Many complex molecules have multiple structural isomers; that is, multiple local minima on their potential energy surface. About twenty-five years ago, it was observed that multiple conformers of tryptophan are present even at the low temperatures of a few Kelvin in a supersonic jet. These conformers have been studied extensively since then with sophisticated spectroscopic techniques. Individual conformers can be identified from their distinct electronic or microwave spectra. Information on the conformational structures can be obtained using microwave or multipleresonance infrared spectroscopy, for example. In similar experiments it was even possible to obtain information on the barriers separating the conformers. The preparation of spatially separated conformers would provide unique possibilities for advanced further investigations. The chemical properties of the individual species and their differences could be directly studied in reactive scattering experiments. Such pure samples would also enable a new class of experiments, such as electron and X-ray diffraction or tomographic imaging experiments of complex molecules in the gas phase. Molecular-frame photoelectron angular distributions, ultrafast time-resolved photoelectron spectroscopy, and ultrafast dynamics studies would also benefit from the availability of these pure samples. For charged species, the separation of molecules with different shapes has been demonstrated by utilizing ion mobility in drift tubes. For neutral molecules, the abundance of the conformers in molecular beams can be partly influenced by selective over-the-barrier excitation in the early stage of the expansion or by changing the carrier gas. Herein, we demonstrate that electrostatic deflection, a classic molecular beam manipulation method that dates back to the 1920s, allows the spatial separation of the conformers of a neutral molecule when it is applied to intense beams of rotationally cold molecules produced by a state-ofthe-art pulsed supersonic expansion source. The idea of exploiting electrostatic deflection to separate quantum states was already conceived by Stern in 1926 for light diatomic molecules, and these ideas were recently extended to proposals for the separation of conformers of large molecules. Polar molecules experience a force in an inhomogeneous electric field. This force is due to the spatial variation in the potential energy of the molecules, and is given by F ! = meff·r ! E. The effective dipole moment meff of a molecule in a given quantum state is the negative gradient of the potential energy with respect to the electric field strength E. This force has been used to decelerate small molecules in a supersonic jet to a standstill and subsequently trap them. Similarly, large neutral molecules have been deflected, focused, and decelerated. Passing polar molecules through a strong inhomogenous electric field will spatially disperse them according to their effective dipole moment. In particular, the conformers of a specific biomolecule all have the same mass m, but differ by the relative orientations of their functional groups. Typically, these functional groups have large local dipole moments associated with them, and the vectorial sum of these local dipole moments largely determines the overall dipole moment of the molecule. Herein, we show that the resulting different overall dipole moments of the conformers can be exploited to select individual conformers using an electrostatic deflector. The cis and trans conformers of 3-aminophenol (Figure 1) are used herein as prototypical structural isomers of complex molecules. From the precisely known rotational constants and dipole moments, the energies of the rotational states of cis3-aminophenol and trans-3-aminophenol are calculated as a function of electric field strength. Figure 1 shows the resulting Stark curves for the lowest rotational states of both species. From Figure 1, it is obvious that the effective dipole moments meff of the states of cis-3-aminophenol are considerably larger than for trans-3-aminophenol, and therefore, a strong spatial separation of the conformers is expected. The results on the separation of the cis and trans conformers of 3-aminophenol are complementary to our previous experiments on the separation of the same species using an ac (alternating current) focusing device, and we will briefly discuss the merits of the individual techniques in the Summary. The experimental setup is shown in Figure 2, and a detailed description is given elsewhere. 3-aminophenol (Sigma–Aldrich, 98%), seeded in 90 bar of helium, is released from a pulsed valve into high vacuum. The molecular beam is [*] F. Filsinger, Dr. J. K pper, Prof. Dr. G. Meijer Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4–6, 14195 Berlin (Germany) Fax: (+49)30-8413-5892 http://www.fhi-berlin.mpg.de/mp/jochen E-mail: jochen@fhi-berlin.mpg.de