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Physical Review A

Lifetime measurements of interference-narrowed sodium Stark resonances

Patrick J. McNicholl, T. H. Bergeman, H. Metcalf

Published May 1, 1988• 24 citations
Publisher

Abstract

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 $^{2}P$ state to \ensuremath{\sim}285 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (${n}^{\mathrm{*}}$\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.

DOI
10.1103/physreva.37.3302

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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 cm\ensuremath−1{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}cm\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.
The populations of n=17,m=0 Stark states of sodium atom in static electric fields have been obtained by two step excitation using tunable lasers. The lifetimes of some selected Stark states and the influences of blackbody radiation on the lifetimes were measured by using delayed electric field ionization. The measured data were compared with the calculated values. The experimental and theoretical results exhibit following behaviours: The total rates of stimulated emission and absorption transitions induced by environmental radiations are of the same order of magnitude in comparison to those of spontaneous transition for the observed n=17 Stark manifold states at room temperature. And this effect is more remarkable for longer lived states. It, along with the effect of the Stark mixing of l sublevels, will reduce the differences among lifetimes of Stark states in a manifold.
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.
We have exploited the interference narrowing of Stark resonances to map electrostatic fields to a previously unattained level of accuracy and spatial resolution. The technique involves stepwise excitation with tightly focused laser light, an atomic beam, and temporal observation of ionization over a region of interference narrowing. Field inhomogeneities as small as 70 ppm have been detected with spatial resolution of 250 \ensuremath{\mu}m in three dimensions. The method is used here with rubidium atoms at fields of 3--4 kV/cm, but any alkali-metal atom over a range from a few tens of V/cm to a few tens of kV/cm is appropriate. Ionization rates may be calculated with the WKB quantum-defect method, but the field-mapping results are independent of the theory. Here this mapping technique is used to evaluate two electrode configurations for precision Stark spectroscopy. Field inhomogeneities produced by an electrode with a narrow slit, when properly constructed, are found to be in good agreement with detailed electrostatic calculations. However, discrepancies between field measurements and calculations for a metal mesh over an aperture indicate the presence of unavoidable crinkles of the mesh. We conclude that the slot is the preferred arrangement, and we present calculations that give the field inhomogeneity as a function of the slot size.
Numerical calculations are presented for the interference spectra of a laser-excited three-level atom, where the strong and the weak atomic transitions are driven by resonant and nonresonant laser fields, respectively. The spectral functions describing the interference spectra for the electric dipole allowed excited state have been considered in the low- and high-intensity limit of the laser field operating in the strong transition. The interference spectra arise from the competition between short-lifetime spontaneous processes and short- and long-lifetime excitations induced by the strong and the weak laser fields, respectively. Both laser fields have been treated as quantized and as classical entities. The computed spectra have been presented graphically for different values of the Rabi frequencies and detunings of the weak laser field. It is shown that the decrease in the intensity of the short-lifetime excitation may provide a measure of the spectral width of the long-lifetime excitation.