Penning ionization of amides by collision with He*(2(3)S) metastable atoms

Penning ionization of amides (formamide, acetamide, and thioacetamide) upon collision with He*(2(3)S) metastable atoms was studied by collision-energy -resolved Penning ionization electron spectroscopy. Partial ionization cros s sections were found to be larger for ionization from molecular orbitals h aving electron density around the C=O and C=S groups. Collision energy depe ndence of partial ionization cross sections indicates that interaction pote ntials are highly anisotropic between He*(2(3)S) atoms and amides. The larg er negative collision energy dependence of sigma (C=O) bands as compared to pi bands shows that attractive interactions are larger around the oxygen a tom for the in-plane direction rather than the out-of-plane direction. On t he contrary, the larger negative collision energy dependence for ionization from non-bonding sulfur orbitals as compared to the a band indicates that an attractive interaction around the sulfur atom was larger for in-plane or out-of-plane sigma (C=S) directions vertical to the C=S axis. Weak interac tions around the nitrogen atoms are different from the strong attractive in teraction around the 'lone pair' orbital region of amines. (C) 2001 EIsevie r Science B.V. All rights reserved.