Direct determination of the sign of the NO dipole moment

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.