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Physical Review A

Direct two-color photoassociative ionization in a rubidium magneto-optic trap

D. Leonhardt, J. Weiner

Published Dec 1, 1995• 22 citations
Publisher

Abstract

We report the observation of direct, two-color photoassociative ionization (PAI) in collisions between $^{85}\mathrm{Rb}$ atoms optically cooled and confined in a magneto-optical trap. We also measure the resulting ${\mathrm{R}}_{2}^{+}$ ion spectrum and compare it to earlier trap-loss fluoresecence spectra. Unlike PAI in Na, which ionizes principally by collisional autoionization, the ${\mathrm{Rb}}_{2}^{\mathrm{*}}$ intermediate photoionizes directly to ${\mathrm{Rb}}_{2}^{+}$ in a fixed optical field. These observations confirm PAI as a sensitive and broadly applicable spectroscopic tool.

DOI
10.1103/physreva.52.r4332

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We report the observation of direct, two-color photoassociative ionization (PAI) in collisions between 85Rb^{85}\mathrm{Rb}85Rb atoms optically cooled and confined in a magneto-optical trap. We also measure the resulting R2+{\mathrm{R}}_{2}^{+}R2+​ ion spectrum and compare it to earlier trap-loss fluoresecence spectra. Unlike PAI in Na, which ionizes principally by collisional autoionization, the Rb2∗{\mathrm{Rb}}_{2}^{\mathrm{*}}Rb2∗​ intermediate photoionizes directly to Rb2+{\mathrm{Rb}}_{2}^{+}Rb2+​ in a fixed optical field. These observations confirm PAI as a sensitive and broadly applicable spectroscopic tool.
Ultrafast and narrow linewidth lasers are combined to study multiphoton photoassociative ionization (PAI) in a Rb magneto-optical trap. Through the use of time of flight spectroscopy and by varying the parameters of the ultrafast laser, the excitation pathway for PAI was determined. It was found that Rb2+{{\mathrm{Rb}}_{2}}^{+}Rb2​+ is formed, not by direct photoionization, but by sequential excitation of Rb2{\mathrm{Rb}}_{2}Rb2​ to an autoionizing state. Determining the excitation and ionization pathway was a necessary first step for future efforts in trying to establish coherent control techniques, first for the efficient production of translationally cold Rb2{\mathrm{Rb}}_{2}Rb2​, and then for transferring population to a desired vibrationally cold molecular state.
We study photo-associative ionization (PAI) for sodium atoms held in a magneto-optical trap (T < 1 mK). The process is investigated using one and two laser frequencies. For a single color PAI, we measure the rate constant as a function of light intensity and the result is compared with calculation based on semiclassical Bloch equation theory. The measured rate constant appears to saturate sooner than theory. In the two-color PAI (TCPAI) we investigate individual steps of the process. Different combinations of laser frequencies prepare different populations in each of the hyperfine ground state and the contribution of each one in this collisional process is investigated. The comparison with theory allows a fairly clear understanding of PAI and TCPAI.
We report measurements of cold photoassociative ionization (PAI) spectra obtained from collisions within a slow, bright Na atomic beam. A high-brightness atom flux, obtained by optical cooling and focusing of the atom beam, permits a high degree of alignment and orientation of binary collisions with respect to the laboratory atom-beam axis. The results reveal features of PAI spectra not accessible in conventional magneto-optical trap studies. We take advantage of this high degree of alignment to selectively excite autoionizing doubly excited states of specific symmetry.
Collisions of ultracold trapped atoms ( T< 1mK) have opened a new field of atomic collisions with abundant opportunities for theory and experiments. Photo-associative ionization (PAI) in sodium atoms has been the first two-body collision Studied in optical traps3. In this process two ground state Na atoms absorb two photons during the course of the collisional encounter. The first absorption at long range put the colliding system on an attractive C3/R3 potential curve, and the two atoms begin to accelerate toward each other. A second absorption promotes the system to a doubly excited Na(3p)+Na(3p) potential curve from which the PAI process takes place at short range.
The direct photodissociation of trapped ^{85}Rb_{2}^{+} (rubidium) molecular ions by the cooling light for the ^{85}Rb magneto-optical trap (MOT) is studied, both experimentally and theoretically. Vibrationally excited Rb_{2}^{+} ions are created by photoionization of Rb_{2} molecules formed photoassociatively in the Rb MOT and are trapped in a modified spherical Paul trap. The decay rate of the trapped Rb_{2}^{+} ion signal in the presence of the MOT cooling light is measured and agreement with our calculated rates for molecular ion photodissociation is observed. The photodissociation mechanism due to the MOT light is expected to be active and therefore universal for all homonuclear diatomic alkali metal molecular ions.