Photoassociation spectroscopy of cold atoms in a bright atomic beam

We report the first cold photoassociation (PA) spectroscopy experiments carried out in an atomic beam. Previous studies have only been performed in magneto-optic traps (MOTs) due to the low density and brilliance of existing cold atom beams. We have, however, successfully applied laser cooling techniques to increase the density of a sodium beam to 1 × 1010 cm−3, and its brightness to 2 × 1024 m−2s−1sr−1, making it, to our knowledge, the brightest cold atomic beam in the world.; The high brightness and strong collimation of the atom beam permits the alignment of the interatomic collision axis with the atomic beam propagation direction. It is the axis against which the polarization of the photoassociating light is referenced. Cold photoassociation spectra is obtained taking the polarization of the PA light as a new parameter, and we have explored the cases of parallel, perpendicular and circular polarizations.; To monitor the photoassociation mechanism two techniques have been used. In the first, photoassociative ionization (PAI), we measure the production of $Na+2$ ions during a two-photon process. In the second, beam-loss, we measure the loss of atoms (fluorescence) in the beam as they are photoassociated into molecules. These investigations are similar to PAI and trap-loss experiments done in MOTs, and are the first to be carried out in an atomic beam.; The PAI polarization-dependent spectra are obtained near the 3 2S_1/2+32P_3/2 molecular potentials asymptote, and shed new light on the importance that the different molecular potentials play during the photoassociation mechanism. In particular, our results show that in the autoionization mechanism from the doubly-excited states, previously identified as having 1_u and $0-u$ symmetry, the 1_u and $0-u$ states play an equally strong role during the PAI process. This conclusion is based on the different excited state populations in these doubly-excited states that arises from the coupling of molecular states with light of different polarization, together with the difference in ion signals measured from the polarization-dependent PAI spectra.; The beam-loss results are obtained near the 32S_1/2 +32P_1/2 molecular potentials asymptote. Here we have identified states of $0-g$ symmetry based on peak separation, width, and rotational structure. However, the results are obtained in the high intensity regime where level shifts and power broadening may alter the spectral features, and no strong-field theory is available to make quantitative comparisons between measured and calculated spectra.