publisher

Abstract

We present the results of non-perturbative, R -matrix Floquet calculations of laser-induced continuum structures (LICS) in helium. We consider the case in which a Nd:YAG laser is used to embed the 1s4s Rydberg state in the continuum. The resulting modification of the photoionization rate of the 1s2s state is then calculated. This LICS has been studied recently by Halfmann et al (1998 Phys. Rev. A 58 R46) and good agreement with their experimental and theoretical findings is obtained. We also investigate the effects of incoherent channels due to higher-order multiphoton processes and LICS involving the 1s4d state. The validity of the simple two-level models that are frequently used for modelling LICS is tested by comparing these results with R -matrix Floquet calculations.

DOI
10.1088/0953-4075/31/18/003

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We present the results of non-perturbative, R -matrix Floquet calculations of laser-induced continuum structures (LICS) in helium. We consider the case in which a Nd:YAG laser is used to embed the 1s4s Rydberg state in the continuum. The resulting modification of the photoionization rate of the 1s2s state is then calculated. This LICS has been studied recently by Halfmann et al (1998 Phys. Rev. A 58 R46) and good agreement with their experimental and theoretical findings is obtained. We also investigate the effects of incoherent channels due to higher-order multiphoton processes and LICS involving the 1s4d state. The validity of the simple two-level models that are frequently used for modelling LICS is tested by comparing these results with R -matrix Floquet calculations.
In recent years our understanding of how laser fields interact with a continuum of levels has progressed substantially. No longer is the continuum viewed as a dissipative environment, best described by a set of kinetic equations. Recent theoretical* and experimental work has shown that strong laser fields involving a continuum of levels may often maintain perfeet coherence. One of the m ore interesting colterent phenomena involving the continuum is the dressing of a continuum by a bound state, termed laser-induced continuum structure (LICS). In the LICS phenomenon, a strong laser field, connecting an unpopulated bound state to the continuum mixes some bound- state character into the continuum. This optically induced resonance can be detected by a second (weaker) laser field, which induces a transition from a populated bound state to the structured continuum.
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.
We report the observation of laser-induced structure (LICS) in the flat photoionization continuum of helium. The structure is a strong and spectrally sharp resonance, showing both enhanced and diminished ionization (a Beutler-Fano profile). Our observations show, in an otherwise unstructured continuum, pronounced ionization suppression (as much as 70%), which is due to population trapping associated with LICS. We also show the effect of dynamic Stark shifts upon the LICS profile. A discussed theoretical model, incorporating significant dynamic Stark shifts, gives a quantitatively accurate description of the line profile for a wide range of pulse intensities.
Summary form only given. We present experimental observations along with theoretical modeling of laser induced continuum structure (LICS) in helium. The excited state 4s/sup 1/S/sub 0/ is coupled to the ionization continuum with strong laser radiation at 1064 nm. The induced continuum structure is detected by ionization of the state 2s/sup 1/S/sub 0/ with a weak laser pulse at 294 nm. Enhancement as well as strong reduction of ionization due to trapping of population can be observed. We demonstrate how LICS structures move about in a systematic way as a result of the AC Stark effect and power broadening. Quantitative comparison between theory and experiment is possible because of the high spectral resolution achieved in this experiment.
Laser-induced continuum structures (LICS) can be produced by a strong laser embedding a low-lying atomic state into a flat atomic continuum, which leads to a tunable asymmetric Fano resonance of adjustable width that can be probed by a second weak laser. In this paper the effects on LICS of transitions caused by the embedding laser between the original atomic continuum and a second atomic continuum are studied in the Markoff approximation by means of Laplace transform methods. Formal results correct to all orders are presented in terms of a T-matrix approach. Numerical calculations are presented which include continuum-continuum coupling correct to the lowest power of the embedding laser intensity. The time evolution of the pure LICS Fano profile is also shown.