Rotational-state-specific guiding of large molecules

A beam of polar molecules can be focused and transported through an ac electric quadrupole guide. At a given ac frequency, the transmission of the guide depends on the mass-to-dipole-moment (m/μ) ratio of the molecular quantum state. Here we present a detailed characterization of the m/μ selector, using a pulsed beam of benzonitrile (C₆H₅CN) molecules in combination with rotational quantum state resolved detection. The arrival time distribution as well as the transverse velocity distribution of the molecules exiting the selector are measured as a function of ac frequency. The μ/Δμ resolution of the selector can be controlled by the applied ac waveforms and a value of up to 20 can be obtained with the present setup. This is sufficient to exclusively transmit benzonitrile molecules in quantum states with the same m/μ value as the absolute ground state. The operation characteristics of the m/μ selector are in quantitative agreement with the outcome of trajectory simulations