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Physical Review A

Polarization-dependent spectra in the photoassociative ionization of cold atoms in a bright sodium beam

Jaime Ramirez-Serrano, W. DeGraffenreid, J. Weiner

Published May 8, 2002• 6 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.65.052719

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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.
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We report beam-loss photoassociation (PA) spectra of cold collisions within a slow, bright Na atom beam near the D1{D}_{1}D1​ resonance line. The high collimation and brilliance of the atomic beam permits the detection of atomic fluorescence loss resulting from the photoassociation of cold sodium atoms. The results show that even at high PA laser intensities, where strong coupling of the colliding atoms to the optical field might obscure spectral features, vibrational and rotational progressions are surprisingly well resolved. We attribute the conservation of narrow spectral features to the properties of the saturated line shapes and to the unconventional collision energy distribution in one-dimensional (1D) single-beam collisions compared to 3D collisions in a gas cell.
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.
\n We report the observation of polarization anisotropy in the optical\nsuppression of photoassociative ionization (PAI) collisions. Binary\ncollisions occur within a tightly collimated and optically cooled velocity\ngroup of a single sodium beam. A laser beam, blue-detuned with respect to\nthe atomic resonance transition, crosses the atomic beam and suppresses the\nPAI rate. We measure this suppression as a function of intensity and\nelectric field vector aligned parallel or perpendicular to the atomic beam\naxis. Quantum close-coupling and multichannel curve-crossing calculations of\nmodel systems yield insight into the nature of the anisotropy and are in\nreasonable agreement with the measurements.\n\n \n
We report cross-section measurements of the associative-ionization process, Na(3p2{\mathit{p}}^{2}p2P3/2{\mathit{P}}_{3/2}P3/2​)+Na(3p2{\mathit{p}}^{2}p2P3/2{\mathit{P}}_{3/2}P3/2​)\ensuremath{\rightarrow}Na2+{\mathrm{Na}}_{2}^{+}Na2+​+e as a function of velocity, alignment, and orientation in the collision energy range from 300 to 5 K. Two independently tunable laser beams select narrow velocity classes of excited sodium in a well-collimated atomic beam. Circular and linear polarization of the excitation lasers permits alignment and orientation of the excited atoms in the two velocity classes. The excitation functions show pronounced differences reflecting different molecular pathways to the final product, and they bridge the energy range between conventional and ``cold'' collision dynamics.
Three-dimensional quantum scattering calculations predict that the degree of optical shielding of cold collisions saturates with increasing intensity very differently from two-state models and is sensitive to the polarization of the shielding light. We report measurements showing how suppression of Na photoassociative ionization (PAI) rates is sensitive to the intensity and polarization of the shielding laser. At higher intensities, circular polarization suppresses PAI rates an order of magnitude more effectively than linear polarization, in qualitative agreement with our model calculations.