Ab initio based calculations of cavity cooling including the ro-vibrational modes of the OH radical

摘要

Ultracold molecules are interesting for the study of cold chemistry, cold collisions, astrochemistry and quantum information processing. Their preparation, in particular of ultracold ground state molecules, however, is still challenging. One class of preparation strategies are optical methods, which have been enormously successful for atoms. Adapted versions of laser cooling, which try to optimize optical pumping into the molecular ground state, have been proposed [1], but their efficiency is limited by the absence of closed transitions in molecules. A more viable avenue seems the use of cavities for cooling, which would avoid the leakage by open transitions. For instance, efficient cavity cooling of the external degrees of freedom has been proposed in Refs. [2]. For atoms, cavity cooling was recently demonstrated [3]. We present a theoretical approach for the simulation of the cooling of internal and external molecular degrees of freedom in a cavity. The idea is to sequentially depopulate excited ro-vibrational levels by vacuum-stimulated Raman scattering into the mode of a high-finesse cavity. As a model system we choose the OH radical. By using a combination of ab initio quantum chemical and experimental data we are able to determine the internal energy level structure and the transition frequencies and strengths needed for our cooling scheme. We take into account the perturbing spontaneous and the coherent Raman processes amplified by the cavity to get a realistic model. This gives us insight in the cooling process and lets us predict the time scales needed to prepare a molecule in its ground state.