GENERATION OF RADICALS ON A METAL-SURFACE FROM PHOTOINDUCED DISSOCIATION OF PHYSISORBED MOLECULES - CH2 FROM H2CO ON AG(111)

Submonolayer formaldehyde (H2CO) has been photodissociated on Ag(111) at 35 K using nanosecond 287 nm laser pulses. The low temperature proh ibits further bimolecular reactions of the dissociation products and a llows their analysis by electron energy loss spectroscopy. One of the dissociation products has been identified as the radical species methy lene (CH2). The dissociation is induced by photoexcited substrate elec trons attached to formaldehyde, forming a transient formaldehyde negat ive ion that dissociates. Due to the influence of adsorbate-substrate bonding on reaction energetics, the dissociation channel of H2CO- on A g appears to be different from that of the gaseous negative ion.