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Optics Letters

Collinear two-step laser excitation: determination of the oscillator strength for the 17 434–34 634-cm^−1 transition in ^238U ii

U. Nielsen, Ove Poulsen, L. Young

Published Dec 1, 1985• 3 citations
Publisher

Abstract

The feasibility of direct determination of optical oscillator strengths in free atoms and ions has been investigated using two-step laser excitation in a collinear fast-beam geometry. Experimental results obtained in the uranium ion are analyzed within the density-matrix formalism. A Rabi frequency Omega = 5.8 +/- 0.8 MHz is found for the 17 434(j=1(1/2))-34 634(J=1(1/2)) transition in (238)U II, corresponding to an oscillator strength f 8.6 x 10(-4) +/- 3.3 x 10(-4) for this transition.

DOI
10.1364/ol.10.000591

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Physical Review A•Jan 1, 1990•12 citations
The feasibility of direct determination of optical oscillator strengths in free atoms and ions has been investigated using two-step laser excitation in a collinear fast-beam geometry. Experimental results obtained in the uranium ion are analyzed within the density-matrix formalism. A Rabi frequency Omega = 5.8 +/- 0.8 MHz is found for the 17 434(j=1(1/2))-34 634(J=1(1/2)) transition in (238)U II, corresponding to an oscillator strength f 8.6 x 10(-4) +/- 3.3 x 10(-4) for this transition.
Laser-induced photoionization and fluorescence signals were simultaneously observed in atomic uranium using a single Nd:YAG-pumped dye laser. These signals were recorded in two specific cases. In the first case, the dye laser was resonant to the first-step transition (0–16900.38 cm−1). In the second case, the laser was near-resonant to the first-step transition with a slight detuning (0.15 cm−1) so that it became two-photon resonant at 33801.06 cm−1. The uranium atoms in the ground state were ionized by a single-color, three-photon photoionization technique resulting in the photoionization signal, and the fluorescence signal was simultaneously obtained from the first excited state involved in the photoionization process. The photoionization and the fluorescence signals in the above-mentioned cases were also estimated theoretically for several values of the photoionization cross section for the transition between the second excited state at 33801.06 cm−1 and the autoionization state at 50701.59 cm−1 using density matrix formalism. From the comparison of theoretically calculated ratios of fluorescence signals in the two specific cases with the experimentally obtained values, the photoionization cross section for the 33801.06–50701.59 cm−1 transition has been obtained, which is found to be (5±1)×10−16 cm2.
We have used two methods to measure the oscillator strength of the transition between the ground and 17992 cm-1 level in 174Yb. The first technique involves exciting the transition with a laser pulse that is nearly time-bandwidth limited, of uniform intensity, and has a reproducible shape from shot to shot. The population left in the excited state after the pulse varies sinusoidally with a period that depends on the integral over time of the electric field amplitude and the transition oscillator strength. These are the Rabi oscillations that are predicted by application of the Schrodinger equation to the two-level atom. The excited-state population is probed using a two-step photoionization to the continuum. The electric field amplitude is determined from the temporal profile and the intensity of the laser pulse. The second method involves observation of the polarization rotation of a set of degenerate sublevels brought about by a time-bandwidth limited light-shift laser. One sublevel (mj = 0) of the J = 1 level at 17992 cm-1 is populated by a linearly polarized laser. A second copropagating light-shift laser, which is linearly polarized at an angle to the first laser, is tuned between 7.5 and 30 GHz off-resonance with the transition. The light-shift laser causes population to be promoted into the mi = ±1 levels by a virtual transition through the ground J = 0, = 0 level. Two linearly polarized photoionizing lasers photoionize the population only from the mi = ±1 levels. The photoion signal oscillates sinusoidally with a period that depends only on the integrated pulse intensity, the laser detuning, and the transition oscillator strength.
We report the experimentally determined oscillator strengths for the 5s6s5s6s5s6s ${^{1}S}_{0}\ensuremath{\rightarrow}5snp$ 1P1{^{1}P}_{1}1P1​ Rydberg transitions of strontium using two-step excitation in conjunction with a thermionic diode ion detector. The absolute photoionization cross section from the 5s6s5s6s5s6s 1S0{^{1}S}_{0}1S0​ excited state has been determined by adjusting the polarization vector of the ionizing laser beam parallel, perpendicular, and at the magic angle with respect to that of the exciting dye laser. The measured absolute value of the photoionization cross section $0.9\ifmmode\pm\else\textpm\fi{}0.2\text{ }\text{Mb}$ at the 5s5s5s threshold is used to extract the fff values of the 5s6s5s6s5s6s ${^{1}S}_{0}\ensuremath{\rightarrow}5snp$ 1P1{^{1}P}_{1}1P1​ $(26\ensuremath{\le}n\ensuremath{\le}73)$ Rydberg transitions. The oscillator strength in the discrete region merges smoothly to the oscillator strength density at the ionization threshold.
We report the first observation of optical nutation in a collinear fast-ion-beam-laser experiment. 35-keV Ba+{\mathrm{Ba}}^{+}Ba+ ions are suddenly tuned into resonance with a strong laser field by post acceleration over a short distance. The degree of excitation as a function of time is monitored by observing the spontaneously emitted light along the ion beam. Oscillations as a function of transition, laser intensity, and laser frequency detuning have been studied. Theoretical curves have been calculated in the density-matrix formalism.
Two methods have been used to measure the oscillator strength of the transition between the ground and 17,992 cm level in XUYb. The first technique involves exciting the transition with a laser pulse that is nearly time-bandwidth limited, of uniform intensity, and has a reproducible shape from shot to shot. The population left in the excited state after the pulse varies sinusoidally with a period that depends on the integral over time of the electric field amplitude and the transition oscillator strength. These are the Rabi oscillations that are predicted by application of the Schrodinger equation to the two-level atom. The second method involves observation of the polarization rotation of a set of degenerate sublevels brought about by a light-shift laser. One sublevel (m/sub j/ = 0) of the J = 1 level at 17,992 cm is populated by a linearly polarized laser. A second copropagating light-shift laser, which is linearly polarized at an angle to the first laser, is tuned between 7.5 and 30 GHz off-resonance with the transition. The light-shift laser causes population to be promoted into the m/sub j/ = +-1 levels through the virtual J = 0, m/sub j/ = 0 level. Two linearly polarized photoionizing lasers photoionize the population only from the m/sub j/ = +-1 levels. The photoion signal oscillates cosinusoidally with a period that depends only on the integrated pulse intensity, the laser detuning, and the transition oscillator strength. Finally, polarization selectivity has been shown experimentally to allow selective photoionization of the odd isotopes of ytterbium using broadband lasers.
Optical nutation in Ba ii has been studied with a rapid Doppler-shifting technique in a cw collinear fast-ion-beam--laser experiment. Time-resolved nutation curves have been recorded as a function of transition, laser irradiance, laser frequency detuning, and laser polarization. Frequency spectra recorded at different, well-specified, times of interaction between the ions and the laser field reveal several interesting effects. Among them is the pronounced influence of the postacceleration potential on the line profiles. Theoretical curves, calculated in the density-matrix formalism, reproduce all the characteristics of the experimental data. Future possibilities of this technique are discussed, e.g., the prospects of determining transition probabilities through the measurements of optical nutation.