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Physical Review Letters

Coherent population trapping probed by charge exchange reactions

M Gieler, F. Aumayr, Laurentius Windholz

Published Dec 14, 1992• 22 citations
Publisher

Abstract

Coherent trapping of atomic population in sodium induced by two-frequency laser light has been studied by using a charge exchange reaction for probing the upper level population. When the two-photon resonance condition occurs, the signal due to electron capture from excited ${\mathrm{Na}}^{\mathrm{*}}$(3p) atoms by ${\mathrm{He}}^{2+}$ projectile ions exhibits the characteristic ``black resonance'' dip, indicating (by nonoptical means) that the fraction of Na atoms in the excited state actually drops to zero.

DOI
10.1103/physrevlett.69.3452

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Coherent trapping of atomic population in sodium induced by two-frequency laser light has been studied by using a charge exchange reaction for probing the upper level population. When the two-photon resonance condition occurs, the signal due to electron capture from excited Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗(3p) atoms by He2+{\mathrm{He}}^{2+}He2+ projectile ions exhibits the characteristic ``black resonance'' dip, indicating (by nonoptical means) that the fraction of Na atoms in the excited state actually drops to zero.
Light-induced drift (LID) is a transport phenomenon in a gas mixture with dramatic manifestations such as the optical piston1 and applications in atomic isotope separation.2 The induced drift velocity is proportional to the excited state fraction, and for Na in Xe high drift velocities have been achieved using two lasers.3 Here one laser excites atoms from one hyperfine ground state and the second laser atoms from the other, avoiding hyperfine pumping. Excited in this way, the Na atom represents a Λ type three-level system, where coherent effects can be important.
We report on an investigation of electromagnetically induced transparency (EIT) in sodium vapour. In our experiment, sodium atoms are excited on the D1-line with laser radiation containing two components with a frequency difference close to that of the two hyperfine ground states of sodium (1.772 GHz). Such an excitation leads to coherent trapping of atomic population in “dark” superpositional states, which dramatically reduces the absorption of light. A frequency transparency window is measured to have a subnatural width, which is a clear indication of coherent population trapping. Dependence of EIT on laser frequencies and intensities, on the magnetic field strength as well as on the temperature of the sodium vapour is studied.
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.
We have calculated the excitation spectra of a single electron in the excited state of a four-level atom interacting with three laser fields at low intensities, where two of them are on two-photon resonance with the three levels of the atom, while the third laser is a weak one operating between the fourth and excited level of the atom. It is shown that light is switched off at certain frequency due to the coherent population trapping while light is turned on at another frequency, where stimulated emission occurs leading to light amplification without population inversion.
We have recently demonstrated rf resonance experiments on Na atoms contained in a magnetic trap.1,2 Sequential rf pulses with appropriate frequencies were applied to a sample of ~2×1010 Na atoms (confined in an ~100-cm3 volume) to transfer population between the four trapped magnetic substates. The relative populations of the magnetic substates were detected by sending a weak narrowly collimated probe laser beam along the axis of the trap and monitoring the induced fluorescence with a photodiode (placed along the wall of the trap enclosure) located at the longitudinal magnetic field minimum. Hence we are sensitive only to light emanating from the small fraction of atoms near the magnetic field minimum.