Penning Ionization of (NH₂)₂C=X (X = O, S) with He*(2³S) Metastable Atoms. Difference of Anisotropic Interaction around N, O, and S Atoms

The anisotropic interaction potential energy surface around urea and thiourea ((NH₂)₂C=X, X = O,S) with the He*(2³S) metastable atom has been studied by two-dimensional (collision-energy/electron-energy-resolved) Penning ionization electron spectroscopy and by model potential energy calculation for the molecule-Li system based on a well-known resemblance between a metastable excited He*(2³S) atom and a ground-state Li(2²S) atom. A different trend was found in the interaction around the C=X (X = O, S) group; an attractive interaction was found for the perpendicular direction around the C=S axis where the 3p orbital of the S atom extends, while the collinear direction along the C=O bond was found to be most attractive. A weak interaction was found around the "nitrogen lone pair" of the amino group in the conjugated systems (urea and thiourea), which is different from the general trend in previous studies that a strong attractive interaction was found around the "lone pair" of target molecules.