Photoassociation of 40Ca at high laser intensities

Abstract

In this thesis I present the results of photoassociation measurements in ensembles of atomic calcium over an intensity range of below 1 W/cm2 up to 600 W/cm2. The temperature of the atoms was reduced to 1 μK by laser and evaporative cooling. Afterwards the two most-weakly bound states in the excited molecular potentials that dissociate to the asymtote 3P1+1S0 have been investigated regarding the shape and absolute rate of the resonances. The investigation showed that the measured rates differ by more than an order of magnitude compared to theoretical calculations. The predictions are based on a scattering formalism by Bohn & Julienne and wave functions derived from coupled-channel calculations. The shape of the resonances, the width in particular, agrees well with the numerical calculations. Especially the power broadening at high intensity is reproduced by the experiment. Further the light shift introduced by the photoassociation laser has been measured which also differs from the predictions. In the further course of the thesis the numerical simulations were improved by considering additional influences like the exact dependence of the Franck-Condon density and the light shift of the photoassociation laser as a function of the collision energy. Additionally the modeling of the experimental setup with a special emphasis on the volume of the optical trap and thus the density of the atomic ensemble was improved and the influence of varying trap depth and photoassociation duration has been studied. Investigating possible explanations for the discrepancy of the measured rates compared to theory, I show that by a modification of the coupling parameter and the spontaneous decay rate of the molecular state the rates measured in the experiment can be reproduced theoretically, with the restriction that at high intensity the calculated line shape no longer agrees with the measurement. In the last chapter an improved experimental apparatus is presented that by using an optical lattice suppresses thermal broadening and thus simplifies the analysis substantially.