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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/textmu) ratio of the molecular quantum state. Here we present a detailed characterization of the m/textmu selector, using a pulsed beam of benzonitrile (C$_6$H$_5$CN) 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 textmu/$Delta$textmu 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 molecules in the absolute ground state of benzonitrile, or rather in quantum states that have the same m/textmu value as the ground state. The operation characteristics of the m/textmu selector are in quantitative agreement with the outcome of trajectory simulations.
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