We report a novel approach for determining the sign of permanent dipole moments, using nitric oxide [NO(v=0)] as an example. State-selected NO (j=|m|=|Omega=1/2) molecules are focused using a hexapole and oriented in a strong dc electric field. The angular distributions of ionic fragments resulting from extreme ultraviolet single-photon and multiphoton dissociative ionization at 400 and 800 nm are measured and indicate that the dipole moment is negative (corresponding to N-O+). The experiments thus rule out an error in the sign of the dipole of NO as the possible source of a remarkable discrepancy between previous theoretical and experimental work on orientation effects in bimolecular collisions involving oriented NO.

Phys. Rev. Lett.

Gijsbertsen, A., Siu, W., Kling, M. F., Johnsson, P., Jansen, P., Stolte, S., & Vrakking, M. J. J. (2007). Direct determination of the sign of the NO dipole moment. Phys.Rev.Lett., 99(Article number: 213003), 1–4. doi:10.1103/PhysRevLett.99.213003