AUTLER-TOWNES SPECTROSCOPY OF THE LITHIUM DIMER MOLECULE

Abstract

This thesis consists of two experimental applications of the Autler-Townes (AT) spectroscopy. In the first experiment, we have determined the electronic transition dipole moment for the 7Li2 A1Σu+ - X1Σg+ system experimentally by using a 4-level continuous wave extended Λ excitation scheme and compared our results with theoretical predictions. 7Li2 is a good test case for the accuracy of the AT splitting based technique to determine the transition dipole moment and its internuclear distance R dependence. The molecule has only 3 electrons per atom. The A1Σu+ - X1Σg+ potential energy curves were well known and thus, one could calculate accurate rovibrational wavefunctions for the simulations. In addition two different quantum mechanical models were available for the comparison: an all-electron valence bond self-consistent-field method and a pseudo-potential molecular orbital method. Our experimental results for the absolute magnitude of the transition dipole matrix elements for rovibronic transitions for different R-centroid values are in excellent agreement with ab initio theoretical calculations of the transition dipole moment. We believe that this technique will become an important method for accurate measurement of the absolute value and R-dependence of electronic transition dipole moments in molecules. The comparison with theory reinforces this view on the accuracy and universality of the AT method. The focus of the second part of this thesis is on experimentally controlling the singlet-triplet character of the 7Li2 molecule by using an external coupling laser field. We have demonstrated experimentally for the first time that the frequency domain quantum control scheme developed by T.Kirova and F. C. Spano (Physical Review A, 71, 063816, 2005) can be used to control the mixing coefficients of a weakly perturbed pair of singlet and triplet rovibrational levels. The coupling field, when tuned to resonance with the rovibronic transition involving the singlet component, causes it to AT split, leading to enhanced mixing of the pair of levels, as predicted by theory.