  We report on the focusing and guiding of the van der Waals complex formed
between benzonitrile molecules (C$_6$H$_5$CN) 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 \pm5%. 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.
