A photoionization experiment on free oriented molecules in the gas phase has been performed. CH3I molecules in a supersonic beam have been oriented with respect to the molecular axis parallel to an external field by use of an electric hexapole. The photoelectrons ejected by vacuum ultraviolet radiation in a region of a very weak field from the lone-pair orbital located at the iodine atom show a pronounced asymmetry depending on whether electrons are emitted parallel or antiparallel to the molecular axis.PACS numbers: 33.80.Eh, 33.60.CvRecent photoemission experiments with atoms, 1 molecules, 2 or solids 3 have shown that details of the photoionization dynamics can be obtained if the photoelectron spectroscopy studies are performed in an angular-and/or even spin-resolved manner. In molecular photoionization, however, still much detailed information was hidden because only randomly oriented molecules could be photoionized in previous gas-phase experiments. Theoretical investigations into the study of photoelectron emission of free oriented molecules became very important and have been successfully performed 4 " 8 after there was experimental evidence 9 that photoemission from molecules chemisorbed on a solid-state surface reflects the orientation of the molecules with respect to the surface. To close the gap between photoionization of randomly oriented molecules in the gas phase and of oriented molecules in the chemisorption requires experimental studies of the influence of the molecular orientation onto the molecular photoemission process. We present for the first time experimental results of an angular-resolved photoelectron spectroscopy study on free oriented molecules in the gas phase.Diatomic closed-shell molecules with permanent electric dipole moment /x rotate perpendicularly to /x, so that their orientation requires enormous electric fields (~~ 10 7 V/cm) to overcome the rotational inertia of the molecules. 10 " 12 However, symmetric top molecules like CH 3 I can have rotational states, where the rotational momentum J has a component parallel to /ut, i.e., symmetric tops in these rotational states can be oriented in lower electric fields, which is a critical requirement in our photoionization experiment. The experimental technique corresponds to an ''electrostatic Stern-Gerlach method" 10 " 14 with use of an electrostatic hexapole. Figure 1 shows the schematic setup of the apparatus. The vacuum ultraviolet (VUV) lamp 15 and the cylindrical-mirror-analyzer electron spectrometer are necessary for photoelectron spectroscopy. The main parts of the source of oriented molecules are the supersonic beam source, the electrostatic hexapole, and the guiding field; in between a buffer field is placed. The interaction energy of a CH 3 I molecule with an electric field E is 13where 9 is the angle between /JL and E, and 7, K, and Mj are the rotational quantum numbers. 14 The force G = -grad H^S tark onto the molecules in the inhomogeneous electric field of the hexapole separates molecules with different orientation of the...