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Physical Review Letters

Observation of a laser-induced resonance in the photoionization spectrum of sodium

Stefano Cavalieri, Francesco Saverio Pavone, M. Matera

Published Dec 23, 1991• 81 citations
Publisher

Abstract

We report the observation of a structure in the ionization spectrum of sodium, induced by a strong radiation field embedding the 5s $^{2}$${\mathit{S}}_{1/2}$ state into the continuum. The structure, probed by single-photon ionization, shows a marked asymmetry, typical of a laser-induced continuum structure.

DOI
10.1103/physrevlett.67.3673

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We report the observation of a structure in the ionization spectrum of sodium, induced by a strong radiation field embedding the 5s ^{2}$${\mathit{S}}_{1/2} state into the continuum. The structure, probed by single-photon ionization, shows a marked asymmetry, typical of a laser-induced continuum structure.
Structure in the continuum of an atomic system attributed to the coupling of a bound state by a strong laser field has been reported elsewhere (1-3). Such experiments have employed a second laser to probe the continuum from the ground state by measuring polarization rotation 0), third harmonic generation (2) or three photon ionization (3). We report here an experiment similar to reference 2 using two photon resonant, third harmonic generation in sodium, perturbed by a second laser which couples the 5s state to the continuum. Under no conditions was any laser induced continuum structure (LICS) observed in these experiments, in contrast to the results reported elsewhere (2).
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.
In this contribution we present an experimental investigation of LICS (Laser‐Induced Continium Structure) in the single‐photon ionization spectrum of cesium and sodium. We report observations of the ionization spectrum of these atoms induced embedding a discrete state (5s for sodium and 8s for cesium) in the continuum by means of an IR laser. The structures, probed by single‐photon ionization from the ground state by an UV tunable laser show a marked asymmetry, typical of the process. We have performed the experiments observing the charge produced in the photoionization or the birefrincence and dichroism induced in the atomic medium. The two techniques allow the study of LICS for different intensity of the embedding laser (from 108 to 1011 W/cm2) giving complementary information on the physical process. The measured line‐shapes are compared with quantitative predictions of theoretical models.
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 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.
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.