Penning ionization of (NH2)(2)C=X (X = O, S) with He*(2(3)S) metastable atoms. Difference of anisotropic interaction around N, O, and S atoms

The anisotropic interaction potential energy surface around urea and thiour ea ((NH2)(2)C=X, X = O,S) with the He*(2(3)S) metastable atom has been stud ied 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 meta stable excited He*(2(3)S) atom and a ground-state Li(2(2)S) atom. A differe nt trend was found in the interaction around the C=X (X = O, S) group; an a ttractive interaction was found for the perpendicular direction around the C=S axis where the 3p orbital of the S atom extends, while the collinear di rection along the C=O bond was found to be most attractive. A weak interact ion was found around the "nitrogen lone pair" of the amino group in the con jugated systems (urea and thiourea), which is different from the general tr end in previous studies that a strong attractive interaction was found arou nd the "lone pair" of target molecules.