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Journal of Physics B Atomic Molecular and Optical Physics

Application of an electro-optical modulator in inelastic collision studies with laser-excited Na*(3p) atoms

M Gieler, F. Aumayr, Richard Gaggl, Christian Neureiter, Laurentius Windholz

Published Jan 28, 1993• 6 citations
Publisher

Abstract

In atomic collision experiments with laser-excited target atoms it is often desirable to maximize the fraction f* of excited atoms. In the case of Na atoms pumped by a single mode CW dye laser radiation several effects including hyperfine pumping limit f* to about 10%. Using two side-bands of laser radiation, produced by means of an electro-optical modulator (phase modulation of laser radiation by a LiTaO 3 crystal in a microwave field) transitions from both hyperfine ground states of Na can be pumped simultaneously. In this way a significant increase of the fraction of excited Na atoms in the 3 2 P states to typically 30% could be achieved. The advantage of using an electro-optical modulator is demonstrated for the case of state-selective single electron capture studies of the He 2+ -Na*(3p) collision system.

DOI
10.1088/0953-4075/26/2/013

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In atomic collision experiments with laser-excited target atoms it is often desirable to maximize the fraction f* of excited atoms. In the case of Na atoms pumped by a single mode CW dye laser radiation several effects including hyperfine pumping limit f* to about 10%. Using two side-bands of laser radiation, produced by means of an electro-optical modulator (phase modulation of laser radiation by a LiTaO 3 crystal in a microwave field) transitions from both hyperfine ground states of Na can be pumped simultaneously. In this way a significant increase of the fraction of excited Na atoms in the 3 2 P states to typically 30% could be achieved. The advantage of using an electro-optical modulator is demonstrated for the case of state-selective single electron capture studies of the He 2+ -Na*(3p) collision system.
Electron scattering by excited sodium atoms has been carried out in a crossed-beam experiment. The atoms have been optically pumped into a well defined hyperfine level (3 2 P 3 2/F=3) by a tunable CW dye laser. For two of the most prominent scattering processes, the 3 2 P to 3 2 S and 3 2 P to 4 2 S transitions, the effect of the anisotropy of the initial state introduced by the laser light is studied in detail and is interpreted in terms of multipole moments describing the optical excitation and the collision process. Possibilities and limitations of the experiment are discussed. Electron collisions may be seen as a test case for the understanding of more involved interaction processes.
We describe optically pumping a beam of sodium atoms to >96% mS and >92% mS, mI state selection. (We have accurately measured the population of every mS, mI state in the optically pumped beam.) For the optical pumping both ground hyperfine states are pumped, using single-mode cw dye-laser radiation tuned to the 3S1/2–3P1/2 transition that is phase modulated in a LiTaO3 crystal to produce first-order sidebands at approximately the 1772-MHz hyperfine splitting of the ground state. The z-directed optical pumping is performed in a z-directed magnetic field of ~5 G. The state-selected atoms then move, in ~1 cm, into an ~200 G, z-directed field. The downstream probe laser beam is scanned through the 3S1/2 (mS, mI) → 3P3/2 (mS′, mI) transitions, which are spectroscopically resolved at 200 G, and the fluorescence intensities portray the residual populations in each of the eight 3S1/2 states.
In a beam experiment the authors have studied the energy pooling reaction between two laser excited Na* (3p) atoms, where in the final state one of the sodium atoms is in the 5s or 4d state and the other in the ground state. They have determined all eight independent partial cross sections and coherence terms, specified according to the various magnetic substate combinations of the colliding Na* (3p) atoms, as a function of the collision velocity. This is the maximum information that can be extracted from the experiment given the symmetry of the detection scheme. By applying a weak magnetic field, to orient the Na* (3p) atoms, the authors were able to distinguish between the two partial cross sections sigma 11 and sigma 1-1 , where the subscripts 11 and 1-1 refer to the initial magnetic substate combinations of the two collision partners. By using an electro-optical modulator the authors have furthermore improved the signal-to-noise ratio to an extent where it was possible to measure all coherence terms.
In studying ionization reactions in a single dense effusive beam of laser-excited Na(3p) atoms, we observe production of both Na{sub 2}{sup +} and Na{sup +} ions. The Na{sup +} results from the familiar associative ionization reaction of two Na(3p) atoms and the Na{sup +} ions under our conditions result mostly from the collisional dissociation of the Na{sub 2}{sup +} by reaction with a third excited Na(3p) atom, rather than from photo- dissociation or photoionization. A CW, single-frequency dye laser tuned to 589 nm resonantly excites the Na(3s) atoms to the 3p state. The atomic beam Na(3s) density is determined from low-intensity 589 absorption measurements. Na{sup +} ion yield depends on the cube of the Na beam density, suggesting successive collisions of three Na(3p) atoms: we believe the Na{sub 2}{sup +} ions polarize the third Na(3p) atom, resulting in a R{sup -4} orbiting collision interaction. A large collisional dissociation cross-section of approximately (4{plus_minus}2)x10{sup -13} cm{sup 2} results, for oven temperatures in the 500-600K range.
Two different atomic species, sodium and potassium, are resonantly excited in a ceel by a CW dye laser and a potassium lamp, respectively. The density of the excited Na (3P) atoms in the cell is measured directly using the absorption-equivalent-width technique on the lines of the diffuse series. To isolate the heteronuclear contribution in the fluorescence spectrum, a modulation detection technique is adopted. The energy-pooling collisions between Na (3P) and K (4P) are observed and cross sections are obtained.
The steady-state distribution of the populations of the hyperfine sublevels of the ground state of alkali atoms is calculated for the case in which the atoms are subjected to a spectrally selective optical pumping on /sup 2/S/sub 1//sub ///sub 2/--/sup 2/P/sub 1/2,3/2/ transitions and a simultaneous pumping by microwave fields which are at resonance with transitions in the hyperfine structure of the ground state, F-italic = 2, M-italic/sub F-italic/ = +- 2, +- 1bold-arrow-left-rightF-italic = 1, M-italic/sub F-italic/ = +- 1. The addition of the microwave pumping is shown to substantially increase the population difference for the O--O transition in the hyperfine structure. During selective optical pumping of the F-italic = 1 level, the population inversion which can be achieved for the O--O transition is limited by the effect of population capture. This capture can be eliminated by using incoherent microwave fields. The quality factor of the O--O resonance is calculated as a function of the parameters of the pump. The outlook for the use of composite pumping in frequency-stabilization systems is discussed.