Penning ionization of amides by collision with He*(2³S) metastable atoms

Penning ionization of amides (formamide, acetamide, and thioacetamide) upon collision with He*(2³S) metastable atoms was studied by collision-energy-resolved Penning ionization electron spectroscopy. Partial ionization cross sections were found to be larger for ionization from molecular orbitals having electron density around the C double bond O and C double bond S groups. Collision energy dependence of partial ionization cross sections indicates that interaction potentials are highly anisotropic between He*(2³S) atoms and amides. The larger negative collision energy dependence of σ_{C double bond O} bands as compared to π bands shows that attractive interactions are larger around the oxygen atom for the in-plane direction rather than the out-of-plane direction. On the contrary, the larger negative collision energy dependence for ionization from non-bonding sulfur orbitals as compared to the σ_{C double bond S} band indicates that an attractive interaction around the sulfur atom was larger for in-plane or out-of-plane directions vertical to the C double bond S axis. Weak interactions around the nitrogen atoms are different from the strong attractive interaction around the `lone pair' orbital region of amines.