Alternating-gradient focusing of the benzonitrile-argon van der Waals complex

We report on the focusing and guiding of the van der Waals complex formed between benzonitrile molecules (C6H5CN) and argon atoms in a cold molecular beam using an ac electric quadrupole guide. The distribution of quantum states in the guided beam is non-thermal, because the transmission efficiency depends on the state-dependent effective dipole moment in the applied electric fields. At a specific ac frequency, however, the excitation spectrum can be described by a thermal distribution at a rotational temperature of 0.8 K. From the observed transmission characteristics and a combination of trajectory and Stark-energy calculations we conclude that the permanent electric dipole moment of benzonitrile remains unchanged upon the attachment of the argon atom to within +/- 5%. By exploiting the different dipole-moment-to-mass (mu/m) ratios of the complex and the benzonitrile monomer, transmission can be selectively suppressed for or, in the limit of 0 K rotational temperature, restricted to the complex. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4748814]