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Physical Review Letters

Interference narrowing at crossings of sodium Stark resonances

J.-Y. Liu, Patrick J. McNicholl, David A. Harmin, J. Ivri, T. H. Bergeman, H. Metcalf

Published Jul 8, 1985• 46 citations
Publisher

Abstract

We have observed a striking line narrowing in the photoionization spectrum of sodium atoms in an electric field. At crossings of two relatively broad resonances, one of them is narrowed by 2 to 3 orders of magnitude as a result of interference between the discrete and continuum coupling amplitudes that mediate their ionization. The observations agree quantitatively with WKB quantum-defect Stark theory and also with a simpler model involving two discrete states and two continua.

DOI
10.1103/physrevlett.55.189

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We have observed a striking line narrowing in the photoionization spectrum of sodium atoms in an electric field. At crossings of two relatively broad resonances, one of them is narrowed by 2 to 3 orders of magnitude as a result of interference between the discrete and continuum coupling amplitudes that mediate their ionization. The observations agree quantitatively with WKB quantum-defect Stark theory and also with a simpler model involving two discrete states and two continua.
We report measurements of the temporal decay of sodium Stark resonances over regions of interference narrowing and compare with calculations using Wentzel-Kramers-Brillouin quantum-defect (WKB-QD) Stark theory. The measurements are performed with a beam of sodium atoms in a uniform electric field of \ensuremath{\sim}3.2 kV/cm excited via the 3 2P^{2}P2P state to \ensuremath{\sim}285 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (n∗{n}^{\mathrm{*}}n∗\ensuremath{\sim}20) below the zero-field ionization threshold, and hence substantially above the classical saddle-point energy. For mixed states at level crossings, the discrete-continuum couplings interfere, yielding resonances that are stabilized against ionization over a small interval of the field. Measurements of the maximum lifetimes of \ensuremath{\Vert}m\ensuremath{\Vert}=1 levels here are not limited by laser bandwidth, as were our previous spectral observations of m=0 resonances. For three narrowing regions studied here, the observed maximum lifetime values of 127--191 nsec are 10--20 % less than values obtained from WKB-QD calculations, while experimental error limits are \ensuremath{\sim}5%. After considering estimates for competing processes, namely radiative decay and transfer by blackbody radiation, a slight discrepancy remains.
We have studied experimentally and theoretically two-step photoionization through the 3P state of sodium in an electric field of up to 3.90 kV/cm. The apparatus uses an atomic beam crossed by two pulsed laser beams, one at 589 nm for the first step, and the other swept through the zero field threshold Eo near 409 nm for the ionization. We observe broad undulations in the photocurrent above Eo that narrow with decreasing energy and eventually sharpen into well-defined but asymmetric transitions to Stark-shifted Rydberg states. We calculate the oscillator strength density for the appropriate transitions using modified hydrogenic wave functions and find excellent agreement for the position and relative strength of all the spectral features. We model the asymmetries as Fano-type interferences between quasi-discrete and continuum ionization channels. Using appropriate core potentials, and taking into account core-induced coupling of both discrete-to-continuum and continuum-to-continuum states, we obtain estimates of Fano's q and Ɖ that agree reasonably well with our measurements.
Stark spectra of barium Rydberg states above the classical field-ionization limit have been recorded at high density of states employing high-resolution laser spectroscopy. The spectra, taken at a binding energy of about \ensuremath{\varepsilon}\ensuremath{\approxeq}-55 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ with respect to zero-field threshold and at electric-field strengths F\ensuremath{\approxeq}100 V/cm exhibit dramatic variations in line profiles with increasing external field strength. The WKB quantum-defect theory of the Stark effect of nonhydrogenic atoms was used to analyze the recorded spectra, and excellent agreement between measured and calculated line profiles was achieved. Core-induced coupling of closed and open hydrogenic Stark channels was found to contribute dominantly to the linewidth of the recorded Stark resonances. In addition, a reformulation of this theory by McNicholl, Ivri, and Bergeman (unpublished) in terms of a configuration-interaction model was employed to analyze a particular avoided crossing of several Stark components. In this way recorded line profiles were parametrized within a limited model to obtain resonance energies, linewidths, and Fano q parameters.
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.
Experimental results on the photoionization spectra of excited rubidium atoms in a well-defined ml=1{m}_{l}=1ml​=1 state in the presence of an electric field are reported. Spectra corresponding to a definite polarization of the ionizing light exhibit characteristic resonances which are ascribed either to the non-Coulombic part of the central potential or to the spin-orbit interaction. The different line profiles are interpreted as being due to interferences between discrete states and continua, the observed structures depending directly on the density of oscillator strengths and not simply on the density of continuum states.