Steffen Spieler, Frank Stienkemeier (Freiburg, Germany)

\tDirect cooling of molecules from room temperature is essential for the study of a variety of cold collisions and reactions. However, it is still a challenge to make cold molecules in the J=0 rotational ground state despite the fact that the rotational ground state is of great interest in many applications. Here, we report that the manipulation of the translational motion of a polar molecule in its rotational ground state is realized by the microwave (MW) dipole force combined with a counter-rotating nozzle. A cold molecular beam of CH₃CN seeded in Kr with the longitudinal velocity of about 100 m s^-1 was created by a pulsed counter-rotating nozzle.  The cold beam was then introduced into a cylindrical MW cavity, in which a standing wave, TM_01p mode MW field, nearly resonant to the J=1?0 rotational transition of CH₃CN was created.   By choosing an appropriate MW frequency, we successfully observed focusing and deflection of the cold beam of CH₃CN due to the lens effect of the MW standing wave. The present result shows that it is possible to decelerate and trap room-temperature polar molecules in the rotational ground state by the MW dipole force.