Simulating Spatial Microwave Manipulation of Polyatomic Asymmetric-Top Molecules Using a Multi-Level Approach

A numerical approach that employs a multi-level dressed state method to determine the AC-Stark shifts of molecular rotational energy levels is described. This approach goes beyond the two-level approximation often employed for simpler molecules, such as ammonia and acetonitrile, and is applicable to a variety of molecules. The calculations are used to develop experiments aimed at focusing, guiding, decelerating and trapping neutral, polyatomic, asymmetric-top molecules by using microwave fields. Herein, numerical calculations are performed for acetonitrile and 4-aminobenzonitrile. Based on these results, trajectory simulations are performed to predict the outcome of microwave focusing experiments in the TE_1,1,p mode of a cylindrically symmetric microwave resonator. Simulations show that, for such an experimental setup, microwave focusing and guiding of 4-aminobenzonitrile requires starting longitudinal velocities close to, or below, 100 m s^-1 , that is, much lower than values obtained with standard molecular beam techniques, such as supersonic expansion. Therefore, alternative beam-generation techniques, for example, buffer-gas-cooled molecular beams, are required to extend microwave manipulation methods to larger and more complex molecules.